Distribution and Turnover of the Heart and Liver Isoenzymes of Aspartate Transaminase in the Developing and Young Chick

Abstract

Abstract The distribution of the isoenzymes of aspartate transaminase has been followed during organogenesis of heart and liver tissues in chicken. The content of the supernatant (cytoplasmic) and mitochondrial isoenzymes of aspartate transaminase was measured by precipitation from tissue extracts with their respective antibodies. Heart shows a steady increase in aspartate transaminase specific activity from early stages of development to adulthood. The ratio supernatant to mitochondrial isoenzyme remains constant. Liver also shows an increase in over-all transaminase specific activity, but displays a less regular pattern. The near-equal distribution of the isoenzymes during the early stages of organogenesis is altered between the 14th and 18th days of egg incubation in the form of an excess of supernatant enzyme. The isoenzymes return to a near-equal distribution in the young adult liver. These findings are consistent with postulates which implicate the supernatant enzyme with glucogenesis. The presence of multiple forms within each isozyme was detected by starch gel electrophoresis. The multiple forms present in the supernatant and mitochondrial isoenzymes from adult chicken heart or liver are also detectable in extracts of embryonic chicken heart or liver. The rates of synthesis and degradation of the isoenzymes were examined by following the decay in specific radioactivity in the enzyme after a single injection of uniformly labeled l-[14C]leucine and l-[guanidino-14C]arginine. The relative rates of degradation were measured by monitoring the radioactivity of the respective isoenzymes after injections of the young chick with 14C- and 3H-labeled l-leucine or l-arginine, or both. Young chicken heart shows a mean half-life of 3.8 days for the supernatant and mitochondrial aspartate transaminases. The half-life of these isozymes in young chicken liver is 2.8 days. The relative rates of degradation as tested by the double isotope technique also show a greater turnover by the liver enzyme. On the other hand, the apparent rate of synthesis is greater for the liver than for the heart isoenzymes. These rates are based on uncorrected values for as yet unknown variations in the reutilization of the radioactive amino acids in chicken heart or liver. These results indicate that the genes coding the two isoenzymes of aspartate transaminase tend to express themselves simultaneously within a given tissue. However, their regulation may vary among the tissues and, in the case of liver, also during the organogenesis of the tissue to accommodate or trigger physiological needs, or both.