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Human Nutrition and Metabolism
Research Communication

Plasma Pyridoxal 5'-Phosphate Concentration Is Correlated with Functional Vitamin B-6 Indices in Patients with Rheumatoid Arthritis and Marginal Vitamin B-6 Status

En-Pei I. Chiang, Pamela J. Bagley, Ronenn Roubenoff, Marie Nadeau and Jacob Selhub⁴

^* Vitamin Metabolism and Aging Laboratory and Nutrition and Exercise Physiology Laboratory Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111, and General Clinical Research Center, New England Medical Center, Boston, MA 02111

⁴To whom correspondence should be addressed. E-mail: jacob.selhub{at}tufts.edu.

Many patients with rheumatoid arthritis (RA) have low plasma pyridoxal-phosphate (PLP) but a normal erythrocyte aspartate aminotransferase activity coefficient ( EAST), a measure of vitamin B-6 status in the erythrocytes, compared with healthy subjects. The goal of the present study was to examine the correlations of PLP levels in these two compartments (plasma and erythrocytes) with other established indices of vitamin B-6 status, and to determine which indicator better reflects functional status of vitamin B-6 in patients with RA. Multiple indices of vitamin B-6 status were measured in 33 patients with RA. Plasma PLP, urinary 4-pyridoxic acid (4-PA), net increase in plasma total homocysteine after a methionine load (tHcy) and net increase in urinary xanthurenic acid after a tryptophan load (XA) were log-transformed to reach normality for statistical analyses. We found that log-plasma PLP levels were inversely correlated with both log-tHcy (r = -0.368, P = 0.035) and log-XA (r = -0.333, P = 0.05). Plasma PLP was not correlated with EAST or urinary 4-PA excretion. In contrast, erythrocyte PLP was inversely correlated with EAST (r = -0.431, P = 0.012) and positively correlated with log-4-PA (r = 0.475, P = 0.005), but erythrocyte PLP was not correlated with the outcomes of a methionine or tryptophan load test. Erythrocyte PLP and log-4-PA, but not plasma PLP, were correlated with dietary intake of vitamin B-6 after adjusting for protein intake (r = 0.420, P = 0.015 and r = 0.333, P = 0.05, respectively). We suggest that in patients with RA, plasma PLP levels are a better diagnostic indicator of functional vitamin B-6 status than erythrocyte PLP levels.

KEY WORDS: • rheumatoid arthritis • pyridoxal 5'-phosphate • methionine load • tryptophan load • xanthurenic acid

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