title: Mechanism-based active site modification of the soybean sterol methyltransferase by 26,27-dehydrocycloartenol.
abstract: 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 microM 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-3beta,24beta-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 microM and a maximum rate of inactivation of 0.29 min(-1). A partition ratio (k(cat)/k(inact)) was calculated to be 0.06.
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, PO Box 1061, Lubbock, TX 79409, USA.
authors.forename: Zhihong
authors.initials: Z
authors.lastname: Song
authors.forename: Wenxu
authors.initials: W
authors.lastname: Zhou
authors.forename: Jialin
authors.initials: J
authors.lastname: Liu
authors.forename: W David
authors.initials: WD
authors.lastname: Nes
mesh_terms: D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D008780:Methyltransferases / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D010840:Phytosterols / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D014315:Triterpenes
chemical_list: D004791:Enzyme Inhibitors; D010840:Phytosterols; D014315:Triterpenes; D008780:Methyltransferases; C033580:soysterol; C100089:cycloartenol