The Synthesis of Phosphoenolpyruvate in the Gluconeogenesis

Abstract

Livers of chick embryos a t different stages of incubation have been fractionated into mitochondrial and "soluble" fractions by centrifugation.The pyruvate carboxylase and phosphoenolpyruvate carboxykinase activities, which constitute the major pathway for the synthesis of phosphoenolpyruvate, have been measured in both preparations.The pyruvate carboxylase activity occurs only in the mitochondria and parallels the gluconeogenetic ability.The bulk of phosphoenolpyruvate carboxykinase activity appears in the mitochondria. Only traces can be detected in the "soluble" fraction. The level of the mitochondrial phosphoenolpyruvate carboxykinase activity varies during the incubation period in accordance with its presumed role as a major source of phosphoenolpyruvate in the gluconeogenesis.The hypothesis that the pathway of synthesis of phosphoenolpyruvate in the process of gluconeogenesis is mainly located in the mitochondria of chick embryo liver is proposed.The sequence of reactions leading to phosphoenolpyruvate synthesis from pyruvate is one of the most intensively studied steps in the overall process of gluconeogenesis and the following sequence of reactions is now generally accepted :. Phosphoenolpyruvate + GDP + CO,.

Ontogenic Profile of Gluconeogenic Key Enzyme Gene Expressions in Embryonic Chicken Liver and Muscle

“…They vary considerably in experimental design, but the enzymes showed maximum activities largely between E14 and E17 in some of these reports (Rinaudo, 1961;Ballard and Oliver, 1963;Felicioli et al, 1967). These results are consistent in tendency with our present data that the expressions of the enzymes had peaks mainly on E13 or E15 and declined thereafter (Fig.…”

“…Also, PEPCK-C gene expression in embryonic chicken liver was inducible by dexamethasone administration with a progressively decreasing responsiveness from E12 until E15 (McCaffrey and Hamilton, 1994). However, Felicioli et al (1967) reported that enzyme activity of PEPCK-C in the liver was very low between E13 and E21. Watford (1985) pointed out that, although PEPCK-C mRNA was considerably increased in the liver of adult chickens deprived of food, its enzyme activity remained very low.…”

“…There have been many studies where the activities of the gluconeogenic key enzymes in embryonic chicken liver are investigated (Rinaudo, 1961;Ballard and Oliver, 1963;Nelson et al, 1966;Felicioli et al, 1967;Evans and Scholz, 1973;Gonzalez-Mujica et al, 2001). They vary considerably in experimental design, but the enzymes showed maximum activities largely between E14 and E17 in some of these reports (Rinaudo, 1961;Ballard and Oliver, 1963;Felicioli et al, 1967).…”

Expression of the Genes for the Mitochondrial and Cytosolic Forms of Phosphoenolpyruvate Carboxykinase in Avian Liver during Development

“…In corporation of 14C-amino acids and 14CC>2 into glucose by liver slices from embryonic chicken is temporally associated with the ac tivity of the gluconeogenic enzymes pyruvate carboxylase, fructose 1,6-diphosphatase, glucose-6-phosphatase and PEPCK [6][7][8], Pre vious studies undertaken to determine the subcellular distribution of the two isozymes of PEPCK in the developing avian liver have, however, given divergent results. In one case [9] substantial amounts of only PEPCK-M activity were observed throughout embryonic development, whereas in another study a ra tio of mitochondrial to cytosolic enzyme ac tivity of approximately 70:30 was noted [10].…”

The pancreatic insulin content and its relationship to plasma glucose and free fatty acid concentrations in the embryo and neonatal chick

“…The maximal activity of both pyruvate carboxylase (EC 6.4.1.1.) and phosphoenol pyruvate carboxykinase (EC 4.1.1.32) occurs on day 17 (Nelson et al, 1966;Felicioli et al, 1967).…”