SUBSTRATE-BASED INHIBITORS OF THE (S)-ADENOSYL-L-METHIONINE-DELTA(24(25))-STEROL TO DELTA(24(28))-STEROL METHYL TRANSFERASE FROM SACCHAROMYCES-CEREVISIAE

A series of 31 side-chain-modified analogs of cholesterol, zymosterol, lanosterol, and cycloartenol and the steroidal alkaloids solasodine a nd solanidine were studied as inhibitors of (S)-adenosyl-L-methionine: Delta(24(25))-sterol methyl transferase (SMT) enzyme activity from Sac charomyces cerevisiae. Two classes of sterol methylation inhibitors we re tested: substrate analogs, including mechanism-based inhibitors, an d transition state analogs. Several novel sterol methylation inhibitor s that contained an aza, aziridine, Or ammonium group in the sterol si de chain were prepared and tested for the first time. The degree and k inetic pattern of methylation inhibition were found to be influenced b y the position and nature of the variant functional group introduced i nto the side chain. The most potent inhibitors of SMT enzyme activity were transition state analog inhibitors (K-i values of 5 to 10 nM) tha t mimicked the structure and conformation of the natural substrate pre sumed to form in the ternary complex generated in the transition state . Steroidal alkaloids were potent competitive inhibitors with a values ranging from 2 to 30 mu M, which is about the K-mapp of zymosterol, c a. 27 mu M. An isosteric analog of the natural substrate, zymosterol, in which the 26/27-gem-dimethyl groups were joined to form a cycloprop ylidene function is shown to be a potent irreversible mechanism-based inactivator of SMT enzyme activity that exhibits competitive-type inhi bition, K-i 48 mu M with a K-inact of 1.52 min(-1). Mechanistic implic ations of these results provide new insights into the topology of the ternary complex involving sterol-AdoMet-enzyme. (C) 1997 Academic Pres s.