MECHANISMS OF THE IN-VITRO ANTIMUTAGENIC ACTION OF CHLOROPHYLLIN AGAINST BENZO[A]PYRENE - STUDIES OF ENZYME-INHIBITION, MOLECULAR-COMPLEX FORMATION AND DEGRADATION OF THE ULTIMATE CARCINOGEN

Mechanisms of the antimutagenic action of chlorophyllin (CHL) towards benzo[a]pyrene (BP) were studied in vitro. In the Salmonella assay, CH L inhibited the mutagenic activity of BP in the presence of an S9 acti vation system and was particularly effective against the direct-acting ultimate carcinogen, benzo[ a]pyrene-7,8-dihydrodiol-9,10-epoxide (BP DE). Spectral studies indicated that the time-dependent hydrolysis of BPDE to tetrols was augmented in the presence of CHL concentrations on the order of 5 mu M. Dose-related inhibition of several cytochrome P4 50-dependent enzyme activities was observed upon addition of CHL to in vitro incubations. Spectral changes for the interaction between CHL a nd cytochrome P450 indicated that CHL does not bind to the active site of the enzyme, but exerts its inhibitory effect indirectly. This was achieved by inhibiting NADPH-cytochrome P450 reductase (K-i similar to 120 mu M With cytochrome c as substrate), and did not involve lowerin g of the effective substrate concentration by complex formation with t he procarcinogen. It is concluded that the in vitro antimutagenic acti vity of CHL towards BP involves accelerated degradation of the ultimat e carcinogen, with inhibition of carcinogen activation occurring only at high CHL concentrations. The latter mechanism is unlikely to occur in vivo following p.o. administration due to the limited uptake of CHL from the gut, but tissue concentrations may be sufficiently high to c ause degradation of BPDE.