Tracer-derived total and folate-dependent homocysteine remethylation and synthesis rates in humans indicate that serine is the main one-carbon donor.
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 approximately 90% of the plasma [U-13C5]methionine enrichment. Methionine-methyl and -carboxyl group fluxes were in the range of previous reports (approximately 25 and approximately 17 micromol.kg(-1).h(-1), respectively). However, the rate of overall homocysteine remethylation (approximately 8 micromol.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 approximately 100% of the methyl groups used for total body homocysteine remethylation under the conditions of this protocol. This contribution represented only a small fraction (approximately 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.
Davis
Steven R
SR
Food Science and Human Nutrition Department, Institute of Food and Agricultural Science, University of Florida, Gainesville, Florida 32611-0370, USA.
Stacpoole
Peter W
PW
Williamson
Jerry
J
Kick
Lilia S
LS
Quinlivan
Eoin P
EP
Coats
Bonnie S
BS
Shane
Barry
B
Bailey
Lynn B
LB
Gregory
Jesse F
JF
D000328:Adult; D002244:Carbon / Q000378:metabolism; D002247:Carbon Isotopes; D005260:Female; D005492:Folic Acid / Q000378:metabolism*; D006710:Homocysteine / Q000378:metabolism*; D006801:Humans; D007930:Leucine / Q000378:metabolism; D008715:Methionine / Q000378:metabolism; D008745:Methylation; D012016:Reference Values; D012694:Serine / Q000737:chemistry / Q000378:metabolism
D002247:Carbon Isotopes; D006710:Homocysteine; D012694:Serine; D002244:Carbon; D005492:Folic Acid; D008715:Methionine; D007930:Leucine