Journal
JOURNAL OF NUTRITION
Volume 141, Issue 5, Pages 835-842Publisher
AMER SOC NUTRITIONAL SCIENCE
DOI: 10.3945/jn.110.134197
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Funding
- NIH [DK072398]
- NIH National Center for Research Resources [1UL1RR029890]
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Methionine is the precursor for S-adenosylmethionine (SAM), the major 1-carbon donor involved in >100 transmethylation reactions. Homocysteine produced from SAM must be metabolized either by remethylation for recycling of methionine or transsulfuration to form cystathionine and then cysteine. Pyridoxal 5'-phosphate (PLP) serves as a coenzyme in enzymes involved in transsulfuration as well as for primary acquisition of 1-carbon units used for remethylation and other phases of 1-carbon metabolism. Because the intake of vitamin B-6 is frequently low in humans and metabolic consequences of inadequacy may be amplified in the postprandial state, we aimed to determine the effects of marginal vitamin B-6 deficiency on the postprandial rates of remethylation, transmethylation, overall transsulfuration, and cystathionine synthesis. Healthy, young adults (4 male, 5 female; 20-35 y) received a primed, constant infusion of [1-(13)C]methionine, [methyl-(2)H(3)]methionine, and [5,5,5-(2)H(3)]leucine to quantify in vivo kinetics at normal vitamin B-6 status and after a 28-d dietary vitamin B-6 restriction. Vitamin B-6 restriction lowered the plasma PLP concentration from 49 +/- 4 nmol/L (mean +/- SEM) to 19 +/- 2 nmol/L (P < 0.0001). Mean remethylation, transsulfuration, and transmethylation rates did not change in response to vitamin B-6 restriction; however, the responses to vitamin B-6 restriction varied greatly among individuals. The plasma cystathionine concentration increased from 142 +/- 8 to 236 +/- 9 nmol/L (P < 0.001), whereas the fractional cystathionine synthesis rate increased by a mean of 12% in 8 of 9 participants. Interrelationships among plasma concentrations of glycine and cystathionine and kinetic results suggest that individual variability occurs in normal postprandial 1-carbon metabolism and in the response to vitamin B-6 restriction. J. Nutr. 141: 835-842, 2011.
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