S-adenosyl-L-methionine : protein-arginine N-methyltransferase has been purified from human term placenta approximately 5, 700 fold with a 11% yield. The final preparation is completely free of any other protein specific methyltransferases.
Histones appear to be a good substrate in accepting methyl group from S-adenosyl-Lmethionine, whereas the relative substrate efficiency of myelin basic protein is less than 5% of that of histone IIA, indicating that the methylation of myelin basic protein is carried out by different enzyme entity.
The enzyme preparation shows two optimum pHs around 7.6 and 8.1, and is stable in the presence of 10% glycerol when stored at -10°C, only 12% activity being lost over 8 weeks. The enzyme is, however, easily inactivated by treating the enzyme preparation at 50°C for 7 min.
The enzyme does not require any divalent cations. The divalent cations Cu²+, Zn2+, Co2+, and Mn2+ are found to be inhibitory, Cu²+ being most potent since not only 90% of the enzyme activity is inactivated at 0. 5 mM but also the activity could not be reversed
by the addition of 1 mM EDTA. The enzyme activity is mainly located in the cytosol and nuclear fractions. Km value for S-adenosyl-L-methionine and K? value for S-adenosyl-L-homocysteine are 5 X 10-6 M and 1. 59 X 10-6 M respectively, and molecular weight of the enzyme deduced from Sephacryl-5-300 chromatography appears to be greater than 1. 5 X 106. The occurrence of the specific enzymes responsible for the methylation of histones, nonhistone chromosomal protein and HnRNP respectively, and the possible multienzyme complex to be formed by these specific enzymes are discussed.
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