The Journal of Nutrition Vol. 128 No. 2 February 1998, pp. 239-245

Orally Fed Digalactosyldiacylglycerol Is Degraded during Absorption in Intact and Lymphatic Duct Cannulated Rats¹

Lena Ohlsson^{*, 2}, Magnus Blom^*, Karin Bohlinder, Anders Carlsson, and Åke Nilsson^*

^* Gastroenterology Division, Department of Internal Medicine, University of Lund, S-221 85 Lund, Sweden, and Scotia LipidTeknik AB, S-113 84 Stockholm, Sweden

	ABSTRACT

Abstract Introduction Methods Results Discussion References

Membrane lipids of green plants digalactosyldiacylglycerol (DGalDG) and monogalactosyldiacylglycerol (MGalDG) are hydrolyzed in vitro by human duodenal contents, pancreatic juice and bile salt stimulated lipase and guinea pig and rat pancreatic lipase-related protein 2 to free fatty acids, di- and monogalactosylmonoacylglycerols and water soluble galactose-containing compounds. The fate of intermediate products is unknown. We have investigated the digestion and absorption of DGalDG in rats. [³H]- and [¹⁴C]-labeled DGalDG in galactolipid dispersions, and 200 g/L soybean triacylglycerol (TG) oil-galactolipid emulsions of different concentrations were fed orally to intact and lymphatic duct cannulated rats. Chyle, gastrointestinal tract, liver and plasma were analyzed for radioactivity in different lipid classes. Recovery of [³H] also was determined in feces. Comparison was made with an emulsion of [¹⁴C]dipalmitoyl-phosphatidylcholine ([¹⁴C]DPPC), soybean TG oil and soybean phosphatidylcholine (PC). Less than 2% of the radioactivity in chyle was found in DGalDG, >70% of the radioactivity in triacylglycerol (TG), and the remaining part in glycerophospholipids. In intact rats, <1.5% of radioactivity in liver and plasma was identified as DGalDG. In experiments where 120 mg galactolipid-phospholipid mixture or 120 mg PC were given in a soybean TG oil-emulsion, the absorption of galactolipid fatty acids was less complete than PC-fatty acids, as indicated by analysis of feces and intestinal contents. Galactolipids are not absorbed intact or as reacylated monoacyl compounds by rats.

	INTRODUCTION

Abstract Introduction Methods Results Discussion References

In all green plants and most fruits, galactolipids are the most abundant lipid class in thylakoid membranes, dominated by monogalactosyldiacylglycerol (MGalDG)³ and digalactosyldiacylglycerol (DGalDG). The concentration of MGalDG is generally higher than that of DGalDG in photosynthetic tissues. This relation is reversed in nonphotosynthetic material (Sprague 1987). Liposomal dispersions of DGalDG have been shown to possess long-term chemical and physical stability, which makes them interesting for pharmaceutical preparations for both oral and parenteral administration (Carlsson et al. 1995). Such an application requires knowledge about their metabolism by mammalian tissues.

The degradation of MGalDG and DGalDG by pancreatic and intestinal tissues from sheep, rats and guinea pigs (Bajawa and Sastry 1974) and human intestinal contents and pancreatic juice (Andersson et al. 1995) showed that the pancreas from all species had high galactolipase activity toward the two galactolipids. In the early study of Bajawa and Sastry (1974), pancreas homogenates were used as the source of enzymes, and they concluded that the major pathway for the galactolipid digestion was hydrolysis of the ester bonds to form free fatty acids (FFA) and intermediate products monogalactosylmonoacylglycerol (MGalMG) and digalactosylmonoacylglycerol (DGalMG), and finally galactose and glycerol. A similar result from the hydrolysis of galactolipids with human duodenal contents recently was reported from our laboratory (Andersson et al. 1995).

