Inhibition of enzymatic reduction of delta 14-double bond of 5 alpha-cholesta-8,14-dien-3 beta-ol and 5 alpha-cholesta-7,14-dien-3 beta-ol by AY-9944

Cholesterol biosynthesis from lanosterol: regulation and purification of rat hepatic sterol 14-reductase

We have previously characterized the membrane-bound sterol 14-reductase (14-reductase) that catalyzes anaerobically NADPH-dependent reduction of the 14-double bond of delta 8,14-diene or delta 7,14-diene sterols that are sterol intermediates in cholesterol biosynthesis in mammals (Paik et al. (1984) J. Biol. Chem. 259, 13413-13423). To elucidate the regulatory mechanism as well as molecular characteristics of the 14-reductase, we extended our investigation on the consequences of alteration of the enzymic activity under various physiological conditions. The enzymic activity of rat hepatic sterol 14-reductase was induced more than 11-fold by feeding 5% cholestyramine plus 0.1% lovastatin (the CL-diet) for 7 days but was severely suppressed by feeding 5% cholesterol or 0.01% AY-9944 (an inhibitor of 14-reductase) for the same period. The increase or decrease in the 14-reductase activity also parallels the same change in the cholesterol synthetic rate in hepatocytes from rats that had been fed either the CL-diet or 0.01% AY-9944. In vitro inhibition studies revealed that AY-9944 acts as a competitive inhibitor of the 14-reductase (Ki = 0.26 microM). A diurnal variation was observed for the 14-reductase with peak activity near the middle of the dark cycle (10 p.m.), which was abolished by administration of cycloheximide. With induced enzyme conditions 14-reductase has been further purified with chromatographic procedures to near homogeneity. Purified 14-reductase appears to be a M(r) = 70,000 protein that is composed of two equally-sized subunits having a M(r) = 38,000. All properties of the purified 14-reductase suggest that the solubilized enzyme is the principal 14-reductase of microsomes. Taken together, our results provide the first evidence in support of a previously unknown regulatory role for the 14-reductase in the overall cholesterol synthetic pathway.

Microsomal delta 8,14-sterol delta 14-reductase in higher plants. Characterization and inhibition by analogues of a presumptive carbocationic intermediate of the reduction reaction

An enzymatic assay for delta 8,14-sterol delta 14-reductase, an enzyme involved in sterol biosynthesis, has been developed for the first time in higher plants. The properties of the microsomal enzyme have been established with respect to cofactor requirements, kinetics and substrate specificity. This enzymatic double-bound reduction is thought to proceed through an electrophilic addition mechanism, involving a C14 putative carbonium ion high-energy intermediate. Using this in vitro assay, ammonium and iminium analogues of this cationic intermediate were shown to be potent inhibitors of the reduction reaction. Thus, compounds of the N-alkyl-8-aza-4 alpha,10-dimethyl-trans-decal-3 beta-ol series strongly inhibited sterol reductase (I50 = 0.07 - 4 microM) (I50/Km = 10(-4) - 10(-3), as did the antimycotic agent 15-azasterol (I50 = 0.03 microM); all of these compounds act as reaction-intermediate analogues of the proposed C14 carbonium ion intermediate. Moreover, the in vitro inhibition of the plant sterol reductase by a series of ammonium-ion-containing fungicides was demonstrated. The relative specificity of these different series of inhibitors toward cycloeucalenol-obtusifoliol isomerase, delta 8----delta 7-sterol isomerase and delta 8,14-sterol delta 14-reductase, was directly studied.

Evidence for the contribution of a sterol 14-reductase to the 14 alpha-demethylation of lanosterol by yeast

Lanosterol was converted to a 14-demethylated metabolite, 4,4-dimethylzymosterol by Saccharomyces cerevisiae microsomes. This metabolism was mediated by a cytochrome P-450 (P-450/14DM). However, a reconstituted system consisting of P-450/14DM and its reductase converted lanosterol to the 14-desaturated derivative of 4,4-dimethylzymosterol, 4,4-dimethyl-5 alpha-cholesta-8, 14,24-trien-3 beta-ol (trienol). When AY-9944 was added to the reaction system with the microsomes, the trienol was formed with corresponding decrease in 4,4-dimethylzymosterol. These observations indicate that the 14 alpha-demethylation of lanosterol by yeast microsomes occurs sequentially via the trienol. Reduction of the trienol to 4,4-dimethylzymosterol is mediated by an AY-9944-sensitive reductase.

The identification of a novel C27 diene, cholesta-5,8-dien-3 beta-OL, in tissues of rats given AY-9944 (trans-1,4-bis(2-dichlorobenzylaminoethyl)cyclohexane) in pregnancy

Treatment of pregnant rats with AY-9944, a drug interfering with the last steps of cholesterol biosynthesis, accumulated cholesterol precursors in brain and liver of newborn animals. Different sterol profiles were found in these organs. Along with cholesterol and cholesta-5,7-dien-3 beta-ol, present in both tissues, liver was found to contain a hitherto unreported sterol, absent in brain. The structure of cholesta-5,8-dien-3 beta-ol was attributed to this compound by mass spectrometric, 1H, and 13C NMR analysis.

Accumulation of ergosta-8,14-dien-3beta-ol by Saccharomyces cerevisiae cultured with an azasterol antimycotic agent

15-Aza-25-methylene-D-homocholesta-8,14-dien-3beta-ol, an antimycotic agent, at a concentration of 75 ng/ml inhibited ergosterol biosynthetis in Saccharomyces cerevisiae strain 3701B resulting in the accumulation of an unusual sterol. Experimental data presented indicate that this sterol is ergosta-8,14-dien-3beta-ol. The accumulation of the compound is supportive of current models of biosynthetic pathways for sterols in yeast and is consistent with inhibition by the azasterol of the delta14 sterol reductase.

Enzymatic conversion of 5alpha-cholesta-7, 14-dien-3beta-ol to 5alpha-cholesta-8, 14-dien-3beta-ol

5alpha-Cholesta-7, 14-dien-3beta-ol, previously shown to be efficiently converted to cholesterol upon incubation with rat liver homogenate preparations under aerobic conditions, has been studied as to its possible conversion to 5alpha-cholesta-8, 14-dien-3beta-ol. Efficient conversion was observed upon incubation in the presence of washed mictosomes of rat liver under anaerobic conditions. This observation is of importance in consideration of possible metabolic pathways in the biosythesis of cholesterol.