P revious studies in vivo and with isolated perfused rat livers have suggested that the deleterious effect of ethanol on hepatic pyridoxal 5′-phosphate metabolism is mediated by acetaldehyde. Inasmuch as acetaldehyde has no effect on the synthesis of pyridoxal phosphate, it has also been postulated that acetaldehyde accelerates pyridoxal phosphate degradation by displacing this coenzyme from binding proteins, which protect it against hydrolysis. To test these hypotheses, studies have been performed with isolated rat hepatocytes, subcellular fractions of rat liver, and human erythrocytes. Ethanol oxidation lowered the pyridoxal phosphate content of isolated liver cells when acetaldehyde oxidation was inhibited by either disulfiram or prior treatment of rats with cyanamide. Additions of 7.5 mM acetaldehyde alone at 40-min intervals to cell suspensions decreased hepatic pyridoxal phosphate content only slightly because acetaldehyde was rapidly metabolized. However, when acetaldehyde oxidation and reduction were inhibited by cyanamide treatment and by 4-methyl-pyrazole and isobutyramide, respectively, a 40% decrease in hepatic pyridoxal phosphate content was observed in 80 min of incubation.In equilibrium dialysis experiments, acetaldehyde, 7.5 and 15 mM, displaced protein-bound pyridoxal phosphate in undialyzed hepatic cytosol and in hemolysate supernate containing added pyridoxal phosphate. In the presence of alkaline phosphatase, acetaldehyde accelerated the degradation of pyridoxal phosphate in dialyzed hemolysate supernate and hepatic cytosol with added pyridoxal phosphate. Acetaldehyde also inhibits tyrosine aminotransferase. The kinetics of inhibition were mixed competitive-noncompetitive with respect to pyridoxal phosphate. These observations support the hypothesis that the deleterious effect of ethanol oxidation on pyridoxal phosphate metabolism is mediated at least in part by acetaldehyde which displaces this coenzyme from protein binding, thereby enhancing its degradation.