Loss of activity of an N1-methyl-4-pyridone-5-carboxamide-forming N1-methylnicotinamide oxidase in livers of rats fed 2-acetylaminofluorene.

Activity of N 1-methylnicotinamide (1-CH3Nmd) oxidase (aldehyde oxidase; aldehyde:oxygen oxidoreductase, EC 1.2.3.1) in the liver of a 39-week-old male Wistar rat, was found separately in three fractions on diethylaminoethyl cellulose column chromatography. Fraction I produced only 1-methyl-2-pyridone-5-carboxamide (2-PY) and was identified as xanthine oxidase (EC 1.2.3.2). Fraction II formed 2-PY preferentially to 1-methyl-4-pyridone-5-carboxamide (4-PY), whereas Fraction III produced only 4-PY. Therefore, it is proposed here tentatively to label these two fractions 1-CH3Nmd oxidase I and 1-CH3Nmd oxidase II, respectively. Although 4-PY- as well as 2-PY-forming activity of 1-CH3Nmd oxidase I increased as the pH value was raised, the maximal activity of 1-CH3Nmd oxidase II was observed at pH 8.5. Km's for 1-CH3Nmd of 2-PY and 4-PY formation of 1-CH3Nmd oxidase I were in the same range (5.88 and 5.43 × 10−4 m, respectively) and were quite different from that of 4-PY formation of 1-CH3Nmd oxidase II (1.02 × 10−4 m). In the liver of the rat that had begun at 6 weeks of age to receive in the diet 0.05% 2-acetylaminofluorene, a potent hepatocarcinogen, for 24 successive weeks followed by a normal diet for 9 weeks, 1-CH3Nmd oxidase activities were observed only in Fractions I and II but not in Fraction III. These two fractions coincided in character with those in Fractions I and II of the control liver, respectively. In Fraction II Km's for 1-CH3Nmd of 2-PY and 4-PY formation (4.13 and 4.13 × 10−4 m, respectively) were in the same range as was the control liver.