Binding of polycyclic planar mutagens to chlorophyllin resulting in inhibition of the mutagenic activity.

Chlorophyllin is known to inhibit the mutagenicity of a variety of compounds. Using highly purified samples of chlorophyllin and its family compounds, we studied the mechanism of the inhibition. Since mutagens with polycyclic planar structures are particularly strongly inhibited, it seemed likely that the inhibition arises by trapping of the mutagens by chlorophyllin through complex formation at the planar surfaces of these molecules. To explore this possibility, we prepared a Sepharose bearing covalently linked chlorophyllin as ligand, and the adsorption of mutagens to this Sepharose was measured. Three different chlorophyllin derivatives were used, i.e., copper-chlorin, iron-chlorin and chlorin, to investigate the role of metal in the center of the chlorophyllin chromophore. Adsorption of 37 different compounds, mostly mutagens, in 0.02 M Tris-HCl buffer at pH 8.0 to these chlorophyllin-Sepharose preparations was studied in a quantitative manner. The results showed that most of the compounds having three or more fused rings were strongly adsorbed with apparent dissociation constants of 10(-5)-10(-6) M, whereas those having two fused rings or one ring were only poorly adsorbed. Since the three Sepharose adsorbents gave similar adsorption profiles, it appeared that the central metal in the chlorophyllin molecule does not play a crucial role in the adsorption. We also measured the inhibitory effect of copper-chlorin against the mutagenicity of some of these compounds using the Salmonella assay. The results showed that those mutagens that were strongly adsorbable to copper-chlorin-Sepharose were subject to efficient inhibition by copper-chlorin, whereas many of those only poorly adsorbed were inhibited only weakly. We concluded that trapping by complex formation plays a role in the antimutagenic actions of chlorophyllin against many mutagens, particularly notable being the actions against ICR-170, quinacrine, aflatoxin B1, Trp-P-1 and Trp-P-2. An unusual behavior of Trp-P-2 in the adsorption process, i.e., a very tight complex formation at an extremely low Trp-P-2 concentration, was found; the implication of this phenomenon in relation to the real environmental setting is discussed.