Protein carboxymethylation: effects of 2% sodium chloride administration on protein carboxymethylase and its endogenous substrates in rat posterior pituitary

Protein carboxymethylase and methyl acceptor proteins have been studied in the posterior pituitary of control and NaCl treated rats. Salt loading stimulates the hypothalamo-neurohypophysial axis causing the secretion of neurohypophysial peptides. Under these conditions there was a progressive decrease of methyl acceptor proteins with the greatest fall occurring between 1 and 2 days after salt loading. Electrophoretic analysis of the methyl acceptor proteins showed a single major peak of methylated proteins accounting for up to 80% of the total radioactivity. The molecular weight (11,000) and the disappearance of this peak after salt loading suggested that this methyl acceptor protein is neurophysin. After prolonged stimulation of the hypothalamo-neurohypophysial axis there was a progressive increase in protein carboxymethylase specific activity.