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Division of Nutritional Sciences, University of Illinois, Urbana, IL 61801 and * Department of Human Nutrition, The Ohio State University, Columbus, OH 43201
3To whom correspondence should be addressed. E-mail: jwerdman{at}uiuc.edu.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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72%), although total hepatic 14C decreased after 24 h. In a comparison of the extrahepatic tissue at 168 h, the 14C was greatest in adipose tissue followed by spleen and then adrenal; 80% of the 14C in the liver was in the cis and all-trans configuration at all time points. At 3 h, the 14C in seminal vesicles was primarily in the all-trans plus 5-cis forms (70%), but by 168 h, 55% of 14C was present as 14C-polar products. Despite the presence of unlabeled lycopene in the prostate, the primary 14C form was in 14C-polar products (67–92%), even at 3 h. The percentage and amount of 14C-polar products in the dorsolateral prostate lobe increased from 3 to 24 h and then reached a plateau. The data suggest that lycopene may be metabolized differently among tissues in rats prefed lycopene.
KEY WORDS: • lycopene • lycopene metabolites • biodistribution • prostate cancer • rats
A primary candidate for biochemoprevention of prostate cancer is lycopene, the red pigment in tomatoes. Lycopene is generally the most abundant carotenoid in human plasma and is highly concentrated in the liver, adrenal glands, testes and prostate (1–4). Additionally, lycopene (2) and oxygen-containing lycopene metabolites (5) accumulate in the human prostate, which suggests that lycopene or its metabolites may have direct effects within prostate tissue. Although lycopene has been shown to be a potent singlet oxygen quencher (6) and to influence gap junctional expression (7), the specific mechanisms by which lycopene influences prostate cancer are unknown.
Studies in rodents and humans clearly indicate that the absorption and metabolism of lycopene is a complex process (8–10). In ferrets, cis isomers of lycopene have been reported to be more bioavailable than the all-trans lycopene form (9), which is the primary form found in raw foods. Metabolic products of lycopene, 2,6-cyclolycopene-diol I and II, have been identified in human milk and serum (11) and in the human prostate (5). Current research focuses on the identification of polar and/or chain-shortened metabolites of lycopene, which may influence health in a manner different from the parent compound (12,13). For example, the potential health benefit of lycopene may be mediated by metabolites that influence gene expression as was shown with the carotenoid metabolites retinoic acid (14) and 4-oxo-retinoic acid (15), which are derived from ß-carotene and canthaxanthin, respectively.
Considering the high interest in lycopene and disease risk, there is a paucity of research on lycopene uptake and tissue distribution. In part, this has been due to a lack of availability of radiolabeled lycopene. In addition, most animal models used in preclinical testing of chemoprevention agents are not ideal for evaluating carotenoid absorption (16); thus, extrapolation of relevance in humans should be done with caution. Rats given N-methyl-N-nitrosourea and testosterone were reported to be a relevant and reliable model for efficacy testing of chemopreventative agents (17,18). Because our laboratory utilized the rat model to evaluate lycopene and prostate cancer risk (19–22), we chose F344 rats for this work. Additionally, we focused attention on the dorsolateral lobe of the rat prostate due to its similarity in histological features and hormonal-responsiveness to the peripheral zone of humans (17). Other species [ferrets and gerbils (9,16,23,24)] might better mimic humans for lycopene absorption studies, but they are not suitable for prostate cancer studies.
The objective of this study was to examine the time course of tissue uptake of lycopene and its metabolism by measuring the appearance of polar 14C-labeled compounds in tissues of F344 rats dosed with 14C-lycopene. Rats were prefed a lycopene-enriched diet to best mimic a human environment in which tomatoes and other lycopene-containing foods are consumed over a lifetime.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The study was approved by the University of Illinois Laboratory Animal Care and Use Committee and followed all necessary protocols to ensure the humane treatment of the rats. F344 male rats weighing 54–70 g (21 d of age) were purchased from Harlan (Indianapolis, IN) and were housed individually in wire-bottomed cages appropriate for their size. The rats were exposed to a diurnal light cycle (0700–1900 h) with a room temperature of 25°C and were monitored daily. They were weighed and provided with fresh AIN-93G lycopene-enriched diet (20) every other day. Rats had free access to food and reverse osmotic water for the duration of the study. Tongue depressors were placed in cages to provide the rats with environmental enrichment.
