METABOLIC-ACTIVATION OF MUTAGENIC TRYPTOPHAN PYROLYSIS PRODUCTS BY RAT-LIVER MICROSOMES

  • 1 January 1980
    • journal article
    • research article
    • Vol. 40  (7) , 2596-2600
Abstract
To determine whether cytochrome P-450 metabolizes [the carcinogenic] tryptophan pyrolysis products, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) to active forms, studies were done using microsomes and the 9000 .times. g supernatant (S-9) from polychlorinated biphenyl mixture-treated rat livers together with Salmonella typhimurium TA 98 as a tester strain. The number of revertants increased with the amount of S-9 fraction added. The highest mutation was seen with an amount of S-9 to 2.5 mg of liver tissue; further increase in the S-9 fraction resulted in a sharp decline in the number of revertants. There was a similar biphasic response when a relatively large amount of S-9 fraction was incubated for an increasing length of time. Microsomes showed parallel biphasic responses, and the 105,000 .times. g supernatant fraction was ineffective in increasing the number of revertants. Most if not all of the ability of the S-9 fraction to convert Trp-P-1 and Trp-P-2 to active forms resided in the microsomes. The involvement of microsomal cytochrome P-450 in this process was further confirmed by the following evidence. The treatment of rats with polychlorinated biphenyl mixture and 3-methylcholanthrene resulted in a marked increase in the ability of microsomes to activate Trp-P-1 and Trp-P-2, and the activation reaction required reduced NADP as a cofactor and was inhibited by carbon monoxide, 7,8-benzoflavone, n-octylamine and 2-diethylaminoethyl-2,2-diphenylvalerate. The mutagenic metabolite formed by microsomal metabolism of Trp-P-2 was fairly stable and survived heating at 90.degree. C for 7 min. With high-performance liquid chromatography, an active metabolite of Trp-P-2 was purified. Preliminary analysis showed this molecule to be an N-hydroxylated derivative of Trp-P-2.

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