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

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Abstract
Hyperhomocysteinemia in humans is associated with genetic variants of several enzymes of folate and one-carbon metabolism and deficiencies of folate and vitamins B12 and B6. 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-13C5]-methionine tracer is used in conjunction with a [3-13C]serine tracer to simultaneously measure rates of total and folate-dependent homocysteine remethylation. In young female subjects, plasma [U-13C4]homocysteine enrichment, a surrogate measure of intracellular [U-13C5]methionine enrichment, reached ∼90% of the plasma [U-13C5]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 [3-13C]serine enrichment based on a conservative correction of plasma [3-13C]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.

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