Tracer-derived total and folate-dependent homocysteine remethylation and synthesis rates in humans indicate that serine is the main one-carbon donor

1 Food Science and Human Nutrition Department, Institute of Food and Agricultural Sciences; 2 Division of Endocrinology and Metabolism, Department of Medicine, and 3 Department of Biochemistry and Molecular Biology, College of Medicine, University of Florida, Gainesville, Florida 32611; and 4 Department of Nutritional Sciences and Toxicology, University of California, Berkeley, California 94720 Submitted 1 August 2003 ; accepted in final form 30 September 2003 Hyperhomocysteinemia in humans is associated with genetic variants of several enzymes of folate and one-carbon metabolism and deficiencies of folate and vitamins B 12 and B 6 . In each case, hyperhomocysteinemia might be caused by diminished folate-dependent homocysteine remethylation, but this has not been confirmed in vivo. Because published stable isotopic tracer approaches cannot distinguish folate-dependent from folate-independent remethylation, we developed a dual-tracer procedure in which a [U- 13 C 5 ]-methionine tracer is used in conjunction with a [3- 13 C]serine tracer to simultaneously measure rates of total and folate-dependent homocysteine remethylation. In young female subjects, plasma [U- 13 C 4 ]homocysteine enrichment, a surrogate measure of intracellular [U- 13 C 5 ]methionine enrichment, reached 90% of the plasma [U- 13 C 5 ]methionine enrichment. Methionine-methyl and -carboxyl group fluxes were in the range of previous reports ( 25 and 17 µmol·kg –1 ·h –1 , respectively). However, the rate of overall homocysteine remethylation ( 8 µmol·kg –1 ·h –1 ) was twice that of previous reports, which suggests a larger role for homocysteine remethylation in methionine metabolism than previously thought. By use of estimates of intracellular [3- 13 C]serine enrichment based on a conservative correction of plasma [3- 13 C]serine enrichment, serine was calculated to contribute 100% of the methyl groups used for total body homocysteine remethylation under the conditions of this protocol. This contribution represented only a small fraction ( 2.8%) of total serine flux. Our dual-tracer procedure is well suited to measure the effects of nutrient deficiencies, genetic polymorphisms, and other metabolic perturbations on homocysteine synthesis and total and folate-dependent homocysteine remethylation. methionine; methylation cycle; cystathionine Address for reprint requests and other correspondence: J. F. Gregory III, Food Science and Human Nutrition Dept., PO Box 110370, Gainesville, FL 32611–0370 (