Plasma concentrations of PL 5#-phosphate (PLP), which is the active coenzyme form of vitamin B-6, are reduced during inflammation. The underlying mechanisms may include altered tissue distribution or increased catabolism via pyridoxal (PL) to pyridoxic acid (PA). Recently, we showed that catabolic enzyme activity could be assessed by substrate product ratios measured in plasma. Objective: We evaluated the ratios PA:PL, PA:PLP, and PA:(PL + PLP) as possible markers of vitamin B-6 catabolism.

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... gn: Cross-sectional and longitudinal data were derived from the Western Norway B-Vitamin Intervention Trial. We analyzed asso ciations of ratios with inflammatory markers and other clinical vari ables by using multiple linear regression and partial correlation. In addition, intraclass correlation coefficients (ICCs) were used to as sess the ability of plasma indexes to differentiate between subjects. Results: PA:(PL + PLP) had the highest ICC of all vitamin B-6 metabolites and ratios tested. In regression models, the inflamma tory markers C-reactive protein, white blood cell count, neopterin, and kynurenine:tryptophan collectively accounted for 28% of the total and . 90% of the explained variation in PA:(PL + PLP). For individual B-6 metabolites, corresponding numbers were 19-25% and 20-44%, respectively, with vitamin supplement intake, smok ing, and kidney function (estimated glomerular filtration rate) as additional predictors. In an analysis of receiver operating character istics, PA:(PL + PLP) discriminated high inflammatory concentra tions with an area under the curve (95% CI) of 0.85 (0.81, 0.89). Conclusions: Broad-specificity enzymes upregulated to reduce oxidative and aldehyde stress could explain increased catabolism of vitamin B-6 during inflammation. The ratio PA:(PL + PLP) may provide novel insights into pathologic processes and potentially predict risk of future disease. Am J Clin Nutr 2014;100:250-5.