The role of cytochrome P450(11 alpha) in detoxification of steroids in the filamentous fungus Rhizopus nigricans.

The evidence was presented that steroid hydroxylating enzyme complex induced by substrate in the filamentous fungus Rhizopus nigricans (R. nigricans) alleviated toxic effect(s) of the steroid on fungal growth. The growth inhibition of fungal mycelium observed in steroid-containing culture(s) became much more obvious when fungal mycelia were grown in the simultaneous presence of inducing steroid and the P450(11 alpha) inhibitor metyrapone. On the other hand, in experiments where we followed the fate of radioactively labelled progesterone added to the mycelial suspension, we noticed that steroid, after being initially accumulated in the microorganism, was, after some time, released from it; the latter phenomenon was not observed if induction of 11 alpha-hydroxylase was prevented by cycloheximide. Results of experiments presented in this communication can be regarded as the first strong indication that the biological role of P450(11 alpha) induction in R. nigricans is in removal of steroids which are toxic for the mycelium.