50 percent of the B₆ intake is excreted as 4-PA, but this proportion can vary somewhat. 4-PA excretion responds almost immediately to changes in dietary B₆ intake (Lui et al., 1985). Because it reflects recent intake, it is of essentially no value in assessing status. Leklem (1990) has suggested a value of greater than 3 µmol/day as indicative of adequate status. This is achieved with intakes of about 1 mg of B₆. However, the use of this cutoff value represents a circular argument; it presupposes that 1 mg/day of B₆ is an adequate intake.

The stimulation (activation) of erythrocyte aspartate aminotransferase (α-EAST) and erythrocyte alanine aminotransferase (α-EALT) by PLP has been used extensively to evaluate long-term B₆ status. These tests measure the amount of enzyme in the apoenzyme form; the ratio of the apoenzyme to total enzyme increases with B₆ depletion. Leklem (1990) has suggested an α-EAST of less than 1.6 and an α-EALT of less than 1.25 as indicative of adequate B₆ status. Variations in values reported in different studies, which may reflect blood storage conditions and time, have interfered with the setting of a well-documented cutoff point. As described in the later section “Women Ages 19 through 50 Years,” aminotransferase activation factors stabilize slowly in response to changes in diet; this leads to an overestimation of the amount of B₆ required to return values to a preset value in depletion-repletion studies.

The absolute EALT and EAST enzyme activities, both holo- and total enzyme, have also been measured in many studies, but the large variation in values limits their usefulness as indicators of status (Raica and Sauberlich, 1964).

One of the earliest markers for B₆ deficiency was the urinary excretion of xanthurenic acid, which is normally a minor tryptophan catabolite. The major pathway of tryptophan catabolism proceeds via the PLP-dependent kynureninase reaction (Shane and Contractor, 1980). The xanthurenic acid pathway also involves PLP-dependent enzymes. However, under conditions of B₆ deficiency, this minor pathway is used to a greater extent, leading to the increased excretion of abnormal tryptophan metabolites. Mitochondrial enzymes involved in xanthurenic acid production probably retain their