Induction of Serine: Pyruvate Aminotransferase in Rat Liver Organelles by Glucagon and a High-Protein Diet1

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Abstract
The effects of the administration of glucagon and a high-protein diet, the conditions for the induction of rat liver serine: pyruvate aminotransferase, on the subcellular distribution of this enzyme were studied. There were two classes of serine: pyruvate aminotransferase activity which were distinguishable by the reactivity with a specific antibody prepared against purified serine: pyruvate aminotransferase. In the liver of fasted rats, the aminotransferase activity was located in peroxisomes, mitochondria and the cytosolic fraction in a ratio of 23 : 49 : 28 as judged by primary subcellular fractionation and subsequent ultracentrifugation on a sucrose density gradient. In mitochondria and the cyto solic fraction, however, the immunoreactive activity accounted for only approxi mately 30% and 50% of the total activity in the respective fractions, while over 90% of the peroxisomal activity reacted with the antibody. The percentage distribution of serine: pyruvate aminotransferase in peroxisomes, mitochondria and the cytosolic fraction was thus calculated to be 40, 30, and 30%, respectively. Only the mitochondrial serine: pyruvate aminotransferase activity was remark ably increased after the administration of glucagon, and when only the immuno precipitable activity was concerned, the increase was as much as approximately 100-fold after 3 daily injections of the hormone. The concomitant rise in the immu noreactive activity in peroxisomes and the cytosolic fraction was several fold, sug gesting the existence of some mechanisms which permit the selective accumulation in mitochondria of serine: pyruvate aminotransferase synthesized under the acute effect of glucagon. In the liver of rats fed on an 88% casein diet for 8 days, the hepatic activity of serine: pyruvate aminotransferase increased approximately 3-fold, but the preferential increase in the activity of any particular subcellular fraction was not as clearly seen as in the case of the liver of glucagon-treated rats. Since rat liver serine:pyruvate aminotransferase has a relatively broad substrate specificity, the enzyme induced under the conditions of glucagon abundance ap peared to participate in the catabolism of various neutral amino acids, such as phenylalanine, asparagine and glutamine, in addition to that of serine.