Mechanism-based active site modification of the soybean sterol methyltransferase by 26,27-dehydrocycloartenol

26,27-Dehydrocycloartenol (26,27-DHC) was shown to be a substrate for the soybean sterol methyltransferase (SMT) as well as a mechanism-based inhibitor of enzyme action. The K m and k cat for 26,27-DHC was 10 μM and 0.018 min −1, respectively. SMT catalyzed 26,27-DHC to two products tentatively identified as 26-homocholesta-9,19-cyclo-23(24) E,26(26′)-dienol and 26-homocholesta-9,19-cyclo-26(26′)-en-3β,24β-diol by GC–MS. Inhibitor treatment was concentration- and time-dependent (pseudo-first-order kinetics). A replot of the half-lives for inactivation versus the inverse of the inactivator concentrations gave an apparent K i of 42 μM and a maximum rate of inactivation of 0.29 min −1. A partition ratio ( k cat/ k inact) was calculated to be 0.06. 26,27-Dehydrocycloartenol was assayed with the sterol methyltransferase from soybean and found to be catalyzed to monol (turnover product 9A) and diol (hydrolysis product assumed to be covalently attached to SMT 8A) structures yielding a partition ratio of 0.06.