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The transsulfuration pathway, which aids in regulating homocysteine concentration and mediates cysteine synthesis, may be sensitive to vitamin B-6 status because cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CGL) require pyridoxal 5'-phosphate (PLP). To assess relations between vitamin B-6 and transsulfuration, we evaluated the effects of dietary pyridoxine (PN) on the hepatic concentration of relevant metabolites and in vitro activity of CBS and CGL. Growing rats were fed AIN-93G- or AIN-76A-based diets that ranged from adequate to deficient in vitamin B-6 (2, 1, 0.5, 0.1, or 0 mg of PN/kg diet, n = 5). This design allowed assessment of the effects of supplemental methionine (AIN-76A) vs. cysteine (AIN-93G) in common research diets over a range of vitamin B-6 levels. CBS activity, assayed in the presence or absence of added S-adenosylmethionine, was independent of diet type and PN level. CGL activity was independent of diet type but proportional to dietary PN. Rats fed deficient (0 and 0.1 mg PN/kg) diets exhibited only approximately 30% of the CGL activity of those fed the 2 mg PN/kg diets. Hepatic cystathionine increased from 20 to 30 nmol/g for the 1-2 mg PN/kg diets to approximately 85 nmol/g for the 0 mg PN/kg diet; however, cysteine was reduced only in B-6-deficient rats consuming the AIN-93G diet (means of 30-40 nmol/g for adequate to 11.6 nmol/g for 0 mg PN/kg AIN-76A diet). In spite of these effects, hepatic glutathione concentration increased in vitamin B-6 deficiency. These results suggest that vitamin B-6-dependent changes in transsulfuration do not limit hepatic glutathione production.

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 hydroxymethyl-transferase (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 (1,2-13C2lgIycine and [5,5,5-2H3]Ieucine 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 <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 ¼mol·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. [ABSTRACT FROM AUTHOR]

Glycine plays several roles in human metabolism, e.g. as a 1-carbon donor, in purine synthesis, and as a component of glutathione. Glycine is decarboxylated via the glycine cleavage system (GCS) that yields concurrent generation of a 1-carbon unit as 5,10-methylenetetrahydrofolate (methyleneTHF). Serine hydroxymethyltransferase (SHMT) catalyzes the inter- conversion of glycine and serine, another 1-carbon donor. The quantitative role of glycine in human 1-carbon metabolism has received little attention. The aim of this protocol was to quantify whole body glycine flux, glycine to serine flux, and rate of glycine cleavage in humans. A primed, constant infusion with 9.26 µmol·kg-1·h-1 [1,2-13C2]glycine and 1.87 µmol·kg-1·h-1 [²H3]leucine was used to quantify the kinetic behavior of glycine in young, healthy volunteers (n = 5) in a fed state. The isotopic enrichment of infused tracers and metabolic products in plasma, as well as breath 13CO2 enrichment, were determined for use in kinetic analysis. Serine synthesis by direct conversion from glycine via SHMT occurred at 193 ± 28 µmol·kg-1·h-1 (mean ± SEM), which comprised 41% of the 463 ± 55 µmol·kg-1·h-1 total glycine flux. Nearly one-half (46%) of the glycine-to-serine conversion occurred using GCS-derived methyleneTHF 1-carbon units. Based on breath 13CO2 measurement, glycine decarboxylation (190 ± 41 µmolµkg-1·h-1) accounted for 39 ± 6% of whole body glycine flux. This study is the first to our knowledge to quantify human glycine cleavage and glycine-to-serine SHMT kinetics. GCS is responsible for a substantial proportion of whole body glycine flux and constitutes a major route for the generation of 1-carbon units. [ABSTRACT FROM AUTHOR]