Altered cytochrome P-450 in a yeast mutant blocked in demethylating C-32 of lanosterol

Molecular aspects of azole antifungal action and resistance

During the past three decades azole compounds have been developed as medical and agricultural agents to combat fungal diseases. During the 1980s they were introduced as orally active compounds in medicine and the number of such azole drugs is likely to expand in the near future. They represent a successful strategy for antifungal development, but as the incidence of fungal infection has increased coupled to prolonged use of the drugs, the (almost) inevitable emergence of resistance has occurred. This was after resistance had already been encountered as a serious problem in the field, where a larger number of azole fungicides had been employed commercially. In this review the molecular basis of how azoles work is discussed together with how fungi overcome the inhibitory effect of these compounds: through alterations in the primary target molecule (cytochrome P45051; Erg11p; sterol 14alpha-demethylase); through drug efflux mechanisms and through a suppressor mechanism allowing growth on 14-methylated sterols. Copyright 1999 Harcourt Publishers Ltd.

The G464S amino acid substitution in Candida albicans sterol 14alpha-demethylase causes fluconazole resistance in the clinic through reduced affinity

Fluconazole selectively inhibits fungal sterol 14alpha-demethylase, a cytochrome P450 enzyme found in plants, animals, fungi, and Mycobacteria. The mutation G464S, observed in the heme-binding domain of sterol 14alpha-demethylase in clinical strains of fluconazole-resistant Candida albicans, is shown here to cause resistance through substantially reducing the inhibitory effect of fluconazole and is associated with perturbation of the heme environment as indicated by spectral data. The protein exhibits 42% of the maximal enzymatic rate of the wild-type protein allowing continued production of the end product of fungal sterol biosynthesis, ergosterol, in resistant strains. This mutation may cause these phenotypes through altering the heme location, thus changing the ability of residues above the heme to bind the drug effectively. This perturbation would also account for the observation of reduced sterol demethylase catalytic activity by changing the location of the 14alpha-methyl group in relation to oxygen-bound heme during the catalytic cycle.

Resistant P45051A1 activity in azole antifungal tolerant Cryptococcus neoformans from AIDS patients

Azole antifungal compounds are important in the treatment of Cryptococcosis, a major cause of mortality in AIDS patients. The target of the azole drugs is P450 mediated sterol 14 alpha-demethylase. We have investigated the P450 system of Cryptococcus neoformans with respect to azole tolerance observed in clinical isolates which were obtained following the failure of fluconazole therapy. The clinical failure was correlated with in vitro tolerance of azole antifungal when compared to wild-type strains. The microsomal P450 system was typical of yeast and fungi and fluconazole tolerance was not associated with defective sterol biosynthesis. The strains had slightly elevated P450 content and slightly reduced azole levels in the cells, but a clear cause for resistance was the increased level of drug needed to inhibit the sterol 14 alpha-demethylase in vitro.

Biochemical approaches to selective antifungal activity. Focus on azole antifungals

Azole antifungals (e.g. the imidazoles: miconazole, clotrimazole, bifonazole, imazalil, ketoconazole, and the triazoles: diniconazole, triadimenol, propiconazole, fluconazole and itraconazole) inhibit in fungal cells the 14 alpha-demethylation of lanosterol or 24-methylenedihydrolanosterol. The consequent inhibition of ergosterol synthesis originates from binding of the unsubstituted nitrogen (N-3 or N-4) of their imidazole or triazole moiety to the heme iron and from binding of their N-1 substituent to the apoprotein of a cytochrome P-450 (P-450(14)DM) of the endoplasmic reticulum. Great differences in both potency and selectivity are found between the different azole antifungals. For example, after 16h of growth of Candida albicans in medium supplemented with [14C]-acetate and increasing concentrations of itraconazole, 100% inhibition of ergosterol synthesis is achieved at 3 x 10(-8) M. Complete inhibition of this synthesis by fluconazole is obtained at 10(-5) M only. The agrochemical imidazole derivative, imazalil, shows high selectivity, it has almost 80 and 98 times more affinity for the Candida P-450(s) than for those of the piglet testes microsomes and bovine adrenal mitochondria, respectively. However, the topically active imidazole antifungal, bifonazole, has the highest affinity for P-450(s) of the testicular microsomes. The triazole antifungal itraconazole inhibits at 10(-5) M the P-450-dependent aromatase by 17.9, whereas 50% inhibition of this enzyme is obtained at about 7.5 x 10(-6)M of the bistriazole derivative fluconazole. The overall results show that both the affinity for the fungal P-450(14)DM and the selectivity are determined by the nitrogen heterocycle and the hydrophobic N-1 substituent of the azole antifungals. The latter has certainly a greater impact. The presence of a triazole and a long hypdrophobic nonligating portion form the basis for itraconazole's potency and selectivity.

