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Dietary vitamin B-6 restriction does not alter rates of homocysteine remethylation or synthesis in healthy young women and men^1,2,3,4

¹ From the Food Science & Human Nutrition Department, Institute of Food and Agricultural Sciences (SRD, JBS, EPQ, JFG), the Division of Endocrinology and Metabolism, Department of Medicine (PWS, BSC), and the Department of Biochemistry and Molecular Biology, College of Medicine (PWS), University of Florida, Gainesville

Background:The effects of vitamin B-6 status on steady-state kinetics of homocysteine metabolism in humans are unclear.

Objective:The objective was to determine the effects of dietary vitamin B-6 restriction on the rates of homocysteine remethylation and synthesis in healthy humans.

Design:Primed, constant infusions of [¹³C₅]methionine, [3-¹³C]serine, and [²H₃]leucine were conducted 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) to establish whether vitamin B-6 status affects steady-state kinetics of homocysteine metabolism in the absence of concurrent methionine intake. Effects of dietary vitamin B-6 restriction on vitamin B-6 status, plasma amino acid concentrations, and the rates of reactions of homocysteine metabolism were assessed.

Results:Dietary vitamin B-6 restriction significantly reduced plasma pyridoxal 5-phosphate (PLP) concentrations (55.1 ± 8.3 compared with 22.6 ± 1.3 nmol/L; P=0.004), significantly increased plasma glycine concentrations (230 ± 14 compared with 296 ± 15; P=0.008), and significantly reduced basal (43%; P < 0.001) and PLP-stimulated (35%; P=0.004) lymphocyte serine hydroxymethyltransferase activities in vitro. However, the in vivo fluxes of leucine, methionine, and serine; the rates of homocysteine synthesis and remethylation (total and vitamin B-6–dependent); and the concentrations of homocysteine, methionine, and serine in plasma were not significantly affected by dietary vitamin B-6 restriction.

Conclusions:Moderate vitamin B-6 deficiency does not significantly alter the rates of homocysteine remethylation or synthesis in healthy young adults in the absence of dietary methionine intake.

Key Words: Vitamin B-6 • homocysteine • human • methionine cycle • one-carbon cycle • pyridoxal 5-phosphate • pyridoxine • remethylation • serine • transmethylation

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