Comparison of two monooxygenase systems with cytochrome P450 in filamentous fungus Rhizopus nigricans

Besides the progesterone inducible steroid hydroxylase which catalyses the transformation of progesterone to 11alpha-hydroxyprogesterone the presence of another monooxygenase system in filamentous fungus Rhizopus nigricans (R. nigricans)--lanosterol demethylase was confirmed. Lanosterol demethylase was not inducible with progesterone. After subcellular fractionation both monooxygenase systems were found in microsomal fraction of fungus R. nigricans. To find out how to differentiate between the hydroxylase and demethylase systems the influence of inhibitors ketoconazole and metyrapone on both monooxygenase systems was studied. Both substances efficiently inhibited fungal steroid hydroxylase. Spectral studies used to characterize the interaction of inhibitors with cytochromes P450 showed that both inhibitors bind to induced fungal preparations whereas in noninduced fungal preparations interaction with P450 was found only with ketoconazole. This indicated the difference in interaction of the two inhibitors on both monooxygenase systems present in fungus R. nigricans.

Purification of cytochrome P450 from filamentous fungus Rhyzopus nigricans

It has been reported in our previous studies that steroid hydroxylation system of Rhizopus nigricans involves cytochrome P450 and NADPH-cytochrome P450 reductase in the electron transport system. Both enzymes are membrane bound and are located in the microsomal preparations of progesterone induced fungal mycelia. In order to identify and characterize the cytochrome P450 component of fungal monoxygenase system, microsomal proteins from induced mycelia were subjected to HIGH Q anion exchange and MONO P (FPLC) anion exchange chromatography. Four fractions containing cytochrome P450 have been resolved on MONO P column. They exhibit CO difference spectra and type II difference spectra with ketoconazole.

Purification and characterization of NADPH-cytochrome P450 reductase from filamentous fungus Rhizopus nigricans

We report here the isolation and partial characterization of a flavoprotein, NADPH-cytochrome P450 (cytochrome c) reductase. The enzyme is a part of steroid 11 alpha-hydroxylating system and is associated with the microsomal fraction of the fungus Rhizopus nigricans. Fungal reductase was solubilized from microsomal membranes with Triton X-100 and purified to apparent homogeneity by affinity and high-performance ion-exchange chromatography. A 350-fold purification of the enzyme with specific activity of 37 mumol cytochrome c reduced/min/mg protein was achieved. A single protein band was obtained on SDS-PAGE analysis with an apparent molecular weight of 79 kDa. Purified reductase contained approximately equimolar quantities of flavin adenine dinucleotide and flavin mononucleotide per mole of the enzyme. Upon induction of the steroid hydroxylating system with progesterone the activity of microsomal NADPH-cytochrome c (P450) reductase increased 10-fold. This is in good correlation with the increase in content of fungal cytochrome P450. Purified fungal flavoprotein was active in a reconstituted system with cytochrome P450 C21 from adrenal gland but could not replace adrenodoxin reductase in the mitochondrial steroid 11 beta-hydroxylating system. We were able to confirm the role of the enzyme by reconstituting steroid 11 alpha-hydroxylating activity from the separated components NADPH-cytochrome P450 reductase and cytochrome P450, partly purified from fungal microsomes.

Effect of polymyxin B on hemoglobin-mediated hepatotoxicity

In previous studies, enhanced toxicity of hemoglobin when combined with endotoxin has suggested a specific binding site. This binding may take place even when endotoxin-free hemoglobin is administered, due to some low levels of endotoxin usually present in the portal vein. To test this hypothesis, we administered endotoxin-free, stroma-free human hemoglobin (SFH) to rabbits in doses shown previously to be hepatotoxic or lethal. A second group of rabbits received the anti-endotoxin antibiotic polymyxin B (PB) to inactivate endotoxin, followed by a dose of SFH. A control group received intravenous PB alone. There was no differences in the incidence of hepatotoxicity among these three groups. To show the binding interactions of endotoxin with PB and endotoxin with hemoglobin, an experiment using toluidine blue was performed. According to this study, specific binding of hemoglobin with endotoxin did not occur. Our results do not agree with previous suggestions that hemoglobin-mediated hepatotoxicity is due to the interaction in vivo of hemoglobin with low levels of endogenous endotoxin.