Norharman microsomal metabolism

The confusion on the mechanism of the comutagenesis and mutagenesis of these pyrolysis products, especially pertaining to the enhancement and inhibition effects of Harman and Norharman,centers around the problem of the lack of certain fixed variables in the experimentation, particularly, the availability of the purified enzymes involved in the metabolic activation, which constitute the cytochrome P-450 mixed function oxidase system. We, therefore, undertake this problem to elucidate the mechanism of microsomal metabolism of these oyrolysis products with the purified mixed function oxidase(MFO) system. 

Microsomal Metabolism of Norharman Crude 3-MC microsomes metabolized norharman in vitro, as shown in Figure 4. The major metabolites of norharman were eluted at earlier retention times than the parent compound. They could be some hydroxylated products as they showed more polar potential in the reverse-phase chromatography. 

Figure 4. Microsomal metabolism of norharman. HPLC profile of ethyl acetate extract. A similar reaction was performed for control, in which microsomes were not added and there were no metabolites eluted. HPLC was performed with a C-18 radial compression module.

Fujino T, Matsuyama A, Nagao M, Sugimura T (1980) Inhibition by norharman of metabolism of benzo[a]pyrene by the microsomal mixed function oxidase of rat liver. Chem Biol Interact 32 1-12... 

Trp-P-2 and Norharman are the mutagen and comutagen shown to be metabolized by the reconstituted, purified rat-liver microsomal cytochrome P-448 system. 

We showed ear11er(18) that the presence of Harman or Norharman did not interfere the reversible noncovalent DNA binding for Trp-P-2, but inhibited irreversible covalent DNA binding in vitro upon the metabolic activation by the crude microsomal fraction isolated from 3-MC pretreated rat livers. 

The alternative explanation must involve the MFO system. The possible mechanism for the inhibitory effect or enhancement effect of Norharman upon covalent DNA binding and mutagenicity must be the results of the net balancing of substrate inhibition and membrane fluidization of the microsomal membrane or of the lipid vesicles in the reconstituted MFO system. A schematic pathway of the metabolism of these tryptophan pyrolysis products is postulated as shown in Figure 8. 

T. Fujino, H. Fujiki, M. Nagao, T. Yahagi, Y. Seino, and T. Sugimura, The effect of norharman on the metabolism of benzo(a)pyrene by rat-liver microsomes in vitro in relation to its enhancement of the mutagenicity of benzo(a)pyrene, Mutat. Res. 58, 151-158 (1978). 

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