Partial purification of cytochrome P-450 of Candida tropicalis and reconstitution of hydroxylase activity

Cytochrome P450 of fungi: primary target for azole antifungal agents

Cytochromes of fungi are essentially similar to those of animals. Cytochromes of fungi constitute two electron transport systems occurring in mitochondria and the endoplasmic reticulum. The former system, called the respiratory chain, contributes to cellular respiration and ATP generation, whereas the later system, named the microsomal electron transport system, is responsible for biosynthesis of several cellular components. The oxidative metabolism of lanosterol, that is included in the biosynthetic pathway of ergosterol, is one of the important functions of the microsomal electron transport system, which is catalyzed by P450(14DM). Many azole antifungal agents avidly combine with P450(14DM) and inhibit the oxidative removal of C-32 (the 14 alpha-demethylation) of lanosterol. This inhibition causes depletion of ergosterol and accumulation of 14-methylsterols in the membrane of fungal cells. Such change in sterol composition disturbs membrane function and results in growth inhibition and death of the fungal cells. Accordingly, P450(14DM) is considered as the primary target for azole antifungal agents. Cytochrome P450, which mediates the 14 alpha-demethylation of lanosterol, is also present in mammalian cells. Mammalian cells contain various species of cytochrome P450 which are responsible for many important cellular metabolic functions. If azole antifungal agents inhibit mammalian cytochrome P450 too, their systemic use may result in potentially significant adverse reactions. The high selectivity of azole antifungal agents for fungal P450(14DM) will be necessary for their systemic application. Binding ability of an azole antifungal agent to P450(14DM) is predominantly determined by the substituent at N-1 of the azole group, and the substituent must interact with the substrate site of the cytochrome. Extensive modification of the N-1 substituents and the screening of newly developed compounds with respect to the selectivity to fungal P450(14DM) with some conventional methods will be necessary. For this project, a biochemical understanding of cytochrome P450 and other cytochromes is important.

Isolation and characterization of cytochrome b5 from Candida tropicalis grown on alkane

Cytochrome b5 from Candida tropicalis grown on alkane has been solubulized in three different ways (sodium cholate, trypsin, osmotic wash). After solubilization of the microsomal membrane with sodium cholate, the purification of cytochrome b5 was achieved by DEAE-cellulose chromatography, hydroxylapatite chromatography, a second DEAE-cellulose chromatography and a Sephadex G-75 gel filtration. The purified protein had an apparent molecular weight of 16 000 +/- 1 000. After solubilization by trypsin treatment or osmotic wash, the purification procedure yielded a protein with an apparent molecular weight of 12 000 +/- 1 000. Though the purified proteins presented molecular weights depending on the technique of solubilization, they exhibited identical optical properties, a great stability with respect to temperature and pH, and were all autooxidable. Redox titrations revealed differences in their midpoint potential values, which were 35 +/- 5 mV for the b5 purified after cholate solubilization, -59 +/- 5 mV for the b5 purified after trypsin treatment and -65 +/- 5 mV for the b5 purified after osmotic wash.

Spectral characterization of cytochrome P-450 of a strain of Candida tropicalis grown on tetradecane

Several properties of the cytochrome P-450 induced in the yeast Candida tropicalis by growth on tetradecane have been studied by differential visible spectroscopy on microsomes. The spectral changes typical of this cytochrome have been obtained by subtraction of an unspecific spectral change, possibly due to the presence of other hemoproteins in microsomes, from the experimental difference spectra. Like the previously described cytochromes P-450 from yeast and mammalian liver, C. tropicalis cytochrome P-450 is in spin-state equilibrium at ambient temperature: about 30% of the originally low-spin cytochrome is converted to the high-spin state upon increasing the ionic strength of the medium, whereas 30% of the originally high-spin cytochrome is converted to the low-spin state upon addition of hydrophobic alcohols. C. tropicalis cytochrome P-450 readily binds nitrogenous ligands, isocyanides and phosphines in the ferric and ferrous state with spectral characteristics similar to those reported for other yeast or mammalian cytochromes P-450. It also reacts sucessively with cumylhydroperoxide and 1,3-benzodioxole to form a high-valent iron-oxo species and an iron-carbene metabolite complex. However it fails to produce any spectral or spin-state change upon addition of hydrophobic non-coordinating compounds such as n-tetradecane, its substrate in vivo.