Diet and 14C-lycopene doses.
An AIN-93G lycopene-enriched semipurified pelleted diet (20) (Dyets, Bethlehem, PA) with a target concentration of 0.25 g lycopene/kg was fed throughout the study. The lycopene concentration in the diet was evaluated by our laboratory, and this amount provides levels of lycopene in tissues comparable to that in humans (22). Lycopene was added to the diet in the form of water-dispersible beadlets containing 10% lycopene (Hoffmann La Roche, Basel, Switzerland). The diet was stored at 4°C. The radioactive 6,7,6',7'-[14C] lycopene (specific activity = 1.83 µCi/mg) suspended in benzene (a gift from Regina Goralczyk, Hoffmann La Roche, Switzerland) was stored in the dark at -80°C. On the day of dosing, the vial was opened and benzene was evaporated completely under argon. Lycopene was reconstituted in chloroform (3 washes of the vial) and added to 25 mL of cottonseed oil. An approximate equivalent amount of nonradioactive lycopene, extracted from the 10% lycopene beadlets, was added as a carrier for a final specific activity of 0.92 µCi/mg lycopene. Chloroform was evaporated before dosing to make a final concentration of 1 mCi 14C-lycopene/11.2 mg of total lycopene. The total amount of 14C-lycopene dosed per rat was 22 µCi, which provided 0.246 mg total lycopene (includes the 14C and nonradioactive lycopene). This amount was only 5% of the 5 mg/d of lycopene obtained from the diet. The dose was 98 ± 0.39% all trans-lycopene with minor cis isomers as impurities. The solubility of lycopene in oil was ensured by observations under the light microscope. 14C-lycopene is very susceptible to isomerization; thus precautions were taken including preparation of the stock solution under yellow lights on the day of dosing, keeping the 14C-lycopene on ice before its addition to oil and purging the 14C in oil (stock solution) with argon.
Study design.
At 21 d of age, the feeding of a lycopene-enriched AIN-93G diet containing 0.25g lycopene/kg diet was initiated. At 51 d of age, rats were gavaged with 22 µCi 14C-lycopene in 0.5 mL of cottonseed oil without overnight food deprivation. Before gavage, each rat was anesthetized using CO2 until loss of consciousness occurred (10–15 s). After the radioactive dose, each rat was moved to a metabolic cage to facilitate urine and feces collection every 24 h (stored at 4°C). At 0, 3, 6, 24, 72 or 168 h (n = 8/time point) post-14C-lycopene dose, rats were anesthetized with CO2 and blood was taken via cardiac puncture. The rats then were killed by CO2 asphyxiation and the adipose, adrenal, heart, spleen, liver, lung, kidney, testes, prostate-seminal vesicle complex, stomach contents, mucosal cells and large and small intestinal contents were excised, weighed and immersed in liquid nitrogen. All harvested tissues and intestinal contents were stored at -20°C, except the seminal vesicles, prostate and bladder (prostate-seminal vesicle complex), which were removed intact as a single unit, immersed in liquid nitrogen and then immediately stored at -80°C.
Analysis of total 14C in tissues, serum and feces.
