Cyclobranol: A substrate for C25-methyl sterol side chains and potent mechanism-based inactivator of plant sterol methyltransferase

Abstract

Cyclobranol 8A, an analog of the cycloartenol substrate 1A for the plant sterol C24-methyltransferase (SMT), was shown to be an acceptor of the soybean SMT1 as well as an inhibitor of enzyme action. The K m and k cat for 8A was 37 μM and 0.006 min −1, respectively. The enzyme-generated product was identified by MS and 1H NMR to be a C24, C25-doubly alkylated Δ 24(28)-olefin 10A. Inhibitor treatment was concentration and time-dependent affording an apparent K i of 25 μM, a maximum rate of inactivation of 0.15 min −1 and a partition ratio ( k cat/ k inact) calculated to be 0.04.