Moderate Dietary Vitamin B-6 Restriction Raises Plasma Glycine and Cystathionine Concentrations While Minimally Affecting the Rates of Glycine Turnover and Glycine Cleavage in Healthy Men and Women

Abstract

Glycine is a precursor of purines, protein, glutathione, and 1-carbon units as 5,10-methylenetetrahydrofolate. Glycine decarboxylation through the glycine cleavage system (GCS) and glycine-serine transformation by serine hydroxymethyltransferase (SHMT) require pyridoxal 5′-phosphate (PLP; active form of vitamin B-6) as a coenzyme. The intake of vitamin B-6 is frequently low in humans. Therefore, we determined the effects of vitamin B-6 restriction on whole-body glycine flux, the rate of glycine decarboxylation, glycine-to-serine conversion, use of glycine carbons in nucleoside synthesis, and other aspects of 1-carbon metabolism. We used a primed, constant infusion of [1,2-13C2]glycine and [5,5,5-2H3]leucine to quantify in vivo kinetics in healthy adults (7 males, 6 females; 20–39 y) of normal vitamin B-6 status or marginal vitamin B-6 deficiency. Vitamin B-6 restriction lowered the plasma PLP concentration from 55 ± 4 nmol/L (mean ± SEM) to 23 ± 1 nmol/L (P < 0.0001), which is consistent with marginal deficiency, whereas the plasma glycine concentration increased (P < 0.01). SHMT-mediated conversion of glycine to serine increased from 182 ± 7 to 205 ± 9 μmol·kg−1·h−1 (P < 0.05), but serine production using a GCS-derived 1-carbon unit (93 ± 9 vs. 91 ± 6 μmol·kg−1·h−1) and glycine cleavage (163 ± 11 vs. 151 ± 8 μmol·kg−1·h−1) were not changed by vitamin B-6 restriction. The GCS produced 1-carbon units at a rate (∼140–170 μmol·kg−1·h−1) that greatly exceeds the demand for remethylation and transmethylation processes (∼4–7 μmol·kg−1·h−1). We conclude that the in vivo GCS and SHMT reactions are quite resilient to the effects of marginal vitamin B-6 deficiency, presumably through a compensatory effect of increasing substrate concentration.