Moderate vitamin B-6 restriction does not alter postprandial methionine cycle rates of remethylation, transmethylation, and total transsulfuration but increases the fractional synthesis rate of cystathionine in healthy young men and women

J Nutr. 2011 May;141(5):835-42. doi: 10.3945/jn.110.134197. Epub 2011 Mar 23.


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.

Publication types

  • Clinical Trial
  • Research Support, N.I.H., Extramural

MeSH terms

  • Adult
  • Breath Tests
  • Carbon Isotopes
  • Cystathionine / blood
  • Cystathionine / metabolism*
  • DNA Methylation
  • Deuterium
  • Diet
  • Female
  • Humans
  • Kinetics
  • Leucine
  • Male
  • Methionine / metabolism*
  • Methylation
  • Monocytes / metabolism
  • Postprandial Period*
  • Pyridoxal Phosphate / blood
  • Reproducibility of Results
  • Severity of Illness Index
  • Vitamin B 6 / administration & dosage
  • Vitamin B 6 Deficiency / blood
  • Vitamin B 6 Deficiency / metabolism*
  • Young Adult


  • Carbon Isotopes
  • Cystathionine
  • Pyridoxal Phosphate
  • Vitamin B 6
  • Methionine
  • Deuterium
  • Leucine