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Significance of Formate as an Intermediate in the Oxidation of the Methionine, S-Methyl-L-Cysteine and Sarcosine Methyl Carbons to CO₂ in the Rat¹

Departments of Nutritional Sciences and Meat & Animal Science, University of Wisconsin, Madison, Wisconsin 53706

Rats were fed diets containing ¹⁴C-methyl-labeled L-methionine (Met), S-methyl-L-cysteine (SMC) or sarcosine (Sar) together with 6% of sodium formate. Conversion of the labeled carbons to formate recovered from the urine and to respired CO₂ were measured during an 8-hour period. An empirical relationship allowing the prediction of the ratio of formate converted to CO₂ to formate excreted in urine as a function of the formate absorbed was derived from experiments in which rats were fed diets containing 2% to 12% of sodium ¹⁴C-formate. The proportions of the total oxidation of the methyl carbons occurring via free formate, corrected for the further conversion of formate to CO₂, were 60% to 87% (Met), 100% to 106% (SMC), and 61% to 71% (Sar). In contrast, other experiments indicated that metabolism via free formate accounted for only about 29% of the total oxidation of the ring-2 carbon of L-histidine, 24% of the 3-carbon of L-serine, and 13% of the 2-carbon of glycine. It is concluded that mechanisms not involving tetrahydrofolate coenzyme inter-conversions must be important in converting the Met, SMC, and Sar methyl groups to formate. It is also proposed that the metabolism of Met via a pathway, independent of S-adenosylmethionine formation may account for the high production of formate from the Met methyl carbon.

KEY WORDS: • methionine • S-methylcysteine • sarcosine • methyl groups • formate

¹ Research supported by the College of Agricultural and Life Sciences; Meat and Animal Science Paper No. 696.

² Present address: Department of Physiology & Biophysics, Colorado State University, Fort Collins, Colorado 80521.

Manuscript received 20 January 1977.