Mechanisms of chlorophyllin anticarcinogenesis against aflatoxin B1: complex formation with the carcinogen

Chlorophyllin (CHL), a food-grade derivative of the green plant pigment chlorophyll, has recently been shown in this laboratory to be a potent inhibitor in vivo of hepatic aflatoxin B1 (AFB1)-DNA adduction and hepatocarcinogenesis (Breinholt et al. (1995) Cancer Res. 55, 57-62). We report here that CHL forms a strong noncovalent complex with AFB1 in vitro (dissociation constant (Kd) by Scatchard analysis = 1.4 (+/- 0.4) microM based on copper chlorin content), which may contribute to its anticarcinogenic activity. Kd values for the related porphyrins chlorine e6, protoporphyrin IX, and zinc protoporphyrin IX were also of the same order of magnitude as that of the commercial CHL. Mole ratio analysis provided evidence that all porphyrins examined associate with AFB1 at an approximate one to one stoichiometric ratio. Energy minimization computer modeling of the complex indicates a favorable association energy of -20 kcal/mol, independent of oxidation state of the 8,9-double bond of AFB1. AFB1 incubated in vitro with liver microsomes in the presence of added CHL showed comparable levels of inhibition in the production of several phase 1 metabolites, including the postulated procarcinogenic metabolite AFB1 8,9-epoxide. Kinetic analysis of microsome-catalyzed AFB1-DNA adduction suggested a CHL inhibition constant near 10 microM chlorin. In vivo, addition of CHL to concentrated AFB1 solutions followed by gavage administration resulted in dose-dependent inhibition of hepatic AFB1-DNA adduction, whereas the same dosages of AFB1 and CHL incorporated into a single bolus of trout diet for gavage provided less protection at all CHL doses.(ABSTRACT TRUNCATED AT 250 WORDS)