New enzymes hydrolyzing galactolipids have been identified in in vitro experiments (Andersson et al. 1995, Bajawa and Sastry 1974). Giller et al. (1992) and Thirstrup et al. (1994) showed that the lipase family consisted of pancreatic lipase subfamilies, namely pancreatic lipase related proteins (PLRP) 1 and 2. These proteins do not have the same lipolytic properties as the classical colipase dependent lipase (PL) with respect to colipase and bile salts. Incubations of the different galactolipids with PLRP1 and 2 from rat (RPLRP1 and 2) and guinea pig (GPLRP2) showed that PLRP2 but not PLRP1 had galactolipase activity, which was higher with GPLRP2 than RPLRP2 (Andersson et al. 1996). Notably Bajawa and Sastry (1974) also found higher galactolipase activity in guinea pigs than in rats. It is not known whether the intermediate products DGalMG and MGalMG are absorbed in a similar way as the lyso-phosphatidylcholine, which in part is reacylated in the mucosal cells (Nilsson 1968, Subbaiah et al. 1970), or whether they are degraded completely to galactose, glycerol and fatty acids.

In this study, we examined the fate of orally given [³H]- or [¹⁴C]DGalDG to determine whether some galactolipid may be absorbed intact or if complete lipolysis and reesterification of the galactolipid fatty acids into chyle triacylglycerols (TG) and phospholipids occur. The capacity of rats to digest a large mass of galactolipids present in a soybean TG oil-emulsion also was examined.

View larger version (27K): [in this window] [in a new window]   Fig 1. Distribution of radioactivity in neutral (a) and polar (b) lipids in chyle from cannulated rats fed 2 mL soybean triglyceride oil emulsion (200 g/L) containing 20 g/L digalactosyldiacylglycerol (DGalDG) to which 66.6 kBq [3H]fatty acid-labeled DGalDG has been added (see MATERIALS AND METHODS). The results are expressed as percent radioactivity distributed in lipid fractions as follows in a: polar lipids, monoglyceride (MG), 1,2-diacylglycerol (DG 1,2), 1,3-diacylglycerol (DG1,3), free fatty acids (FFA) and triaclylglycerol (TG); and in b: lyso-phosphatidylcholine (LPC), phosphatidylcholine (PC), digalactosylmonoacylglycerol (DGalMG) with phosphatidylinositol (PI), phosphatidylethanolamine (PE), DGalDG with phosphatidic acid (PA) and neutral lipids at the time intervals indicated and are presented as means ± SEM, n = 4.

View larger version (23K): [in this window] [in a new window]   Fig 2. Percent of given dose [3H] (a) and [14C] (b) radioactivity and triglycerides in chyle from cannulated rats fed 2 mL soybean triglyceride (TG) oil emulsion (200 g/L) containing DGalDG (20 g/L) to which 66.6 kBq [3H]fatty acid-labeled DGalDG or 3.7 kBq [14C]DGalDG has been added (see MATERIALS AND METHODS). The results are expressed as percent recovered radioactivity and percent TG of given dose at the time intervals indicated and are presented as means ± SEM, n = 3.

	MATERIALS AND METHODS

Abstract Introduction Methods Results Discussion References

Sources of galactolipids.  Two emulsions containing 200 g/L of soybean triglyceride (TG) oil and 20 or 40 g/L, respectively, of a galactolipid mixture (GL mixture) from oat grains and one emulsion containing 40 g/L of soybean-phosphatidylcholine (soy-PC) all were obtained from Scotia LipidTeknik, Stockholm, Sweden. The GL mixture consisted of 50% DGalDG (g/100 g), 47% phospholipids and 3% of minor glycolipids, including MGalDG and sterylglycosides. The fatty acid composition in natural galactolipids from oat is 55% 18:2, 22% 16:0, 17% 18:1, 2.5% 18:3 and 1% 18:0.

