Bacterial mutagenesis from 2-aminofluorene mediated by washed rat liver microsomes was elevated 2- to 3-fold by addition of the hepatic soluble protein fraction. Enhancement was observed at 2-aminofluorene concentrations between 1 and 20 µg/assay but not at 30 to 50 µg/assay. The soluble protein fraction (without added microsomes) did not activate 2-aminofluorene for bacterial mutagenesis. However, mutagenesis by N-hydroxy-2-aminofluorene or 2-nitrosofluorene was enhanced by the soluble protein fraction, but only when reduced nicotinamide adenine dinucleotide phosphate was also present. On the basis of chemical assay, reduced nicotinamide adenine dinucleotide phosphate reduced 2-nitrosofluorene to N-hydroxy-2-aminofluorene and protected the hydroxylamine from oxidation, thus indicating that it was the mutagenicity of N-hydroxy-2-aminofluorene (but not 2-nitrosofluorene) which was enhanced by the soluble protein fraction. Without the added soluble protein fraction, mutagenesis by N-hydroxy-2-aminofluorene or 2-nitrosofluorene was unaffected by reduced nicotinamide adenine dinucleotide phosphate.

We succeeded in partially purifying a protein fraction with properties of the enhancement activity. The partially purified fraction, which represents a 14-fold increase in specific activity, was assigned a molecular weight of 33,500 by gel filtration through Sephadex G-100. This fraction was resolved into three components by polyacrylamide gel electrophoresis; the molecular weights of the three components were determined by sodium dodecyl sulfate-polyacrylamide gel (10%) electrophoresis to be 33,000, 27,000, and 16,250. The mechanism of mutagenesis enhancement remains unknown.

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This research was supported in part by the College of Agricultural and Life Sciences, University of Wisconsin-Madison (CALS Research Division Project 2293); the University of Wisconsin-Madison Graduate School; the Wisconsin Agricultural Experiment Station; the United States Department of Agriculture; and the University of Wisconsin-Madison Food Research Institute.

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