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 K(m) and k(cat) for 8A was 37 microM and 0.006 min(-1), respectively. The enzyme-generated product was identified by MS and (1)H NMR to be a C24, C25-doubly alkylated Delta(24(28))-olefin 10A. Inhibitor treatment was concentration and time-dependent affording an apparent K(i) of 25 microM, a maximum rate of inactivation of 0.15 min(-1) and a partition ratio (k(cat)/k(inact)) calculated to be 0.04.