Plasma pyridoxal 5′-phosphate concentration is correlated with functional vitamin B-6 indices in patients with rheumatoid arthritis and marginal vitamin B-6 status

Many patients with rheumatoid arthritis (RA) have low plasma pyridoxal-phosphate (PLP) but a normal erythrocyte aspartate aminotransferase activity coefficient (a 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 (DeltatHcy) and net increase in urinary xanthurenic acid after a tryptophan load (DeltaXA) were log-transformed to reach normality for statistical analyses. We found that log-plasma PLP levels were inversely correlated with both log-DeltatHcy (r = -0.368, P = 0.035) and log-DeltaXA (r = -0.333, P = 0.05). Plasma PLP was not correlated with a EAST or urinary 4-PA excretion. In contrast, erythrocyte PLP was inversely correlated with a 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.