Collected tissues were analyzed for 14C using liquid scintillation counting (LS-6500, Beckman Instruments, Fullerton, CA). Depending on the particular tissue and expected lycopene concentration, 0.1–0.6 g of tissue was placed in a 20-mL glass vial with 1 mL of tissue solubilizer-II (Research Products International, Mount Prospect, IL). The vials then were incubated in a water bath at 50°C for 4–6 h (depending on the tissue matrix). Next, the tubes were mixed on a vortex and 200–1000 µL of 30% hydrogen peroxide (Fisher Scientific, Fair Lawn, NJ) was added to samples for decolorization. Concentrated acetic acid (Sigma, St. Louis, MO) was then added in the range of 100–500 µL to the sample vial. Higher amounts of acetic acid were added when more hydrogen peroxide was used. Finally, 15 mL of scintillation cocktail fluid (Biosafe II, RPI, Mount Prospect, IL) was added to the 20 mL sample vial, mixed on a vortex and analyzed for total radioactivity. Fecal samples were removed from the freezer and placed in a mortar under the hood overnight for drying. The total feces from the 24-h collection period were weighed, ground using a mortar and pestle, and 0.01-g aliquots were weighed into 20 mL glass vials in triplicate. The release of the radioactivity from the tissue matrix was facilitated as described above. The assumption that serum accounts for 5.46 g/100 g total body weight was used to calculate the total serum radioactivity (25).
Prostate and seminal vesicle dissection.
Each prostate-seminal vesicle complex was dissected under a dissecting microscope (Olympus, Lake Success, NY) in 4°C cold HBSS (Sigma, St. Louis, MO) into three lobes: dorsolateral, anterior and ventral. The anterior and ventral lobes were then pooled for analysis. The complete dissection process required a maximum of 35 min. Due to the small amount of prostate tissue obtained from each rat, it was necessary to pool all prostate tissue samples to obtain an accurate analysis of 14C.
Serum preparation.
To maximize accuracy and reliability for recovery of radioactivity from serum samples, results were obtained by pooling serum samples from two rats.
Tissue lycopene extraction.
The tissue extraction procedure was followed with slight modifications as previously utilized by our laboratory (20). Briefly, analysis of the hepatic tissue was accomplished by placing 0.5 g of hepatic tissue into a 50-mL tube and thoroughly mincing. KOH/ethanol solution (12 mL, 1:5) containing 0.01% BHT was added and mixed on a vortex. The samples then were placed in a water bath at 60°C for 30 min for complete saponification. The samples were immediately placed on ice and 3 mL of double deionized water was added, providing for separation of layers. Lycopene was extracted four times by adding 7 mL of hexane. After extraction, the sample was dried in a Speedvac concentrator (model AS160; Savant, Farmingdale, NY), flushed with argon and stored at -20°C for 
2 d before HPLC analysis. A similar procedure was used for extrahepatic tissues. The tissue extraction procedure was examined previously and optimized to reduce the potential for degradation and/or isomerization of lycopene (26). The addition of an antioxidant (BHT), the minimal exposure to heat during saponification and maintaining the sample on ice during the extraction procedure minimize isomerization and oxidation. It is important to note that this particular procedure will isolate primarily the nonpolar compounds and will not extract the most polar metabolic products. The quantification of the hexane insoluble polar metabolites was not carried out in the current study, but future studies will focus on the identification of these polar products utilizing alternative extraction procedures.
HPLC analysis of lycopene isomers and 14C-labeled products.
Separation of lycopene isomers and polar 14C-containing products was conducted using HPLC methods previously utilized by our laboratory (20,27). A Waters 991 detector (Millipore, Milford, MA) with monitoring from 250 to 550 nm, a Rainin Dynamics gradient pump system model SD-200, a Varian Prostar model 210 (Woburn, NC) and a C30, 4.6 x 150 mm column (YMC, Wilmington, NC) were used for detection of lycopene isomers and polar 14C-products.
Assignment of all-trans, cis lycopene peaks and 14C-labeled products.
Peaks eluting from the HPLC were identified on the basis of their spectra, standards and literature (11,19–20,27). We chose to group trans and cis isomers of lycopene and metabolic products as follows: polar products eluting from 1 to 5.5 min, unidentified metabolites eluting from 6 to 10 min, minor cis isomers eluting from 10.5 to 20 min, major cis isomers eluting from 20.5- 26 min and all-trans and 5-cis lycopene co-eluting from 27 to 30 min.
Collection of 14C.
After the separations, the HPLC eluent was collected every 30 s into 7-mL glass vials and dried under the hood. Once evaporated, the vials were filled with 6 mL liquid scintillation counting fluid (Biosafe II, RPI, Mount Prospect, IL) and manually shaken; 14C was detected using a Beckman, LS-6500 (Bakersfield, CA).