Genetic and physiological analysis of azole sensitivity in Saccharomyces cerevisiae

Ketoconazole and fluconazole are azole antifungal agents which inhibit cytochrome P-450 mediated sterol C14 demethylation during ergosterol biosynthesis. We report on the activity of these antifungals on a variety of Saccharomyces cerevisiae strains grown under differing conditions known to affect cyt P-450 levels. Only slight increases in resistance to azoles were observed under conditions which induce the yeast cyt P-450 from undetectable levels. Strain variation was observed, with some strains exhibiting a fungicidal, and others a fungistatic response. Two cyt P-450 deficient mutants examined exhibited resistance to treatment with fluconazole and ketoconazole. This was attributed, at least in part, to an additional defect in sterol delta 5,6 desaturation and possibly to reduced cellular levels of azole drug.

A single amino acid substitution converts cytochrome P450(14DM) to an inactive form, cytochrome P450SG1: complete primary structures deduced from cloned DNAS

Genes for lanosterol 14-demethylase, cytochrome P450(14DM), and a mutated inactive cytochrome P450SG1 were cloned from S. cerevisiae strains D587 and SG1, respectively. A single nucleotide change resulting in substitution of Asp for Gly-310 of cytochrome P450(14DM) was found to have occurred in cytochrome P450SG1. In this protein the 6th ligand to heme iron is a histidine residue instead of a water molecule, which may be the ligand for the active cytochrome P450(14DM). Molecular models of the active sites of the cytochrome P450(14DM) and cytochrome P450SG1 were built by computer modeling on the basis of the known structure of that of cytochrome P450CAM whose crystallographic data are available. The mechanisms which may cause a histidine residue to gain access to the heme iron are discussed.

Isolation and analysis of ketoconazole resistant mutants of Saccharomyces cerevisiae

Nine mutants of Saccharomyces cerevisiae which are resistant to ketoconazole, have been isolated and characterized. In each case the mutation is nuclear in origin and allelic to a previously described mutation, erg3, which gives rise to a block in the delta 5-6 desaturation step of ergosterol biosynthesis. The significance of this second site mutation to the point of inhibitory action of ketoconazole, that is the P-450-mediated C-14 demethylation of lanosterol, is discussed.

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.

Characterization of a cytochrome P450 deficient mutant of Candida albicans

A previously described Candida albicans nystatin resistant mutant blocked in 14 alpha-demethylation of lanosterol was shown to also lack all traces of cytochrome P450 as determined by carbon monoxide difference spectra. This strain does not require ergosterol for growth and reverted to an ergosterol producing, cytochrome P450 containing strain indicating no other lesions. Cytochrome P450 mutants described in Saccharomyces cerevisiae are auxotrophic for ergosterol or contain a second mutation in 5,6 desaturation of the sterol B ring. These results suggest that a cytochrome P450 lesion in these yeasts have different phenotypes and may reflect different sterol requirements for the two organisms.

Isolation of a cytochrome P-450 structural gene from Saccharomyces cerevisiae

We have transformed a Saccharomyces cerevisiae host with an S. cerevisiae genomic library contained in the shuttle vector YEp24 and screened the resultant transformants for resistance to ketoconazole (Kc), an inhibitor of the cytochrome P-450 (P-450) enzyme lanosterol 14 alpha-demethylase. Two plasmids were isolated which transformed yeast to both increased resistance to Kc and increased levels of total P-450. Hybrid-selection and immunoprecipitation experiments showed that these plasmids, pVK1 and pVK2, contained the structural gene for an S. cerevisiae P-450. This conclusion was confirmed by the nucleotide sequence of a portion of pVK2, which revealed an open reading frame encoding a characteristic P-450 heme-binding region.

