Application of sterol synthesis inhibitors to investigate the sterol requirements of protozoa and plants.

T h e major sterols found in higher plants. algae. fungi and many protoma are typically substituted at C-24 in the sidechain by a methyl or ethyl group. Depending upon the organism, the C-24 alkyl sterol can have either the 2311or 24s-configuration and the extra carbons are derived by transmethylation reactions involving S-adenosylmethionine and appropriate A'4and A''''x -sterol substrates 11 -41. Why plants. fungi and protozoa should expend considerable energy t o biosynthesize 24-alkyl sterols is an intriguing question and despite much speculation there is as yet apparently no satisfactory answer. I n vertebrate animals, cholesterol, with ;in unsuhstituted side-chain, plays a vital role in cellular membrane structures ;and i t also acts as a precursor t o essential cornpounds such as the bile acids and steroid hormones. Likewise, in fungi. sterol is considered t o perform a vital membrane function. Although sterols are apparently not a major component o f chloroplast membranes, it is generally assumed that sterol plays an important structural role in other cellular mcmbrancs in plants 15-71. In higher plants the sterols act also as precursors to a diverse range of steroidal saponins and alkaloids, usually classed as secondary metabolites, for which precise physiological and biochemical tunctions have not been assigned. However, the brassinolides are now emerging as an exciting group o f steroid derivatives possessing 'hormonal' properties in plants and which may play a profound role in plant development 18.91. lvidence that sterols may play an important 'metabolic' role in the cell proliferation process. in addition to the purely structural function of sterol during production of new cell membranes. has been emerging in recent years from three lines o f study employing fungi. protozoa and higher plants. Investigations with anaerobically grown yeast and yeast mutants have established multiple roles for sterol during yeast growth. Under strict anaerobic conditions, o r in the presence of 2.3-iminosqualene. sterol biosynthesis is blocked at the squalcne epoxidase step. Under these conditions growth of Succharomyc~es cerevisicre did not occur unless exogenous sterol was supplied t o the culture I 1012 I and a 24S-methyl sterol was shown t o be essential for maximal growth. Other work with yeast mutants which arc sterol auxotrophs has established a dual role for the sterol required to support growth [ 1.3171. One is for 'bulk' sterol. which is presumably needed t o satisfy the demands o f mcmbrane elaboration. This can be satisfied by a variety o f sterols and is not dependent upon the presence o f a C-24 methyl gruup in the side-chain. T h e other need is for traces o f a A'-sterol, preferably possessing a 24-methyl group (e.g. ergosterol). to promote cell proliferation. This latter sterol function has been variously described as 'sparking', metabolic, hormonal or synergistic. In addition, other levels of sterol function in yeast membranes have bcen described as 'critical domain' and 'domain' I IS, 161. It should be noted that in wild-type S . cerevisiue all these functions would normally be met by ergosterol biosynthesized by the yeast. With another fungus. Gihhereh fiijikiiroi, it seems that other sterols in addition to ergosterol are required for growth and they apparently fulfil a variety of physiological roles [ I 81. T h e essential role for sterol in fungal growth is also highlighted by the development of antifungal drugs which speci-