Adenylate cyclase of catfish hepatocyte membranes: basal properties and sensitivity to catecholamines and glucagon

Abstract

Some characteristics of adenylate cyclase of catfish ( Ictalurus melas) liver membranes were studied, and the effects of catecholamines and of glucagon were tested. The enzyme has an optimum temperature of 40 °C, and a K m for ATP of 0.16 mM at 30 °C, and requires Mg 2+ for its activity. The enzyme activity is inhibited with a Ca 2+ concentration higher than 5 × 10 −5 M, and enhanced with F − higher than 10 −4 M. The response of adenylate cyclase to GTP is biphasic, with a maximum of activity at 10 −5 M GTP. Catecholamines (epinephrine, norepinephrine, isoproterenol, phenylephrine) enhance cyclase activity. Propranolol inhibits the increase in enzyme activity induced by catecholamines, whereas phentolamine is ineffective. This indicates that catecholamines (phenylephrine included) activate adenylate cyclase through a β-adrenergic mechanism. Glucagon (mammalian) has a smaller effect than epinephrine in increasing the enzyme activity of catfish hepatocyte membranes. This fact is the opposite of that observed for the cyclase activity of rat liver membranes.