Determination of 7-methylguanine by immunoassay.

Antisera to 7-methylguanine (7-meGua) were obtained using a novel analogue of 7-meGua which was bound covalently to methylated bovine serum albumin (mBSA) and was used as an antigen. Rabbit antisera at high dilution (1 in 10(5] recognized 7-meGua-ovalbumin, and the free base itself was detected at low levels (1 pmol/well) in a competitive enzyme-linked immunosorbent assay (ELISA). A number of purines have been screened for cross-reactivity, and only those of closely related structure (e.g., 7-methylxanthine) cross-reacted to any appreciable extent (50% inhibition at 250 pmol/well). The ELISA procedure was used to quantify 7-meGua in DNA following neutral thermal hydrolysis. Currently, the sensitivity of this assay is of the order of 10-100 mumol 7-meGua/mol Gua.