Spectral properties of a novel cytochrome P-450 of a Saccharomyces cerevisiae mutant SG1. A cytochrome P-450 species having a nitrogenous ligand trans to thiolate

An altered cytochrome P-450 (SG1 P-450) was partially purified from Saccharomyces cerevisiae mutant SG1 which is defective in lanosterol 14 alpha-demethylation. Oxidized SG1 P-450 showed a Soret peak at 422 nm and the alpha peak was lower than the beta peak. This spectrum was considerably different from those of known low-spin P-450s, indicating a unique ligand structure of SG1 P-450. The absorption spectrum of ferric SG1 P-450 was superimposable on that of the imidazole complex of ferric P-450, suggesting the presence of a nitrogenous ligand such as histidine of the apoprotein at the 6th coordination position. SG1 P-450 was immunochemically indistinguishable from cytochrome P-450 of S. cerevisiae catalyzing lanosterol 14 alpha-demethylation (P-45014DM) but had no lanosterol 14 alpha-demethylase activity.

Biochemical targets for antifungal azole derivatives: hypothesis on the mode of action

The selective interaction of low concentrations of azole derivatives and other nitrogen heterocycles with cytochrome P-450 may be at the origin of the inhibition of ergosterol biosynthesis. From the depletion of ergosterol and the concomitant accumulation of 14 alpha-methylsterols, alterations in membrane functions, the synthesis and activity of membrane-bound enzymes, mitochondrial activities, and an uncoordinated activation of chitin synthase may result. Since chitin synthesis is more important in the hyphal form than in the budding form of C. albicans, the uncoordinated activation of chitin synthesis may be more trouble for the hyphal growth than for yeast budding. The assumption is made that from this difference the greater sensitivity of hyphal growth to azole antifungal agents may originate. It is also assumed that the higher degree of lipid unsaturation may be related to an inhibition of ergosterol biosynthesis. The inhibition of fatty acid desaturation and elongation induced by higher doses of miconazole and ketoconazole and the longer contact times might be related to interference with membrane fluidity, or it might due to chelation of the iron used in the oxidation reduction sequence during desaturation. The decreased availability of ergosterol and the accumulation of 14 alpha-methylsterols also may provide the environment needed to inactivate membrane-bound enzymes; e.g., cytochrome c peroxidase. However, it is still too speculative to correlate effects on membrane components with miconazole-induced changes in properties of all oxidases; e.g., the NADH-dependent, cyanide-insensitive oxidase. The accumulation of toxic concentrations of hydrogen peroxide, resulting from an increased NADH-oxidase activity and disappearance of the peroxidase and catalase activity, may contribute to the degeneration of subcellular structures. The complete disappearance of catalase observed at concentrations of miconazole greater than or equal to 10(-5) M may originate from direct effects on the cell. At these high concentrations reached only by topical application, direct membrane damage resulting from interaction of miconazole with lipids was observed. These direct interactions result in an inhibition of membrane-bound enzyme and mitochondrial activities and in leakage of intracellular components. The direct interactions were much less pronounced in cells treated with ketoconazole. This correlates with the smaller area occupied in the membrane per ketoconazole molecule (30 A2), compared with that occupied in the membrane per miconazole molecule (90 A2).(ABSTRACT TRUNCATED AT 400 WORDS)

Differences in the cytochrome P-450 enzymes of sterol C-14 demethylase mutants of Saccharomyces cerevisiae

A number of nystatin-resistant strains of S. cerevisiae have been isolated which are defective in lanosterol C-14 demethylation, a reaction normally catalysed by cytochrome P-450. In this paper two of these strains have been compared and found to have differences in their reduced-CO difference spectra indicating different distortions in the enzyme molecule. Nystatin resistance in the C-14 demethylation deficient SG1 in shown to be determined by a single gene, and a sterol 5,6-desaturase defect does not appear to be required for viability of SG1, was reported for the C-14 demethylase deficient isolate JR4 by Taylor et al. (1983). There are at least two discernable mutant phenotypes for the yeast cytochrome P-450 structural gene which give a C-14 demethylase defect.