Plasma pyridoxal 5'-phosphate (PLP) concentrations are considered to be the most reliable single indicator of vitamin B-6 nutritional status and are thought to reflect tissue PLP and pyridoxamine 5'-phosphate (PMP) levels. We investigated the relationship between dietary level of pyridoxine hydrochloride (PN-HCl) and concentrations of PLP in blood and PLP and PMP in liver and brain of mice. Female heterogeneous stock mice, 60 to 90 d old, were fed purified diets containing 0.5, 1.0, 2.0, 3.0, 5.0, or 7.0 mg PN-HCl/kg diet for 5 wk. PLP and PMP concentrations were determined by a spectrophotometric apotryptophanase assay. PLP content of plasma, erythrocytes, whole blood, liver and brain and PMP levels in liver and brain were highly correlated with dietary level of PN-HCl (r values ranged from 0.81 to 0.94, n per correlation = 32 to 43). By using the entire range of dietary levels of PN-HCl, both plasma and erythrocyte PLP were found to be significantly correlated with tissue PLP and PMP concentrations. For any one dietary level, however, correlations between plasma or erythrocyte PLP and tissue PLP and PMP concentrations were low and nonsignificant. These results suggest that plasma PLP levels may be suitable to determine vitamin B-6 status of populations, but not to reliably predict tissue concentrations of PLP or PMP in individuals.
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