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Inhibition of sterol-14 alpha-demethylase, a cytochrome P450 (CYP51, Erg11p), is the mode of action of azole antifungal drugs, and with high frequencies of fungal infections new agents are required. New drugs that target fungal CYP51 should not inhibit human CYP51, although selective inhibitors of the human target are also of interest as anticholesterol agents. A strain of Saccharomyces cerevisiae that was humanized with respect to the amino acids encoded at the CYP51 (ERG11) yeast locus (BY4741:huCYP51) was produced. The strain was validated with respect to gene expression, protein localization, growth characteristics, and sterol content. The MIC was determined and compared to that for the wild-type parental strain (BY4741), using clotrimazole, econazole, fluconazole, itraconazole, ketoconazole, miconazole, and voriconazole. The humanized strain showed up to >1,000-fold-reduced susceptibility to the orally active azole drugs, while the topical agents showed no difference. Data from growth kinetic measurements substantiated this finding but also revealed reduced effectiveness against the humanized strain for the topical drugs. Cellular sterol profiles reflected the decreased susceptibility of BY4741:huCYP51 and showed a smaller depletion of ergosterol and accumulation of 14 alpha-methyl-ergosta-8, 24(28)-dien-3beta-6 alpha-diol than the parental strain under the same treatment conditions. This strain provides a useful tool for initial specificity testing for new drugs targeting CYP51 and clearly differentiates azole antifungals in a side-by-side comparison.

Sterol 14alpha-demethylase encoded by CYP51 is a member of the cytochrome P450 (CYP) superfamily of enzymes and has been shown to have an essential role in sterol biosynthesis in eukaryotes, with orthologues recently being described in some bacteria. Examination of the genome sequence data for the proteobacterium Methylococcus capsulatus, a bacterial species known to produce sterol, revealed the presence of a single CYP with strong homology to CYP51, particularly to a form in Mycobacterium tuberculosis. This M. capsulatus CYP51 protein represents a new class of CYP consisting of the CYP domain naturally fused to a ferredoxin domain at the C terminus via an alanine-rich linker. Expression of the M. capsulatus MCCYP51FX fusion in Escherichia coli yielded a P450, which, when purified to homogeneity, had the predicted molecular mass approximately 62 kDa on SDS/PAGE and bound lanosterol as a putative substrate. Sterol 14alpha-demethylase activity was shown (0.24 nmol of lanosterol metabolized per minute per nanomole of MCCYP51FX fusion) by gas chromatography/mass spectrometry with the activity dependent upon the presence of ferredoxin reductase and NADPH. Our unique findings describe a new class of naturally existing cytochrome P450, which will provide pivotal information for CYP structure/function in general.

The widely accepted catalytic cycle of cytochromes P450 (CYP) involves the electron transfer from NADPH cytochrome P450 reductase (CPR), with a potential for second electron donation from the microsomal cytochrome b5/NADH cytochrome b5 reductase system. The latter system only supported CYP reactions inefficiently. Using purified proteins including Candida albicans CYP51 and yeast NADPH cytochrome P450 reductase, cytochrome b5 and NADH cytochrome b5 reductase, we show here that fungal CYP51 mediated sterol 14alpha-demethylation can be wholly and efficiently supported by the cytochrome b5/NADH cytochrome b5 reductase electron transport system. This alternative catalytic cycle, where both the first and second electrons were donated via the NADH cytochrome b5 electron transport system, can account for the continued ergosterol production seen in yeast strains containing a disruption of the gene encoding CPR.