Preparation of [³H]fatty acid-labeled DGalDG.  A half gram of the GL mixture was tritiated by catalytic reduction of double bonds in fatty acids using tritium gas. The labeling was performed by Amersham Tritium Labeling Service (Cardiff, UK). The specific radioactivity was 1.11-2.22 TBq/mmol per double bond reduced. The individual [³H]fatty acid-labeled galactolipids were purified by two-dimensional thin layer chromatography (TLC) on silica gel 60 plates according to Bratt and Åkerlund (1993). The mobile phase was in the first direction chloroform:methanol:water 65:25:4 and in the second direction chloroform:methanol:acetic acid:water 85:15:10:3.5. The galactolipid spots were eluted sequentially with chloroform:methanol:water 65:25:4 and 50:50:10, pure methanol and methanol:water 1:1. To examine the distribution of [³H] among different fatty acids of the [³H]DGalDG after catalytic hydrogenation with tritium-gas, an aliquot was subjected to transmethylation in 2% (mL/100 mL) sulfuric acid in toluene:methanol 1:1 at 65°C for 4 h. The transmethylated fatty acids of [³H]DGalDG was separated on high performance liquid chromatography (HPLC) equipped with a reversed phase C18 column and a mobile phase of acetonitrile:H₂O 90:10. The distribution of fatty acids were 35.4% as 18:0, 19.3% as 18:1, 4.5% as 18:3, 4.3% as 16:0 and 2.7% as 18:2. Other peaks were unidentified. About 89% of the radioactivity was found in the fatty acids (Blom et al. 1996).

View larger version (17K): [in this window] [in a new window]   Fig 3. Percent recovery of [3H]radioactivity in different tissues from intact rats fed [3H]DGalDG. Values are expressed as percent radioactivity per whole tissue 2 h after oral administration of 2 mL GL (20 g/L) (galactolipids) dispersed in 2.5% (mL/100 mL) glycerol in water containing 96.2 kBq [3H]DGalDG. The results are presented as means ± SEM, n = 4. *n = 1.

View larger version (21K): [in this window] [in a new window]   Fig 4. Percent recovery of [3H]radioactivity in different tissues from intact rats fed [3H]DGalDG. Data are expressed as percent radioactivity per whole tissue 2 h after administration of 3 mL galactolipid mixture (40 g/L) in soybean triglyceride oil emulsion (200 g/L) containing 298.2 kBq [3H]fatty acid-labeled DGalDG. The results are presented as means ± SEM, n = 4.

Preparation of [¹⁴C]-labeled DGalDG.  Euglena gracilis algae were subcultured and grown 1 wk to reach maximum growth rate. The pH of the medium was made slightly alkaline with sodium hydroxide before adding 0.1 MBq sodium [¹⁴C]bicarbonate. The culture then was placed in sunlight indoors. After 0.5, 1, 3 and 6 h, 50 mL algae culture was centrifuged in a test tube. The lipids were extracted from the green pellet according to Bligh and Dyer (1957). The individual galactolipids were purified by the two-dimensional TLC system (Bratt and Åkerlund 1993), and the spots of DGalDG were eluted from the gel as described above. Of the radioactivity in DGalDG, 87% was found in the fat soluble part. The distribution of fatty acids were 26% 18:3, 19% 18:1, 17% 16:0, 12% 18:2 and 7% 16:4 (Sastry 1974).

Galactolipid dispersion.  [³H]DGalDG dissolved in chloroform:methanol 1:1 in a glass tube was dried under nitrogen. GL mixture (see above) dispersed in 2.5% (mL/100 mL) glycerol in water were added to give a total concentration of 2% (g/L). The radioactive liposomal dispersion was equilibrated for 36 h at 4°C to achieve maximum swelling of the galactolipid. This mixture was sonicated with a Branson Sonifier 250 (KEBO Lab AB, Stockholm, Sweden; output power 30-40 w for 3 × 2 min, pulse off time 4 min over nitrogen at 0°C) to reduce the liposomal size.

