Phenylhydrazine was found to be a potent inducer of microsomal haem oxygenase activity in rat liver and kidney, but not in spleen. The phenylhydrazine-mediated increase in haem oxygenase activity was time-dependent. Maximum activity was attained 12h after treatment in the liver, and 24h after treatment in the kidney. The increases in the activity of haem oxygenase in the liver and the kidney could be inhibited by cycloheximide. Furthermore, the increases could not be elicited by the treatment of microsomal preparations in vitro with phenylhydrazine. In consonance with the increased haem oxygenase activity, a marked increase (16-fold) was observed in the serum total bilirubin concentration in phenylhydrazine-treated rats. The mechanism of haem degradation promoted by phenylhydrazine in vivo appears to differ from that in vitro; only in the former case is bilirubin formed as the end-product of haem degradation. When rats were given zinc-protoporphyrin (40 mumol/kg) 12h before and after phenylhydrazine treatment, the phenylhydrazine-mediated increases in haem oxygenase activity in the liver and the kidney were effectively blocked. Treatment of rats in vivo with the metalloporphyrin also inhibited the activity of splenic haem oxygenase, and promoted a major decrease in the serum bilirubin levels. In phenylhydrazine-treated animals, the microsomal content of cytochrome P-450 was significantly decreased in the absence of a decrease in the microsomal haem concentration. The decrease in cytochrome P-450 content was accompanied by an increased absorption in the 420nm region of the reduced CO-difference spectrum, suggesting the conversion of the cytochrome to an inactive form. The marked depletion of cellular glutathione levels suggests that this conversion may be related to the action of active intermediates and free radicals formed in the course of the interaction of phenylhydrazine with the haem moiety of cytochrome P-450.

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