Implication of rifampicin-quinone in the irreversible binding of rifampicin to macromolecules.

1. When [3H]rifampicin is incubated with rat liver microsomes or rat liver homogenate, minor amounts are bound irreversibly to protein. This effect does not depend on the presence of NAD, NADH, NADP or NADPH. 2. Rifampicin is autoxidized at physiological pH. The product of autoxidation, rifampicin-quinone, if incubated with albumin, shows a much greater irreversible binding to the protein than the parent compound rifampicin. Hence it is concluded that rifampicin may bind irreversibly to proteins in a non-enzymic reaction after autoxidation to rifampicin-quinone. 3. Rifampicin-quinone also binds irreversibly to RNA and poly-L-lysine, if incubated with these compounds. This suggests that free amino groups of protein or RNA are involved in the binding. 4. 48 h after dosage of [3H]rifampicin (33 mg/kg) to rats, 29-2 +/- 4-1 (S.D.) pmol are bound irreversibly to 1 mg liver RNA, 15.8 +/- 8-1 pmol to 1 mg liver protein and 5-0 +/- 0-47 pmol to 1 mg protein in brain tissue. 5. Microsomal NADPH-cytochromcin-quinone to rifampicin. The KM of this reaction is 10(-4) M. Induction of the NADPH-cytochrome c reductase by pre-treatment of rats with 20 mg/kg rifampicin over 5 days results in a corresponding increase of increase of rifampicin-quinone reduction. 6. These results suggest that microsomal NADPH-cytochrome c reductase prevents accumulation of higher amounts of possibly toxic rifampicin-quinone by reduction to rifampicin.