Bhattacharyya D, Foote R S, Boulden A M, Mitra S
University of Tennessee-Oak Ridge Graduate School of Biomedical Sciences.
Eur J Biochem. 1990 Oct 24;193(2):337-43. doi: 10.1111/j.1432-1033.1990.tb19343.x.
O6-Methylguanine-DNA methyltransferase, present in most organisms, removes mutagenic and carcinogenic O6-alkylguanine from DNA by accepting the alkyl group in a stoichiometric reaction. The protein has been partially purified from human placenta. It reacts with second-order rate constants of 2.20 x 10(8) and 0.067 x 10(8) lmol-1 min-1 at 37 degrees C for duplex and single-stranded DNA substrates, respectively. The corresponding value for the alkylated base in synthetic poly(dC, dG, m6dG) is 0.02 x 10(8) l mol-1 min-1. The native protein is monomeric with a molecular mass of 22-24 kDa. Methylation of the protein does not lead to a gross change in its conformation but causes a slight reduction in its isoelectric point of 6.2. Although DNA protects the protein from heat inactivation, both duplex and single-stranded DNAs inhibit its activity in a concentration-dependent manner. The transferase reaction rate is also strongly inhibited by salt with about 20% of the maximum rate observed in physiological ionic strength. This inhibition is nonspecific with respect to the ions of univalent salts.
O6-甲基鸟嘌呤-DNA甲基转移酶存在于大多数生物体中，通过在化学计量反应中接受烷基，从DNA中去除诱变和致癌的O6-烷基鸟嘌呤。该蛋白已从人胎盘中部分纯化。在37℃下，对于双链和单链DNA底物，其反应的二级速率常数分别为2.20×10⁸和0.067×10⁸ l·mol⁻¹·min⁻¹。合成的聚（dC，dG，m6dG）中烷基化碱基的相应值为0.02×10⁸ l·mol⁻¹·min⁻¹。天然蛋白为单体，分子量为22 - 24 kDa。蛋白的甲基化不会导致其构象发生重大变化，但会使其等电点从6.2略有降低。虽然DNA可保护该蛋白免受热失活，但双链和单链DNA均以浓度依赖的方式抑制其活性。转移酶反应速率也受到盐的强烈抑制，在生理离子强度下观察到的最大速率约为20%。这种抑制作用对于单价盐的离子是非特异性的。
