Patients with cirrhosis and other hepatic diseases frequently exhibit lower concentrations of plasma pyridoxal 5'-phosphate (PLP), which is derived primarily from liver. To determine the biochemical basis for this abnormality, the enzymes of vitamin B6 metabolism--pyridoxal kinase, pyridoxine (pyridoxamine) 5'-phosphate oxidase, PLP phosphatase(s), and pyridoxal oxidase(s)--were analyzed in liver. The activities of the two biosynthetic enzymes, pyridoxal kinase and pyridoxine (pyridoxamine) 5'-phosphate oxidase were similar for both. The phosphatase activities were significantly higher (mean +/- SD of 9.55 +/- 8.03 versus 3.97 +/- 2.36 nmol X min X mg protein, p less than 0.05) for cirrhotics. Pyridoxal oxidase activities appeared slightly lower for cirrhotics. There was considerable variation in many indices of liver function, which suggests that the defects contributing to altered vitamin B6 metabolism may be complex and individualistic. These analyses have shown that cirrhotics are capable of apparently normal PLP synthesis and that increased hepatic dephosphorylation may be responsible for low levels of plasma PLP.