Purification and Properties of the Enzyme*

An enzyme, “protein methylase I,” which methylates its endogenous protein has been purified from calf thymus. Acid hydrolysis of the methylated endogenous protein gives rise to two methylated amino acid derivatives. Both derivatives are very sensitive to alkali treatment, and one of the decomposition products is identified as methylurea. These findings, as well as other evidence, indicate that one of the compounds is guanidino-methylated arginine and the other is a-amino-, guanidino-methylated arginine. Various histones comprise approximately 70% of the total endogenous methylated protein, and the radioactivity is widely distributed among the various histones when the enzyme is labeled with S-adenosyl-L-methionine-methylJ4C. Protein methylase I has a pH optimum of 7.4 and a K, of 2.1 X 10e5 M for S-adenosyl-L-methionine.