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

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 Km and kcat 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 Δ24(28)-olefin 10A. Inhibitor treatment was concentration and time-dependent affording an apparent Ki of 25 μM, a maximum rate of inactivation of 0.15 min−1 and a partition ratio (kcat/kinact) calculated to be 0.04.