Statistical analysis.
The experiment was conducted as a complete randomized design with time as the main effect measured. Mean differences were analyzed by one-way ANOVA using the general linear modeling procedure. The prostate and seminal vesicle tissues were pooled within groups, resulting in one data point; thus statistical analysis was not performed for these tissues. When significant differences were found (P < 0.05), group differences were further analyzed by the post-hoc Tukey’s studentized range test. Data with unequal variances were analyzed using multiple comparison and a sums of squares types I and III, which corrects for unequal variances. Results were expressed as means ± SEM. All statistical analyses were conducted with SAS (version 8.1; SAS Institute, Cary, NC).
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The concentration of hepatic lycopene was 138 ± 13 nmol/g. The amount of lycopene found in the serum was 562 ± 72 nmol/L. In a comparison of the ventral plus anterior prostate, dorsolateral prostate and seminal vesicles, the dorsolateral prostate lobe had the highest lycopene concentration at 3.4 ± 0.9 nmol lycopene/g tissue, followed by the ventral (1.7 ± 0.09 nmol/g) and seminal vesicles (0.7 ± 0.06 nmol/g).
Retention and recovery of 14C dose.
Each rat was dosed with 22 µCi of lycopene in 0.5 mL of cottonseed oil. The amount of 14C retained in the body (including tissues and serum) at 3, 6, 24, 72 and 168 h was estimated to be 3, 4, 6, 4 and 3%, respectively. The recovery of the 14C dose (including tissues, gastrointestinal contents, feces and urine) at 168 h was 74 ± 4.8% (data not shown).
Total 14C in gastrointestinal tract (GI) over time.
At the end of 1 wk, the stomach contents contained the greatest amount of total 14C in the GI tract, followed by the large intestinal contents, the mucosal cells and, lastly, the small intestinal contents (Fig. 1).
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The most prominent route of 14C elimination in rats was through the feces, followed by a small amount eliminated in the urine (Fig. 2A and B). Within the first 24 h, 36 ± 0.6% of the 14C dose was excreted from the body through feces, whereas 1 ± 0.02% was excreted through urine. At the end of 1 wk, 68 ± 4.6% of the dose was eliminated through feces. The majority of 14C excretion was achieved within the first 48 h (54 + 3.4%); however, radioactivity was still detected up to 168 h later in both the feces and urine, with 2.1 ± 0.6% of the dose excreted between 72 and 168 h. All of the 14C in urine was assumed to be absorbed and metabolized 14C-labeled metabolic products.
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Serum 14C was maintained between 3 and 24 h (0.028 ± 0.004 µCi) with a reduction (P < 0.05) to 0.009 ± 0.001 µCi at 72 h (Fig. 3A and B). In the hepatic tissue, maximal accumulation was reached at 24 h; 1.13 ± 0.09 µCi was detected on a whole-tissue basis, but radioactivity decreased to 0.50 ± 0.06 µCi by 168 h (P < 0.01). The greatest amount of total 14C at all time points occurred in the liver.
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All extrahepatic tissues examined contained measurable amounts of 14C that varied depending on tissue and time (Fig. 4A and B). A 14C increase (P < 0.05) in the spleen was observed at 72 h but decreased (P < 0.05) by 168 h. It is interesting to note that due to differences in tissue weights, the spleen and adrenal on a per gram basis contained similar amounts of radioactivity, as did the liver (data not shown). For example, at 168 h, spleen, adrenal and liver radio activities were 0.02, 0.02 and 0.04 µCi/g, respectively. One week post-14C dosing, the greatest amount of 14C was retained in the liver, followed by the adipose tissue, the adrenal gland and the spleen.
