Journal
JOURNAL OF NUTRITION
Volume 139, Issue 3, Pages 452-460Publisher
OXFORD UNIV PRESS
DOI: 10.3945/jn.108.099184
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Funding
- NIH [R01 DK072398]
- General Clinical Research Center [M01-RR00082, 5PO1DK058398-08]
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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-C-13(2)]glycine and [5,5,5-H-2(3)]leucine to quantity in vivo kinetics in healthy adults (7 males, 6 females; 20-39 y) of normal vitamin B-6 status or marginal vitamin B-6 A 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 mu mol.kg(-1).h(-1) (P < 0.05), but serine production using a GCS-derived 1-carbon unit (93 +/- 9 vs. 91 +/- 6 mu mol.kg(-1).h(-1)) and glycine cleavage (163 +/- 11 vs. 151 +/- 8 mu mol.kg(-1).h(-1)) were not changed by vitamin B-6 restriction. The GCS produced 1-carbon units at a rate (similar to 140-170 mu mol.kg(-1).h(-1)) that greatly exceeds the demand for remethylation and transmethylation processes (similar to 4-7 mu mol.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. J. Nutr. 139: 452-460, 2009.
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