Sites of methylation of DNA bases by the action of organophosphorus insecticides in vitro.

Methylation in vitro of DNA by three methyl-14C-labelled organophosphorus insecticides has been studied. The ability of methylbromphenvinphos, methylparathion and malathion to methylate N-7 of guanine in DNA can be expressed as 100:40:15. Among the methylation products, no O6-methylguanine, a known mutagen, was found. Both in the reaction with dsDNA and with ssDNA 7-methyl-guanine was the main methylation product. However, all methyl derivatives of adenine (3-methyladenine, 1-methyladenine and 7-methyladenine) constituted about 40% and 50% of all methylation products in the case of dsDNA and ssDNA, respectively. The only methyl derivative of pyrimidine we have identified was 3-methylcytosine. In the case of dsDNA 3-methylcytosine appeared in small amounts but in the alkylated ssDNA 3-methylcytosine C constituted about 20% of all alkylation products.