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The all-trans and major cis isomers of lycopene were the primary isomeric forms found in the hepatic tissue (Fig. 5C, peaks 1 and 2) compared with the other minor cis isomers (peaks 3) or polar products (peaks 4 and 5). Hepatic 14C was made up of 60% all-trans- plus 5-cis- lycopene, 20% major cis isomers and 20% polar products of lycopene. The combined all-trans and 5-cis 14C peak appeared to increase over time. At 3 h, an estimated 21% of hepatic 14C was in the major cis isomer form and it decreased to 13% by the end of 168 h (data not shown).
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The 14C profile of a lipid extract from the seminal vesicles and prostate lobes of the group of rats killed 3 h postdosing was examined (Fig. 6). In the seminal vesicle, 40% of the 14C was in the form of all-trans, 5-cis and major cis isomers of lycopene (Fig. 6A, peaks 1 and 2). However, a large percentage (38%) was also present as polar products of lycopene (Fig. 6A, peak 5). The lowest amount of 14C was present as minor cis isomers and unidentified or polar products (Fig. 6A, peaks 3 and 4). The 14C profile of the seminal vesicle was similar to that of hepatic tissue (Fig. 5) although a greater percentage of polar products was present in seminal vesicles.
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Presence of polar products in hepatic tissue and dorsolateral prostate.
The total 14C from a hexane extract of pooled prostate and seminal vesicle for each time point was evaluated (Fig. 7). Seminal vesicle weights were more than three times that of the prostate gland (mean pooled group prostate weight was 0.44 g and mean pooled group seminal vesicles weight was 1.59 g); because of this size difference, the data are represented as µCi 14C/g tissue. The concentration of 14C at 6 h in the dorsolateral and ventral plus anterior lobes of the prostate was 2–3 times greater than in the seminal vesicles and was maintained for 168 h. A comparison of the percentage accumulation of 14C-polar products in the hepatic tissue, seminal vesicles, serum, ventral and dorsolateral prostate lobes over time was carried out (Fig. 8). Of these tissues, polar products were highest at most time points in the dorsolateral prostate. Hepatic tissue did not appear to further accumulate 14C-labeled polar products. Serum 14C-labeled polar products appeared to accumulate over the 168 h.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The current study had the two following major objectives: 1) to examine the time course distribution of a single dose of 14C in rats that were prefed lycopene and 2) to define the time course associated with lycopene isomerization and appearance of 14C-polar products in rats that were prefed lycopene. Lycopene was prefed to rats in an attempt to create a lycopene fed state comparable to that of typical Western men. This design was advantageous and necessary to evaluate the time course distribution and degradation of newly consumed lycopene in an in vivo model.
The mean hepatic nonradiolabeled lycopene concentration achieved after 30 d of consuming 0.25 g lycopene/kg diet was 138 ± 13 nmol/g. This concentration was greater than what would be expected on the basis of other studies performed in our laboratory (19,20) and those of Ferreira et al. (34). The hepatic lycopene concentrations detected in three studies performed by Boileau et al. and Ferreira et al. were 16–61 nmol/g (19), 40 nmol/g (20), 36–89 nmol/g (21) and 14 nmol/g in rats fed for 6, 8, up to 63 wk and 9 wk, respectively. In contrast, the hepatic lycopene concentration detected by Zhao et al. in rats fed for 10 wk with a tomato oleoresin as the source of lycopene in their diet ranged from 78 to 224 nmol/g (35); the concentration achieved after 16 d of consuming 0.3 g lycopene/kg diet was 145 nmol/g in another study (36). A comparison of serum values in the current study (mean of 536 nmol/L) with other values in rats of 82–295 (19), 434 (20), 74–117 (18) and 299–532 nmol/L (35) and with serum lycopene concentrations in human trials, 70–1790 (37), 317–438 (5), 600-1900 (2) to 664-1300 nmol/L (38) suggests that the lycopene status in rats in the present study was more comparable to humans who frequently consumed tomato products.
