Characterization and substrate specificity of a protein carboxymethylase in the pituitary gland

Protein carboxymethylase, an enzyme capable of methylating proteins and polypeptides, was purified from bovine pituitary. The anterior pituitary hormones, luteinizing hormone, follicle-stimulating hormone, adrenocorticotropic hormone, growth hormone, thyroid-stimulating hormone, and prolactin, were found to be substrates for this enzyme. The posterior pituitary hormones, oxytocin and vasopressin, did not serve as substrates. With luteinizing hormone as the substrate, protein carboxymethylase had a pH optimum near pH 5.5. A limiting K(m) of 1.47 muM for S-adenosyl-L-methionine was obtained with luteinizing hormone as the methyl acceptor. Possible roles of this enzyme in the posterior and anterior pituitary are discussed.