Glyoxylate metabolism in fish liver

Abstract

L-Alanine-glyoxylate aminotransferase (EC 2.6.1.44) activity has been detected in the liver of spotted sea trout (Cynoscion nebulosus). The enzyme converts glyoxylate to glycine while the amino donor, alanine, is converted to pyruvate. This was confirmed by radioisotope double-labeling experiments. Enzyme markers showed the activity to be mainly mitochondrial, although the cytosol exhibited some activity that could not be explained as resulting from mitochondrial leakage. The mitochondrial activity showed strong preference for alanine over serine, tyrosine and glycine. The activity found in the cytosol also preferred alanine, but glutamate was next in preference. The mitochondrial enzyme used L-alanine almost twice as much as DL-alanine. The reaction could be catalyzed by mitochondria even after thorough washing or by the cytosol after dialysis, provided that in both cases, a suitable amino donor was added. However, the addition of either amino-oxyacetate, isonicotinic acid hydrazide and, to a lesser extent, hydroxylamine, inhibited the reaction. Thus, the enzyme required pyridoxal phosphate for the reaction and seemed to be tightly bound to it.