title: 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.
abstract: 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.
authors: Food Science and Human Nutrition Department, Institute of Food and Agricultural Sciences, College of Medicine, University of Florida, Gainesville, FL 32611-0370, USA.
authors: Yvonne
authors: Y
authors: Lamers
authors: Bonnie
authors: B
authors: Coats
authors: Maria
authors: M
authors: Ralat
authors: Eoin P
authors: EP
authors: Quinlivan
authors: Peter W
authors: PW
authors: Stacpoole
authors: Jesse F
authors: JF
authors: Gregory
mesh_terms: D000328:Adult; D001944:Breath Tests; D002247:Carbon Isotopes; D003540:Cystathionine / Q000097:blood / Q000378:metabolism*; D019175:DNA Methylation; D003903:Deuterium; D004032:Diet; D005260:Female; D006801:Humans; D007700:Kinetics; D007930:Leucine; D008297:Male; D008715:Methionine / Q000378:metabolism*; D008745:Methylation; D009000:Monocytes / Q000378:metabolism; D019518:Postprandial Period; D011732:Pyridoxal Phosphate / Q000097:blood; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D025101:Vitamin B 6 / Q000008:administration & dosage; D026681:Vitamin B 6 Deficiency / Q000097:blood / Q000378:metabolism*; D055815:Young Adult
chemical_list: D002247:Carbon Isotopes; D003540:Cystathionine; D011732:Pyridoxal Phosphate; D025101:Vitamin B 6; D008715:Methionine; D003903:Deuterium; D007930:Leucine