tissues at very high intakes of B6 (Merrill et al., 1984). When this capacity is exceeded, free PLP is rapidly hydrolyzed and nonphosphorylated forms of B6 are released by the liver and other tissues into circulation. At pharmacological doses of B6, the high capacities for PLP-protein binding of muscle, plasma, and erythrocytes (hemoglobin) allow them to accumulate very high levels of PLP when other tissues are saturated (Lumeng et al., 1978).
PLP in the liver can be oxidized to 4-PA, which is released and excreted. The major PLP-binding protein in plasma is albumin. PLP is the major form of the vitamin in plasma and is derived entirely from liver as a PLP-albumin complex (Fonda et al., 1991; Leklem, 1991). Tissues and erythrocytes can transport nonphosphorylated forms of the vitamin from plasma. Some of this is derived from plasma PLP after phosphatase action. In tissues, conversion of the transported vitamin to PLP, coupled with protein binding, allows accumulation and retention of the vitamin. B6 in tissues is found in various subcellular compartments but primarily in the mitochondria and the cytosol.
Normally, the major excretory product is 4-PA, which accounts for about half the B6 compounds in urine (Shultz and Leklem, 1981). Other forms of the vitamin are also found in urine. With large doses of B6, the proportion of the other forms of the vitamin increases. At very high doses of PN, much of the dose is excreted unchanged in the urine. B6 is also excreted in feces but probably to a limited extent (Lui et al., 1985). Microbial synthesis of B6 in the lower gut makes it difficult to evaluate the extent of this excretion.
Pharmacokinetic analyses of urinary excretion of a tracer dose of labeled PN and its metabolites have suggested a two-compartment model for body B6 stores (Johansson et al., 1966). With this approach, body stores have been estimated at 365 µmol (61 mg) or 7.7 µmol/kg in a healthy 20-year-old woman and 660 µmol (110 mg) or 8.8 µmol/kg in a 25-year-old man. Overall body half-lives were about 25 days (Shane, 1978). Intake (and excretion) was estimated to be 1.5 mg (9 µmol) for the woman and 3.4 mg (20 µmol) for the man. The two-compartment model has been questioned because muscle stores most of the body’s B6; the pool in muscle appears to turn over very slowly. This fact may have not been considered, resulting