Plasma glutathione and cystathionine concentrations are elevated but cysteine flux is unchanged by dietary vitamin B-6 restriction in young men and women.

Cysteine synthesis from homocysteine is catalyzed by two pyridoxal 5'-phosphate (PLP)-dependent enzymes. This suggests that vitamin B-6 status might affect cysteine and glutathione homeostasis, but it is unclear whether this occurs in humans. We assessed the effects of vitamin B-6 status on static and kinetic parameters of cysteine and glutathione metabolism in healthy female (n=5) and male (n=4) volunteers (20-30 y) before and after 4 wk of dietary vitamin B-6 restriction (<0.5 mg vitamin B-6/d). Rates of reactions related to cysteine metabolism were measured from blood sampled during primed, constant infusions of [(13)C(5)]methionine, [3-(13)C]serine, and [(2)H(2)]cysteine that were conducted after an overnight fast at baseline and after the dietary protocol. Vitamin B-6 restriction reduced the concentration of PLP (55.1+/- 8.3 vs. 22.6+/-1.3 nmol/L; P=0.004) and increased concentrations of cystathionine (124%; P<0.001) and total glutathione (38%; P<0.008) in plasma. Concentrations of plasma homocysteine, cysteine, cysteinylglycine, and C-reactive protein (an indicator of systemic inflammation) were not affected by dietary vitamin B-6 restriction. The rate of cysteine synthesis via transsulfuration was below detection limits in this protocol. Neither the fractional synthesis rate of cystathionine nor whole-body cysteine flux was affected by vitamin B-6 restriction. These data indicate that glutathione homeostasis is altered by dietary vitamin B-6 deficiency and appears to be unrelated to cysteine flux under conditions of minimal amino acid intake as evaluated in this study.