Substrate-Based Inhibitors of the (S)-Adenosyl-l-Methionine:Δ24()- to Δ24()-Sterol Methyl Transferase fromSaccharomyces cerevisiae

Mechanism-based Enzyme Inactivators of Phytosterol Biosynthesis

“…In work with the yeast SMT, 25-azalanosterol was discovered to bind to the active center with a K d value of 4 µM similar to either the K d values of zymosterol or AdoMet at 4 µM [20]. The expectation for tight binding was borne out by the results of the 24-and 25-heteroatom substituted sterols assayed with SMT1 and SMT2, the carbocation mimic was bound to the Michaelis complex many orders more tightly than the sterol (or AdoMet) substrate generating a K m /K i ratio of approximately 1000 [16,23,38,[40][41][42][43][44][45][46]. The efficacy of 25-azacycloartenol, a substrate mimic, and solasodine have been compared in vitro and in vivo with the pathogenic yeastlike microbe P. wickerhamii which synthesizes ergosterol by the cycloartenol-cyclolaudenol route shown in Scheme 1.…”

“…Indeed, microbial growth was affected less by solasodine treatment. However, while the azasteroids can be fungicidal and powerful inhibitors of SMT, as noted by Ator et al [41] clinical experience with triparanol indicate a range of side effects of sufficient severity to require the withdrawal of the drug from the market. Consequently, the clinical importance of these types of compounds has not been examined further and their study has been restricted to deciphering the substrate requirements for SMT catalysis.…”

“…The compounds shown in Scheme 4 have been tested and found to be noncompetitive inhibitors against the native substrate of the SMT [22,23,28,39]. In spite of numerous side chain modifications [38,[40][41][42][43][44][45][46][47], only a limited number of structural variants exhibit binding affinities to the enzyme active site that are significantly greater than those of the native substrate.…”

“…The first set of mechanismbased inhibitors were designed with sulfur or an aziridine function and they were considered to contain latent reactive functions that will be specifically activated by the SMT. The effectiveness of sulfur and aziridine-containing steroid derivatives on inhibition of enzyme activity is related to the structure of the sterol nucleus [7,41,46] whereas the mode of action is related to the structure and position of the functional group in the side chain. Two hypothetical mechanisms have been considered for the mode of action of 36 and 40 (Scheme 5).…”

Cholesterol import fails to prevent catalyst-based inhibition of ergosterol synthesis and cell proliferation of Trypanosoma brucei

“…All sterols isolated from the parasite except the new compounds described here were prepared and characterized in our earlier publications (21)(22)(23)(24)(25)(26)(27). The transition state analog inhibitor, 25-azalanosterol (25-AL), was prepared as described (23).…”