Abstract

Protein carboxymethylase (S-adenosyl-l-methionine:protein-O-methyltransferase, EC 2.1.1.24) transfers methyl groups from S-adenosylmethionine to protein carboxyl groups. This cytosolic enzyme is found in highest concentration in secretory tissue and methylates membrane proteins. Stimulation of the parotid gland by catecholamines rapidly and reversibly increases protein carboxymethylase activity and methyl acceptor capacity of proteins in parotid homogenates. Isoproterenol was effective at concentrations causing amylase release in vivo and in vitro. Both enzyme activity and methyl acceptor capacity of proteins increased within 5 min, continued to increase for 30 min and then declined to control values within 60 min. The response to isoproterenol was stereospecific. The action of isoproterenol could be blocked by the beta adrenergic antagonist propranolol, while the alpha adrenergic agonist phenylephrine did not stimulate the enzyme or increase methyl acceptor proteins. Methyl acceptor proteins have been partially characterized by polyacrylamide gel electrophoresis. Although many proteins in the parotid are methylated, only two groups of methylated proteins increase after stimulation by isoproterenol.