Sterol 14 alpha-demethylation is one of the key steps of sterol biosynthesis in eukaryotes and is catalyzed by cytochrome P450 sterol 14 alpha-demethylase (other names being CYP51 and P45014DM) encoded by ERG11. This enzyme activity is supported by an associated NAPDH-dependent reductase encoded by NCPR1 (NCP1), which is also associated with the endoplasmic reticulum. A diglycine linker recognition site (Gly-Gly-Ile-Glu-Gly-Arg-Gly-Gly) for the protease factor Xa, also containing a thrombin recognition site, was inserted just beyond the N-terminal hydrophobic segment of Candida albicans Erg11p. This modified enzyme was heterologously expressed at a level of 2.5 nmol of Erg11p/mg of protein as an integral endoplasmic reticulum protein. Following purification, treatment of the modified protein with factor Xa or thrombin resulted in sequence-specific cleavage and production of a soluble N-terminal truncated Erg11p which exhibited spectral characteristics identical to those of the purified full-length, wild-type form. Furthermore, reconstitution of the soluble enzyme with soluble yeast Ncpr1p, expressed and purified as an N-terminal deletion of 33 amino acids encompassing its membrane anchor, resulted in a fully functional and soluble eukaryotic Erg11p system. The complex was disrupted by high-salt concentration, reflecting the importance of electrostatic forces in the protein-protein interaction. The results demonstrate the membrane anchor serves to localize Erg11p to the ER where the substrate is located, but is not essential in either Ncpr1p or Erg11p activity. The possibility of cocrystallization of an active soluble eukaryotic 14 alpha-demethylase can be envisaged.

Sterol delta22-desaturase has been purified from a strain of Candida glabrata with a disruption in the gene encoding sterol 14alpha-demethylase (cytochrome P-45051; CYP51). The purified cytochrome P-450 exhibited sterol delta22-desaturase activity in a reconstituted system with NADPH-cytochrome P-450 reductase in dilaurylphosphatidylcholine, with the enzyme kinetic studies revealing a Km for ergosta-5,7-dienol of 12.5 microM and a Vmax of 0. 59 nmol of this substrate metabolized/min/nmol of P-450. This enzyme is encoded by CYP61 (ERG5) in Saccharomyces cerevisiae, and homologues have been shown in the Candida albicans and Schizosaccharomyces pombe genome projects. Ketoconazole, itraconazole, and fluconazole formed low-spin complexes with the ferric cytochrome and exhibited type II spectra, which are indicative of an interaction between the azole moiety and the cytochrome heme. The azole antifungal compounds inhibited reconstituted sterol delta22-desaturase activity by binding to the cytochrome with a one-to-one stoichiometry, with total inhibition of enzyme activity occurring when equimolar amounts of azole and cytochrome P-450 were added. These results reveal the potential for sterol delta22-desaturase to be an antifungal target and to contribute to the binding of drugs within the fungal cell.

Triadimenol and tebuconazole are potent inhibitors of the sterol 14 alpha-demethylation reaction in fungi which is catalysed by CYP51, a haem-thiolate containing enzyme belonging to the cytochrome P450 monooxygenase superfamily. Using CYP51 from the phytopathogen Ustilago maydis, a comparison of the sensitivity of the fungal enzyme to triadimenol and tebuconazole has been carried out. U. maydis CYP51 was purified to homogeneity as determined by SDS-PAGE and specific haem content. Catalytic activity was investigated following reconstitution with its respective NADPH cytochrome P450 reductase and proposed endogenous substrate, 24-methylenedihydrolanosterol. Addition of the triadimenol and tebuconazole induced type II spectral changes in the enzyme, with saturation occurring at equimolar azole concentrations. Inhibition of reconstituted activities showed a one-to-one sensitivity of the fungal CYP51 as judged by IC50 values. The implications for fungicide mode of action and treatment are discussed.

D0870 was 12 to 15 times more active than fluconazole in experiments to determine the MIC for growth arrest for two isolates of Candida albicans. A biochemical comparison of in vitro sterol biosynthesis in cell extracts showed only a twofold superiority of D0870 over fluconazole. A large differentiation (10-fold) in 50% saturating concentrations obtained by examining the binding of the azoles to microsomal P-450 was observed in a type II binding spectrophotometric assay, possibly reflecting the differential affinity for more than one P-450 enzyme. Additional mechanisms besides affinity for the target enzyme sterol 14 alpha-demethylase, such as differential intracellular accumulation of drug, may contribute to the differences in antifungal activity.