Oral feeding of DGalDG.  Male Sprague Dawley rats, weighing 200-225 g, were purchased from Möllegaard, Ejby, Denmark. In all experiments, rats were food-deprived over night. The rats had free access to water before they were fed 2 or 3 mL dispersion or emulsion containing 96.2-296 kBq [³H]DGalDG, galactolipid-enriched soybean TG oil-emulsions or 3 mL soy-PC-enriched soybean TG oil-emulsion with 55.5 kBq [¹⁴C]dipalmitoyl-phosphatidylcholine ([¹⁴C]-DPPC). The emulsions were fed by gastric intubation with a soft feeding tube to ~7 cm from the level of the teeth while the rats were slightly anesthetized. Two and 4 h later the rats were killed by aortic puncture. Each group contained four rats. Blood and gastrointestinal organs were weighed immediately and extracted of lipids according to Bligh and Dyer (1957) in chloroform:methanol 1:1 containing 0.005% butylated hydroxytoluene (BHT). All animal experiments were reviewed and approved by the ethics committee.

Oral feeding of DGalDG for feces collection.  Rats were food-deprived over night with free access to water before they were fed one of the following emulsions under light ether anesthesia: 2 mL dispersion containing 2% (g/L) GL mixture (totally 20 mg DGaDG) and 7.4 kBq [³H]DGalDG; 3 mL of galactolipid-enriched soybean TG oil-emulsions containing 2% (g/L) GL mixture (totally 30 mg DGaDG) and 7.4 kBq [³H]DGalDG; 3 mL soy-PC-enriched soybean TG oil emulsion with 4% (g/L) PC (totally 120 mg PC) and 7.4 kBq [¹⁴C]-DPPC; or 3 mL soybean TG oil-emulsion enriched with 4% (g/L) GL (totally 60 mg DGalDG) and 13.3 kBq [¹⁴C]DGalDG (from algae). They were put in a cage with a net floor. Four hours after feeding the rats had free access to nonpurified diet (Altromin Nr 1324, Ringsted, Denmark). One day later feces were collected and extracted as mentioned before. The radioactivity in lipid soluble and water soluble phases was determined as described. Each group contained four rats.

Preparation of radioactive chyle.  Two emulsions were prepared by mixing a soybean TG oil (200 g/L) emulsion containing DGalDG (20 g/L) with either 66.6 kBq [³H]DGalDG or 3.7 kBq [¹⁴C]DGalDG using light pulsed sonication (see above) with the sample cooled on ice. Mesenteric lymph duct cannulations were performed according to Warshaw (1972), and radioactive chyle was collected after feeding stomach cannulated rats 2 mL of the emulsion during 1 h. Chyle was collected for 8 h in preweighed tubes on ice containing EDTA to a final concentration of ~2 mmol/L.

Lipid analysis.  The radioactivity of the lipid extracts was determined, and the distribution between lipid classes analyzed by silica gel G TLC for separation of neutral lipids in a mobile phase of petroleum ether:diethylether:methanol:acetic acid 80:20:2.25:1 and phospholipids in chloroform:methanol:acetic acid:water 50:40:6:0.6. The spots were identified with iodine vapor and scraped into scintillation vials to which 1 mL methanol:water 1:1 and 9 mL scintillation cocktail (Ready Gel from Beckman Instruments AB, Bromma, Sweden) was added before measuring the radioactivity in a Packard Tri-Carb 2100 Liquid Scintillation Counter using automatic external standard for quench correction. TG in chyle was determined with Triglyceride GPO-PAP test-kit from Boehringer Mannheim Scandinavia AB (Bromma, Sweden).

Statistical analyses.  Unless otherwise stated, data present in this paper were means ± SEM of at least three observations. Unpaired two-tailed Student's t test was used for statistical analysis in results from experiments with intact rats fed a high mass of phosphatidylcholine in soybean triacylglycerol oil-emulsion.