For most tissues, the distribution of 14C appeared to be time and tissue dependent. Of the original dose, 7% was absorbed and 6% was retained in 168 h. Surprisingly, a small quantity of 14C was detected in the stomach even after 168 h. The dose was provided in 0.5 mL of oil and because lycopene is a neutral lipid, some of the dose may have adsorbed onto the stomach lining, causing a slow release of lycopene. Early 14C fecal excretion was likely a result of unabsorbed 14C-lycopene, whereas for the later collection periods (24–48 h), mucosal sloughing and bile excretion probably accounted for the majority of 14C in feces. Much of the 14C found in the feces at 72 h and beyond had probably been absorbed, metabolized and excreted into bile.
To achieve the objectives of the study, a time course 14C analysis of major tissues was performed. On the basis of previous studies in rats, hepatic tissue was expected to be the primary depot for lycopene (19,20,35). We expected 14C to increase in the hepatic tissue and eventually reach a maximum concentration, which then would be maintained 1 wk postdosing. Our results did in fact show an increase of total 14C in hepatic tissue with a peak value of 1.13 ± 0.09 µCi at 24 h, but surprisingly, the amount of 14C-lycopene decreased to 0.50 ± 0.06 µCi at 168 h. Because 14C in serum did not increase, it is likely that 14C was excreted via bile or urine. A possible explanation for the decline in 14C-lycopene was that metabolic processes were triggered to metabolize lycopene, eventually leading to the excretion of this compound. In addition, rat tissues took up and retained less 14C than expected. Adipose tissue, for instance, did not continue to accumulate additional 14C over time as expected because lycopene is a nonpolar compound that would be expected to accumulate in a high lipid-containing tissue.
The only other study examining the accumulation and distribution of 14C-lycopene in rats was conducted by Mathews-Roth et al. in 1990 (10). They used male (n = 3) and female (n = 3) Sprague-Dawley rats that were not prefed lycopene and that were administered a single oral dose of 20 µCi of 14C lycopene. The rats were killed at 4, 8, 24, 48 and 72 h. The authors detected 14C in all of the tissues measured and reported that the liver accumulated the largest amount of 14C. Serum 14C peaked between 4 and 8 h and then decreased. In the current study, 14C in serum was still elevated at 24 h. The variation in peak serum radioactivity found in these studies is most likely a result in differences in tissue lycopene concentrations before administration of the radioactive dose of lycopene.
From the results of this study, we hypothesize that the prefeeding of lycopene induced enzymes involved in lycopene metabolism and clearance. Breinholt et al. (39) reported that specific detoxification enzymatic activities were induced with increased lycopene intake in female Wistar rats. The plasma lycopene concentration in the Breinholt study ranged from 16 to 72 nmol/L, which was lower than the lycopene concentration in our study (mean of 536 nmol/L). The Breinholt study demonstrated the dose-dependent induction of hepatic benzyloxy resorufin O-dealkylase and the induction of hepatic ethoxy resorufin O-dealkylase at the two highest serum lycopene levels (67 and 71 nmol/L). Their data suggest that lycopene has an isozymic specificity towards CYP1A and CYP3A4. Additionally, glutathione transferase was induced at 67 nmol lycopene/L serum but not at the lower concentrations. Results of the Breinholt study suggest that the concentrations of dietary lycopene in the current study may have modulated certain lycopene-metabolizing enzymes, resulting in the production of 14C-labeled polar products. In contrast, other studies reported that several hepatic phase I and phase II detoxification enzymes are induced by the oxycarotenoids, but not by the more nonpolar carotenoids, lycopene and ß-carotene (36,40). The interaction between detoxification enzymes and carotenoids seems to exist, but further research is warranted to define the precise enzymatic systems involved.
5,6-Dihydroxy-5,6-dihydrolycopene was identified as a metabolite of lycopene in human serum. It was suggested that this product is formed by phase I detoxification enzymes, autooxidation or by lipoxygenases (41,42). Recently, dialdehyde 2,7,11-trimethyl-tetradecahexaene-1,14-dial, another oxidation product of lycopene, was shown to be bioactive due to its ability to upregulate gap junctions in vitro (8). Another laboratory identified lycopene cleavage products produced by autooxidation of lycopene at each of the 11 conjugated double bonds to form apo-lycopenals (14', 12', 10', 8', 6'), 3, 7, 11-trimethyl-2, 4, 6, 10-dodecatetraen-1al, 6, 10, 14-trimethyl-3, 5, 7, 9, 13-pentadecapentaen-2-one and acycloretinal (43). Similar cleavage products were found in Tween, liposomes and pig liver homogenates. Pig liver homogenates were shown to convert acycloretinal to acycloretinoic acid, suggesting that biological systems may be able to cleave and oxidize lycopene into short-chain carboxyl compounds and that acycloretinoic acid can be formed by enzymatic conversion of acycloretinal.
