FORMATION AND PERSISTENCE OF N-7-METHYL-GUANINE AND O-6-METHYL-GUANINE IN DNA OF CHICK-EMBRYO BRAIN-CELLS IN OVO FOLLOWING ADMINISTRATION OF N-NITROSO-N-METHYLUREA

Previously, O-6-methyl-guanine-DNA alkyltransferase (AT) of the chicke n embryo has been investigated in vitro. In the present studies, the e ffect of in vivo (in ovo) treatment with methylnitrosourea (MNU) was e xamined at a developmental stage of 15 days and doses of 1.25-20 mg/eg g, yielding about 1-16 mmol MNU/kg embryo weight. At intervals of 1-24 h, DNA of the brain was prepared. N-7-methylguanine and O-6-methylgua nine were quantified by combining a rapid method of DNA isolation, hig h-pressure-liquid-chromatography (HPLC) and electrochemical detection of the guanine-alkyl adducts. In parallel, the AT activity of brain ho mogenates was determined. Within the range of the detection limits (N- 7-methylguanine: 16 nM, O-6-methylguanine: 2.5 nM), no repair of the g uanine adducts, being about 500 nmol O-6-methyl- and 1800 nmol N-7-met hyl-adducts per mmol guanine 1 h following administration of 10 mg MNU /egg, was evident. The rather low acute toxicity of MNU in the chicken embryo at the 15th day of development DL(50/24) h being > 4 mg MNU/em bryo, argues, therefore, for an additional repair mechanism, e.g. cell replacement repair.