Characterization of protein carboxyl methylase activities in rat testis: presence of testis specific charge isomers.

The physicochemical and molecular properties of testicular Protein Carboxyl Methylase (PCM) have been studied. The testicular enzyme was stable at high ionic strength, independent of metal ions and resistant to reducing or alkylating agents, and displayed a narrow pH optimum of pH 5.8-6.0. The enzyme had a molecular weight of approximately 25,000 daltons and a Km for S-adenosyl-L-methionine (SAM) of 1.3 microM. Varying concentrations of gelatine (exogenous methyl acceptor protein) did not change the affinity of the enzyme for SAM. The molecular properties of the enzyme were characterized using gel filtration, hydroxyapatite and DEAE-cellulose chromatography, and isoelectric focusing. Three charge isomers exhibiting PCM activity were isolated with pI values of 6.1, 6.7, and 7.35 while comparative data from the pituitary gland revealed primarily the presence of the most acid isozyme (pI 6.1). These results suggest that the different testicular charge isozymes may be involved in the selective methylation of specific testicular proteins.