	RESULTS

Abstract Introduction Methods Results Discussion References

Experiments with lymphatic duct cannulated rats.  The total recovered [³H] radioactivity in chyle was 12.3 ± 2.1% during 8 h, and 90% of this radioactivity was found in neutral lipids, mainly in TG (Fig. 1a,b, and Fig. 2). The polar lipids were dominated by PC, and we detected little or no radioactivity as intact galactolipids (Fig. 1b). The amount of TG in chyle was 63.8 ± 7.6% of the given dose. This figure includes the TG formed from endogenous fatty acids and thus overestimates the recovery of TG (Fig 2). When a second group of rats was fed [¹⁴C]DGalDG containing emulsion, the results were similar with those of the rats given [³H]DGalDG with respect to total recovered radioactivity, 15.3 ± 7.6%, and distribution to neutral and polar lipids (data not shown). The amount of recovered TG was 20.6 ± 2.3% of the given dose (Fig. 3). Although the lymph duct cannulation experiments indicated that no intact galactosylglycerols appeared in chyle lipids, the incomplete recovery in these experiments left open the possibility of portal transport. We therefore performed a series of studies on intact rats in which uptake of orally administered soybean TG oil emulsions in gastrointestinal (GI) organs, plasma and liver was investigated.

Experiments in intact rats.  When tissue lipids were analyzed for radioactivity, the highest level was found in the stomach after both 2 h (Fig. 4) and 4 h (data not shown). The radioactivity in the upper half of the small intestine was 4.4 ± 0.4% dose and that of the lower half was 7.2 ± 0.8% dose. Almost no radioactivity migrated as DGalDG in the tissues examined, except for small amounts in the contents of the upper small intestine (Table 1, Fig. 4) and in the upper intestinal tissue. The latter may have been due to difficulties in removing all contents from the intestinal brush border (Table 1). Analysis of radioactivity in different lipid classes of the examined organs showed that more radioactivity was found in phospholipids, mainly phosphatidylcholine (PC) and phosphatidylethanolamine (PE), than in neutral lipids (Table 1, Table 2). After 4 h this difference increased. In all examined tissues except liver and plasma, recovery of water-soluble radioactivity accounted only for ~10% of the recovery of the given dose (data not shown). The ratio of water:lipid soluble radioactivity was 1:3 in liver and 1:1 in plasma, indicating a rapid breakdown of the labeled fatty acids by -oxidation (data not shown). Because almost no intact DGalDG was found in the tissues (Table 1), we decided to investigate the effects and capacity of rats to digest a high concentration of GL.

Experiments with intact rats fed high mass of galactolipid in soybean triacylglycerol oil-emulsion.  About 30% of the given radioactivity was recovered in the gastrointestinal tract with a preponderance of stomach content after 2 h (Fig. 5) and 4 h (data not shown). The recovery was lower in all tissues examined compared to the rats fed lower mass of DGalDG in a glycerol dispersion (Fig. 4). The small intestine was divided in four parts. Most of the radioactivity in the upper two parts migrated as polar lipids and in the lower parts as neutral lipids (NL) after 2 h (Table 3). After 4 h, the radioactivity in NL increases in the upper parts but decreases in the lower half. We detected a preferential distribution of radioactivity in polar lipids to PC and PE after 4 h (Table 3). The main hydrolysis products from DGalDG, i.e., DGalMG, migrated together with phosphatidylinositol (PI) in this TLC system (see Materials and Methods). About 6-9% of the radioactivity was found in the DGalMG-PI spot, which we believe originates from PI and not DGalMG because the main fatty acid in [³H]DGalDG, stearate, can be incorporated into PI. A high amount of radioactivity as DGalDG in the intestine indicated the presence of undigested DGalDG from the intestinal contents. In the intestinal contents, 8-20% of the radioactivity was located in PI and DGalMG. The largest amount was probably in DGalMG as a product of the digestion of DGalDG.

