Activities of enzymes concerned with pyruvate and oxaloacetate metabolism in the heart and liver of developing sheep.

1. In order to assess whether the potential ability of heart ventricular muscle and liver to metabolise substrates such as alanine, aspartate and lactate varies as the sheep matures and its nutrition changes, the activities of the following enzymes were determined in tissues of lambs obtained at varying intervals between 50 days after conception to 16 weeks after birth and in livers from adult pregnant ewes: lactate dehydrogenase (EC 1.1.1.27), alanine aminotransferase (EC 2.6.1.2), pyruvate kinase (EC 2.7.1.40), pyruvate carboxylase (EC 6.4.1.1), phosphoenolpyruvate carboxykinase (GTP)(EC 4.1.1.32), malate dehydrogenase (EC 1.1.1.37), aspartate aminotransferase (EC 2.6.1.1) and citrate (si)-synthase (EC 4.1.3.7). 2. In the heart a most marked increase in alanine aminotransferase activity was found throughout development. During this period the activities of citrate (si)-synthase, lactate dehydrogenase and pyruvate carboxylase also increased. There were no substantial changes in the activities of aspartate aminotransferase, malate dehydrogenase or pyruvate kinase. Pyruvate kinase activities were five times greater in the heart compared with those found in the liver. No significant activity of phosphoenolpyruvate carboxykinase (GTP) was detected in heart muscle. 3. In the liver the activities of both alanine aminotransferase and aspartate aminotransferase increased immediately following birth although the activity of alanine aminotransferase was lower in livers of pregnant ewes than in any of the lambs. As with alanine aminotransferase the highest activities of lactate dehydrogenase were found during the period of postnatal growth. No marked changes were observed in malate dehydrogenase or citrate (si)-synthase activities during development. A small decline in pyruvate kinase activity occurred whilst the activities of pyruvate carboxylase and phosphoenolpyruvate carboxykinase (GTP) tended to rise during development.     

Enzyme activities in the sheep placenta during the last three months of pregnancy

In order to assess the extent to which metabolism within the sheep placenta may influence the transfer of metabolites between mother and foetus at different stages of gestation the activities of enzymes concerned with some aspects of carbohydrate, amino acid and ketone body metabolism were determined in placental cotyledons resected from ewes during the last three months of pregnancy.The activities of pyruvate kinase (EC 2.7.1.40), lactate dehydrogenase (EC 1.1.1.27), malate dehydrogenase (EC 1.1.1.37), ATP citrate (pro-3S)-lyase (EC 4.1.3.8), citrate (si)-synthase (EC 4.1.3.7), acetyl-Co A synthetase (EC 6.2.1.1), acetyl-CoA acetyltransferase (EC 2.3.1.9) and 3-keto acid CoA-transferase (EC 2.8.3.5) per gram wet weight cotyledon do not change during the period studied. The activities of alanine aminotransferase (EC 2.6.1.2), aspartate aminotransferase (EC 2.6.1.1), isocitrate dehydrogenase (NADP⁺) (EC 1.1.1.42), ornithine-oxoacid aminotransferase (EC 2.6.1.13) and 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30) show an increase in activity between the third and fourth months of pregnancy whilst the activities of arginase (EC 3.5.3.1) and possibly pyruvate carboxylase (EC 6.4.1.1) show an increase in activity between the fourth and final months of pregnancy. Ornithine decarboxylase (EC 4.1.1.17) activity declines to one tenth of its activity during this later period. The absence of detectable activities of phosphoenolpyruvate carboxykinase (EC 4.1.1.32) and ornithine carbamoyltransferase (EC 2.1.3.3) indicate that gluconeogenesis and urea synthesis from ammonia do no occur in the sheep placenta.It appears that the ability of the placenta to metabolise several substrates is achieved by the time the placenta reaches its maximum size at approximately 90 days.