Abstract
The most striking difference among animal species regarding gluconeogenesis is that of the intracellular location of phosphoenolpyruvate carboxykinase. In an adult chicken liver the enzyme is located entirely within the mitochondrial matrix. The situation is essentially reversed in rats, mice and hamsters, having a major activity in the cytosol. Most other mammalian species show substantial activities of both types of hepatic phosphoenolpyruvate carboxykinase. The functional, structural and genetic relationships of the two varieties of phosphoenolpyruvate carboxykinase are not well understood. In this report phosphoenolpyruvate carboxykinases occurring in both the cytoplasm and mitochondria of the liver of an adult bullfrog and its larvae were studied with respect to their identity and turnover in vivo. The results obtained are as follows :
(1) Immunological and incorporation studies show that the liver of an adult bullfrog and its larvae contains a single type of phosphoenolpyruvate carboxykinase which is mainly localized in the mitochondrial fraction.
(2) The mitochondrial enzyme activity increases at the climax of natural and 3, 5, 3′-tri iodothyronine - induce d metamorphosis.
(3) Labelling experiments in vivo of the enzyme with [8H] leucine show (j) that the cytosolic enzyme is labelled rapidly whereas the mitochondrial enzyme is labelled after a lag period of six minutes and 0 that the relative rate of synthesis of phosphoenolpyruvate carboxykinase is about 3 times higher in the liver of a metamorphosing tadpole than that of a normal tadpole. Since no difference is observed in the degradation rate of the enzyme between normal and metamorphosing tadpoles, the increase in the enzyme activity at the metamorphic climax is based upon an acceleration of the enzyme synthesis.
