Inhibition of hepatic gluconeogenesis by phenethylhydrazine (phenelzine)

Abstract The hepatic effects of phenethylhydrazine (phenelzine), an antidepressive drug occasionally causing hypoglycemia, were examined. Phenelzine (1 mM) inhibits gluconeogenesis from l -lactate, pyruvate and propionate, but not from fructose in experiments with isolated perfused rat livers. Ketogenesis from endogenous sources as well as from hexanoate or fructose is likewise inhibited. Gluconeogenesis, urea formation, and net formation of l -lactate + pyruvate from l -alanine are inhibited in experiments with isolated hepatocytes. Phenelzine leads to a reduction of the cytoplasmic, but not of the mitochondrial NAD + /NADH system. Cross-over plots of intrahepatic metabolites revealed forward cross-overs between l -malate and phosphoenolpyruvate (PEP) and fructose-1,6-bisphosphate (FDP) and fructose-6-phosphate (F-6-P) in the presence of phenelzine, when l -lactate was glucogenic precursor. With pyruvate as substrate forward crossover points were between pyruvate and l -malate and between FDP and F-6-P. The concentrations of l -glutamate, l -aspartate, and α-oxoglutarate changed in the presence of phenelzine in a way compatible with an inhibition of the aspartate aminotransferase reaction. The overall concentrations of acetyl-CoA decreased in the presence of phenelzine, when pyruvate was substrate. PEP-car☐ykinase was inhibited in vitro by phenelzine, due to trapping of oxaloacetate by phenethyl-hydrazone formation. The same mechanism was found for aspartate aminotransferase when tested in the direction of l -aspartate formation. In the direction of oxaloacetate formation a competitive inhibition was observed ( K i app= 7.2 · 10 −4 M), probably due to an interaction of phenelzine with the enzyme linked pyridoxal-5'-phosphate (PLP) as indicated by aldimine formation of phenelzine with PLP in vitro . Phenelzine (1mM) inhibited significantly the incorporation of carbon from the C-1 and from the C-2 position of the lactate-pyruvate pool into CO 2 , glucose, and (C-2 only) fatty acids, whereas the incorporation into the glyceride-glycerol fraction increased. The incorporation of hydrogen from 3 H 2 O into total lipids and glyceride-glycerol was strongly, that into fatty acids completely, inhibited under the same conditions. Phenelzine did not inhibit acetoacetate reduction by isolated rat liver mitochondria with succinate. citrate or isocitrate as substrate, but it inhibited strongly when pyruvate, and slightly when l -malate, was the substrate. The ability of phenelzine to form hydrazones with 2-keto acids increased in the sequence α-oxoglutarate l -lactate, but not from pyruvate, was inhibited by 0.1 mM 2-phenethylhydrazonopropionate. It is concluded that phenelzine, if at all, affects gluconeogenesis only partly via its hydrazone derivatives. It acts mainly by restricting oxalacetate formation in the cytosol due to an inhibition of aspartate aminotransferase. In addition, phenelzine inhibits pyruvate oxidation. This effect is mainly responsible for the observed inhibition of fatty acid synthesis from carbohydrates. The mechanism of action precludes the use of this or similar drugs in the treatment of diabetes.