On the nature of three forms of phosphoenolpyruvate carboxykinase occurring in the cytosol of chicken liver.

Chicken liver mitochondria contained two forms of phosphoenolpyruvate (PEP) carboxykinase (designated as Mt-I and Mt-II) and the chicken liver cytosol fraction contained three forms of PEP carboxykinase (designated as Sol-I, Sol-II, and Sol-III). Mt-I and Mt-II were purified to homogeneity. They both had a molecular weight of 72,000 as judged by the sodium dodecyl-sulfate-polyacrylamide gel electrophoresis method, whereas an apparent molecular weight of 42,000 was obtained for both enzymes when estimated by Sephadex G-100 gel filtration. Studies with a rabbit antibody against the purified Mt-I revealed that Mt-I, Mt-II, Sol-I, and Sol-II are immunochemically identical, whereas Sol-III is immunochemically different from any of the other proteins. Genetic analogy between Mt-I and Mt-II was also suggested by gel electrophoretic analysis of their peptide maps. The content of Sol-III in the adult chicken liver was very small, whereas the livers of chick embryo and very young chick contained a considerable amount of Sol-III. The level of Sol-III in the adult chicken, however, could be significantly increased by the administration of hydrocortisone or isoproterenol. Apparently Sol-III is a cytosol-specific PEP carboxykinase which is similar to the cytosolic PEP carboxykinases of various mammals. Sol-III showed a molecular weight of about 60,000 as estimated by gel filtration. Studies with combined use of the antibody and [3H]leucine revealed that some portion of PEP carboxykinase (Sol-I plus Sol-II) appearing in the liver cytosol corresponds to a precursor in transit to the mitochondria, although major portions of Sol-I and Sol-II appear to be accounted for by release of the mitochondrial enzymes.