Enzymatic methyl esterification of pituitary polypeptides.

Methyl esterification of pituitary polypeptides by protein methylase II (S-adenosylmethionine:protein-carboxyl O-methyltransferase, EC. 2.1.1.24) has been investigated. Ovine lutropin and adrenocorticotropin (alpha1-39-ACTH) were found to be good methyl acceptor substrates, followed by beta-lipotropin. While the alpha-subunit of lutropin had nearly equal the methyl accepting activity of lutropin, the beta-subunit was devoid of accepting activity. The maximum amount of esterification occurred between 15 and 30 min at 37 degrees C) depending on the methyl acceptor molecule. The rate of the methyl esterification of adrenocorticotropin fragments was also studied. While alpha7-38-ACTH had less than half of alpha1-37-ACTH methyl accepting capacity, alpha1-17-ACTH did not serve as methyl acceptor. However, when a mixture of the two fragments was preincubated, the resulting mixture had full alpha1-39-ACTH activity.