View larger version (22K): [in this window] [in a new window]   Fig 5. Percent recovery of [14C]radioactivity in different tissues from rats fed [14C]phosphatidylcholine (PC). Data are expressed as percent radioactivity per whole tissue 2 h after administration of 3 mL soy-PC (40 g/L) in soybean triglyceride oil emulsion (200 g/L) containing 56.5 kBq [14C]dipalmitoyl-PC. The results are presented as means ± SEM, n = 3.

Of the radioactivity in the liver, 30% was distributed to the neutral lipids after both 2 and 4 h. Liver contained the highest levels of radioactivity in PE, 11-14% (2-4 h; P < 0.05) than all other organs investigated.

The plasma contained very little radioactivity: 0.7% of the dose after 2 and 4 h (Fig. 6) of which 60% migrated as neutral lipids.

Experiments with intact rats fed a high mass of phosphatidylcholine in soybean triacylglycerol oil-emulsion.  The total recovery of radioactivity was higher with [¹⁴C]PC than with [³H]DGalDG (Fig. 5) and most of the radioactivity was found in stomach. There was also considerable radioactivity in the contents of the lower parts of the small intestine (Fig. 5). The radioactivity in plasma, liver, intestinal contents and the upper part of the intestine was distributed mainly between NL and PC. The amount of radioactivity in PC in liver (P < 0.1) and plasma (P < 0.05) increased by 47% from 2 to 4 h (Table 4). The only notable amounts of radioactivity in lyso-PC were in the contents of the small intestine (Table 4).

Experiments with collected feces from intact rats.  Because the total recovery of radioactivity in the investigated organs was low (30-40%), we also studied the recovery of radioactivity in feces from intact rats fed the different GL and PC enriched emulsions (Table 5). From rats given the high load 120 mg galactolipid mixture, the recovered radioactivity after 24 h was 45%, of which one-third was obtained as water soluble activity (Table 5). When 20 mg GL was given, the recovered radioactivity was 9.5% of which 2.0% was water soluble, whereas collected feces from rats given the PC emulsion contained only 0.6% of the given dose of radioactivity of which 0.12% was water soluble (Table 5).

	DISCUSSION

Abstract Introduction Methods Results Discussion References

This study shows that radiolabeled DGalDG in triolein or glycerol emulsions given orally to rats, did not appear as intact DGalDG to an appreciable extent in chyle or in any of the examined tissues except the gastrointestinal tract.

Earlier studies have indicated that homogenates of pancreas from different species and human duodenal contents hydrolyzed galactosylglycerides to the corresponding galactosylglycerols and fatty acids (Andersson et al. 1995, Bajawa and Sastry 1974). In human duodenal contents incubated with [³H]DGalDG, accumulation of intermediate galactosylmonoglycerols was observed (Andersson et al. 1995). The hydrolysis of one of the ester bonds was faster than that of the remaining ester bond, although the position of the ester that is hydrolyzed first is not known (Andersson et al. 1996). It was therefore possible that an uptake of intermediate products might occur, analogous to the uptake of lysophospholipids after digestion of glycerophospholipids by pancreatic phospholipase A2 (Nilsson 1968, Subbaiah et al 1970).

After absorption, lyso-PC either is reacylated or degraded further. Acylation increases with the amount of dietary fat (Nilsson 1968, Subbaiah et al 1970).

To determine whether any absorption and reacylation of DGalMG occurs, we incorporated the galactolipids with soybean TG.

When a galactolipid and [³H]DGalDG containing soybean TG oil-emulsion was given orally to rats with lymph duct cannulas, little or no [³H] appeared in DGalDG in chyle. The same results were obtained when the emulsion contained [¹⁴C]DGalDG in which the esterified fatty acids were in their native form and not hydrogenated. The recovery of [³H] after 6-8 h slightly exceeded that of fatty acids. Most of the fatty acid mass recovered in chyle TG originates from the TG of the given emulsion. The time course of recovery of TG fatty acids thus reflects that for the digestion and absorption of the given TG. The time course for the digestion of galactolipids seems to be somewhat slower. Furthermore, the [³H]DGalDG administered contained [³H] stearic acid, which has some preference for incorporation into mucosal phospholipid pools, and some of it therefore is retained in the mucosa before being secreted in chyle.

