A Comparative Study Of Nadp-linked Malic Enzyme In Vertebrates
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NADP-linked malic enzyme has been considered part of a lipogenic set of cytosol enzymes which includes the NADP-linked HMP shunt dehydrogenases, CCE, acetyl CoA carboxylase and fatty acid synthetase. In the present study, the activity of NADP-linked dehydrogenases have been measured in selected tissues from a variety of avian and mammalian species representing different dietary and phylogenetic types. The pattern of distribution of malic enzyme, G6PD and ICDH does not appear to be dependent on the dietary habits of the species. In birds, however, a difference emerges between two phylogenetic groups. In general, all birds have a high liver malic enzyme activity; the passerines have an enzyme activity many fold in excess of that of mammalian liver. In some non-passerine liyers, however, malic enzyme activity was nearer to that typycal .of mammals. G6PD was extremely . inactive in all passerine livers examined, but it was present in somewhat higher activities in livers of non-passerines. In rat liver, malic enzyme and G6PD levels were highly responsive to dietary manipulations, being elevated in conditions favouring fatty acid synthesis and depressed in livers of starved rats, whereas ICDH did not respond to such changes. In pigeon liver, these enzymes were little affected by fasting and refeeding. Malic enzymes were purified from rat and pigeon liver by similar procedure and assayed under identical conditions. Kinetically, the two enzymes were found to be similar. Phenobarbitone, an inducer of microsomal drug metabolising systems, caused a. substantial increase in the activity of hepatic malic enzyme in normally fed rats and in rats in which the basic level of the enzyme had been reduced by starvation or by feeding the animal with a high protein diet, but not in animals where the basic level had already been elevated by feeding a diet rich in sucrose. Phenobarbitone had relatively little effect on HMP shunt dehydrogenases, whereas methyl cholanthrene, a compound which evokes an induction of microsomal drug metabolising systems qualitatively different from that produced by phenobarbitone, induced not only malic enzyme levels but also G6PD and 6PGD significantly. ICDH and CCE were not affected by either of the two compounds.