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 methyltransferase.
Histories appear to be a good substrate in accepting methyl group from S-adenosyl-L-methionine, 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 10,% 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 ions Cu2+, Zn 2+, and Mn 2+ 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 mMEDTA.
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 5x10-6 M and 1. 59 x 10-6M respectively, and molecular weight of the enzyme deduced from Sephacryl-S300 chromatography appears to be greater than 1. 5 X 106.
The exogenous protein methylase I activity does not correlate with placental age, whereas a gradual increase in the endogenous protein methylase I activity occurs with advancing gestation up to term.
From the above results it is discussed that the occurrence of the specific enzymes responsible
for the methylation of histones, nonhistone chromosomal proteins and HnRNP respectively the possible multienzyme complex formed by these enzymes and the possibility that differ protein methylase I may be involved in cell proliferation and cell differentiation respectively.
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