is the active coenzyme species. In animals fed graded levels of pyridoxine (PN), the plasma PLP concentration correlated well with tissue B6 (Lumeng et al., 1978).
Protein-bound PLP in the plasma is in equilibrium with free PLP. Binding of PLP to protein protects it from hydrolysis by alkaline phosphatase. Conditions of increased plasma phosphatase activity can lead to reduced plasma PLP. Hydrolysis of plasma PLP is required before it can be transported into tissues.
In the controlled depletion-repletion study (Kretsch et al., 1991) in which 2 of 11 young women placed on a diet containing less than 0.05 mg of B6 exhibited abnormal electroencephalogram patterns, plasma PLP dropped to about 9 nmol/L. Similar PLP values were also observed in the other 9 depleted but asymptomatic subjects. This suggests that PLP concentrations of about 10 nmol/L represent a suboptimal concentration associated with clinical consequences in some subjects. Although fewer than half the subjects in this study exhibited signs of deficiency, more subjects might have shown signs if the depletion diet had been continued longer than 12 days.
Leklem (1990) has suggested a plasma PLP concentration of 30 nmol/L as the lower end of normal status. Results from a large number of studies involving various population groups (Brown et al., 1975; Driskell and Moak, 1986; Lindberg et al., 1983; Lumeng et al., 1974; Miller et al., 1975, 1985; Rose et al., 1976; Tarr et al., 1981) have shown that a substantial proportion of individuals in these populations, in some cases half, have plasma PLP concentrations below 30 nmol/L, but there are no confirming clinical or other data to suggest B6 deficiency. Other investigators have proposed a cutoff of 20 nmol/L for plasma PLP as an index of adequacy (Lui et al., 1985). The more conservative cutoff of 20 nmol/L is not accompanied by observable health risks but it allows a moderate safety margin to protect against the development of signs or symptoms of deficiency. A cutoff for PLP of 20 nmol/L was selected as the basis for the average requirement (EAR) for B6 although its use may overestimate the B6 requirement for health maintenance of more than half the group.
A recent random sampling of the Dutch population indicated a 3 to 7 percent prevalence of plasma PLP concentrations of less than 19 nmol/L in various life stage and gender groups (Brussaard et al., 1997a, b). The prevalence was slightly higher in men aged 50 to 79 years. Although plasma PLP values in this population correlated with dietary variables, some of the fundamental tests for B6 status,