The data thus indicated that no absorption and incorporation of intact DGalDG into chyle lipoproteins, or absorption, reacylation and incorporation into chyle lipids of DGalMG took place regardless of the degree of fatty acid unsaturation. The recovery of radioactivity in chyle was incomplete. The released radioactive fatty acids might be absorbed via the portal route, which can explain some of the low recovery of radioactivity. The studies therefore were extended to include tissues and feces after feeding [³H]DGalDG to intact rats.

When rats were fed 20 mg DGalDG in a soybean TG oil-emulsion or dispersion, <2% of the radioactivity in liver and plasma was found in [³H]DGalDG (Table 1). The reason for including an experiment in which galactolipids are dispersed in water with glycerol is because galactolipids show a long-term physical stability in this form. They also may be present in oral oil-in-water emulsions, especially if the emulsifier is added in excess. (The average particle size of both the sonicated liposomes and the 20% oil-in-water emulsions was ~0.15 µm). Glycerol was added to make the liposomal dispersion isoosmotic with normal saline solution.

The data indicate that no intact or reacylated galactolipids are transported to tissues by the portal route. Rather, fatty acids from the hydrolyzed galactolipids are reesterified into TG and phospholipids in the enterocytes by the preferred pathway for the 16-18 carbon fatty acids. The formed TG then are transported by the TG-rich lipoproteins.

When large amounts (120 mg) of GL containing [³H]DGalDG and PC containing [¹⁴C]-DPPC in soybean TG oil-emulsions were compared, higher recovery of radioactivity was seen in all organs in rats given the PC-enriched emulsion. These findings might be explained by a higher capacity of the intestine to digest and absorb PC. The [¹⁴C]-labeled palmitic acid, however, is reacylated more readily into PC as absorbed [¹⁴C]palmitoyl-lysoPC, and this will influence the further transport and retention in tissues of [¹⁴C]-palmitic acid.

Analysis of feces (Table 5) showed a 45% retrieval of radioactivity when 60 mg [³H]-labeled DGalDG was administered but only 11% for the [¹⁴C]-labeled DGalDG. The different hydrolysis rates of saturated and unsaturated fatty acids may explain these results, and this explanation agrees with results of other studies in which recovery of TG in chyle was lower with butterfat and higher in saturated fatty acids rather than other oils (Degrace et al. 1996).

The main conclusion thus is that DGalDG is not absorbed into blood or chyle as intact DGalDG or as reacylated DGalMG. Although the capacity to absorb DGalDG is less than that for PC in rats, it is likely to be sufficient for any amount that is normally ingested. However, because the galactolipase activity with PLRP2 was higher in guinea pigs than in rats, the question remains whether the capacity to digest galactolipids is different among species such as humans, guinea pigs and rats.

In many ways, the metabolism of galactolipids rats was similar to that of phosphatidylcholine, acylglycerides and long chain polyunsaturated fatty acids (Carroll 1958, Garg et al. 1988, Soichura et al. 1981). However, to get the complete mechanism for the degradation of DGalDG, complementary studies must be done with radioactive labeling in the galactose part of the galactolipid molecules.

	ACKNOWLEDGMENTS

This study was supported by grants from Scotia LipidTeknik AB and The Swedish Nutrition Foundation. We thank Ning Xu, Department of Clinical Chemistry, Malmö, Sweden, and Li Zhou, Gastroenterology Division, Department of Internal Medicine, University of Lund, Sweden, for performing cannulations of the mesenteric lymph duct as well as Lena Carlsson at the Department of Plant Physiology, Lund, Sweden, for kindly providing Euglena gracilis algae.

	FOOTNOTES

Manuscript received 5 May 1997. Initial reviews completed 25 June 1997. Revision accepted 15 October 1997.

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