The Role of Acetaldehyde in Mediating the Deleterious Effect of Ethanol on Pyridoxal 5′-Phosphate Metabolism

Abstract

A B S T R A C T Previous 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-methylpyrazole 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-noncompeti-

Plasma content of B6 vitamers and its relationship to hepatic vitamin B6 metabolism.

“…Immediately following isolation, 95+1% (-SE, n = 8) of the cells exeluded trypan blue, andl 84+2% of the cells continued to exclude the dye following 2 h of incubation. Cells prepared in this manner exhibited high rates of gluconeogenesis (12), maintained intracellular K+ concentrations at 130-150 me(l/liter of cell water (13), and were active in protein synthesis (14) ancl amino acid transport (11).…”

Alcohol-associated severe hyperhomocysteinaemia

“…21 Cystathionine b-synthase and cystathionine lyase are pyridoxal-5 0 -phosphate-dependent enzymes, and as such alcohol metabolism can compromise homocysteine transsulphuration.…”

B-Vitamin dependent methionine metabolism and alcoholic liver disease

“…One of the principal clinical manifestations of vitamin B6 deficiency in alcoholism is sideroblastic anemia as was found by bone marrow examination in 23 % of a sequential series of hospitalized anemic chronic alcoholic patients [48] . Vitamin B6 circulates in the form of protein bound pyridoxal phosphate, and the most widely accepted etiology for vitamin B6 deficiency is the degrading effect of the acetaldehyde product of alcohol metabolism on this compound with subsequent urinary wastage of the free vitamin [49,50] . By contrast, circulating levels of vitamin B12 are often elevated in ALD [21,24] , even though its absorption from the intestine is compromised by chronic alcoholism [51] .…”