Role of electronic factors in binding and reduction of azo dyes by hepatic microsomes

The factors which regulate the binding and reduction of azo dyes by rat liver microsomes have been investigated. Azo dyes having both electron-donating and -withdrawing substituents were reduced more readily both enzymically and chemically (dithionite) than those containing electron-donating substituents alone, which is consistent with less negative oxidation-reduction potentials of the former. A linear correlation between Vmax and Km was seen for substrates having only electron-donating substituents, suggesting a possible inverse relation between binding affinity and rate of reduction. This relationship was not apparent for substrates having both electron-donating and -withdrawing substituents. A lower Km was seen with substrates having a greater number of heteroatoms bearing nonbonding electrons in either electron-donating or -withdrawing groups. Furthermore, more basic dyes, which have a higher density of nonbonding electrons, showed an inverse correlation with both Vmax and Km. The requirement of nonbonding electrons was also observed with the binding of the fully reduced amine metabolites to microsomal cytochrome P-450. Type II binding spectra were observed for both aniline and 4-chloroaniline, but not with aniline derivatives bearing electron-withdrawing substituents such as methyl anthranilate and methyl-4-aminobenzoate. Electron-withdrawing substituents increase delocalization of nonbonding electrons on the amino residue; consequently, these are no longer available for binding to the enzyme. The binding constant of the reduced metabolite, aniline (36 microM), relative to the Km values of substrate azo dyes (range 0.31-1.73 microM) implies that the more weakly bound amine metabolites are readily released from the binding/catalytic site.(ABSTRACT TRUNCATED AT 250 WORDS)