Enzymes in facultative anaerobiosis of molluscs—III. Phosphoenolpyruvate carboxykinase and its role in aerobic-anaerobic transition

Abstract

1. 1. In this paper, the regulatory properties of p-enolpyruvate carboxykinase (E.C. 4.1.1.32) from oyster adductor tissue are further studied. 2. 2. Competitive inhibition patterns are obtained for metal ions with ITP as the inhibitor. In both instances 0·25 mM ITP increases the apparent K m values of the metal ion by tenfold. 3. 3. In the presence of Zn 2+ with ITP as the inhibitor and IDP as the variable substrate, a linear non-competitive inhibition pattern is obtained. At all concentrations of ITP tested the K m (IDP) remains unchanged at about 0·06 mM. In contrast in the presence of Mn 2+ a linear, mixed competitive ITP inhibition pattern is obtained. In this instance, 0·25 mM ITP increases the K m (IDP) several fold. 4. 4. In the presence of either metal ion, ITP inhibition is competitive with respect to p-enolpyruvate. With Zn 2+, 0·5 mM ITP increases the t m (p-enolpyruvate) by about twofold while in the presence of Mn 2+, 0·25 mM ITP increases the K m (p-enolpyruvate) at least by fivefold. 5. 5. In the presence of IDP or GDP, GTP also inhibits the p-enolpyruvate carboxykinase reaction. 6. 6. Alanine has a slight stimulatory effect at low p-enolpyruvate concentrations and markedly deinhibits ITP (or GTP) inhibited p-enolpyruvate carboxykinase. 7. 7. From the data presented it is speculated that co-ordinated changes in the intracellular concentrations of H +, ITP, GTP and alanine control p-enolpyruvate carboxykinase activity in vivo.