Short-chain cleavage products of lycopene potentially can be formed by the ß-carotene 15,15'-mono-oxygenase enzyme II, which normally cleaves ß-carotene at the 9', 10' double bond to form retinal (44). It has also been suggested that ß-carotene 15,15'-mono-oxygenase enzyme I, the central cleavage enzyme, might centrally cleave lycopene to produce acycloretinoic acid, although acycloretinoic acid appears to have little biological activity (45,46).
The second objective of this study was to define the time course associated with lycopene isomerization and metabolism. Initially, serum 14C was predominantly in the all-trans and 5-cis lycopene form. Within the first 24 h, however, a shift from all-trans and cis lycopene to oxidative/metabolic products was observed in serum. At 72 and 168 h, there was a decrease in total serum radioactivity presumably as 14C continued to be lost via urine and feces (via bile). The more polar products of 14C were highest at 24 h. The minor cis isomers of 14C in serum were the dominant forms at 168 h. The increase in lycopene metabolites in serum after prefeeding of tomato products was reportedly observed in human serum (8).
Hepatic tissue contained 80% all-trans and cis isomers of 14C and 20% metabolic and polar products of 14C. Although the total 14C declined in liver after 24 h, the percentage of polar products did not change. Although polar products did not accumulate in the liver over time, it is possible that polar products produced in the liver might have been transported via serum for excretion in urine or accumulated in other tissues.
Unexpectedly, the 14C in the dorsolateral lobe of the prostate, which is the major site of prostate cancer development in rats and is homologous to the peripheral region in the human prostate (17), was high in metabolic, polar products. At 3 h, 69% of the radioactivity was as metabolic, polar products and by 1 wk, these 14C-polar products in the prostate increased to 82% of the total (Fig. 8). In contrast, the seminal vesicles had a lycopene isomer distribution pattern similar to that of hepatic tissue, except that more metabolic products were observed over time. The ventral and anterior prostate lobes had a chemical distribution similar to that of the dorsolateral lobe.
The nonradioactive lycopene concentration found in the dorsolateral prostate lobe ranged from 2.4 to 6.6 nmol/g, which is higher than the lycopene concentrations reported in rats [range from 0.05 to 0.24 nmol/g (19) to 0.09–0.18 nmol/g (35)] and human literature data [0.2–0.4 nmol/g (38) to 0.63–0.91 nmol/g (2)]. Analysis of nonradioactive lycopene in the prostate lobes and seminal vesicles did not reveal many polar products (primarily all-trans and cis isomers of lycopene; data not shown).
The data presented provide an initial insight into the absorption, biodistribution and metabolism of lycopene in rats prefed lycopene. Although studies in humans are lacking, it may be that life-long consumption of tomato products and other lycopene-rich foods may be important to obtain its health benefit. The results suggest differential metabolism of newly absorbed lycopene in the prostate. It is possible that a concentration threshold of lycopene in certain tissues is necessary to trigger the induction of lycopene-metabolizing enzymes. Further research is required to determine the chemical structure of these lycopene polar or chain-shortened products and whether they accumulate in the human prostate or other human tissues. Our laboratory is currently investigating some of these questions. The mechanisms associated with lycopene metabolism within tissues and the biochemical function of lycopene metabolites may have critical relevance to the health effects of lycopene.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 This material is based upon work supported by the IFAFS/USDA under Award no. 00–52101-9695. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the U.S. Department of Agriculture.
Manuscript received 19 June 2003. Initial review completed 16 July 2003. Revision accepted 12 September 2003.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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