Evidence for a mevalonate shunt in a tracheophyte

Abstract

Primary leaf tissue from light and dark grown wheat seedlings incubated with [2-14C]acetate and [2-3H]mevalonolactone (MVA) synthesized doubly labelled sterols and long chain fatty alcohols (LCFA). While [2-3H]MVA was incorporated into LCFA as efficiently as into sterols, [5-3H]MVA was metabolized only to sterols. Mevinolin, a specific inhibitor of HMG–CoA reductase, completely inhibited [2-14C]acetate incorporation into sterols but it did not completely prevent [2-3H]MVA from being incorporated into LCFA. In the presence and absence of mevinolin,3H in the purified LCFA was found associated primarily with C22, C24, and C26(components isolated from the subcellular membranes), while14C was present additionally in C28(the major LCFA isolated from the epicuticular wax). Substantial14C and3H was incorporated into the membrane-bound 24-desalkyl and 24-alkylsterols with no loss of label associated with increasing the side chain length, that is, by alkylation at C24. The results demonstrate for the first time that: (i) the MVA shunt operates in a tracheophyte; (ii) preferential utilization of acetate, formed by the shunt and presumably compartmentalized, may exist in wheat for the synthesis of LCFA having a chain length distribution more suitable for membrane than wax construction; and (iii) the MVA shunt is not a minor vestigial lipid pathway but may, under certain physiological and developmental conditions, represent a pathway for routing isopentenyl pyrophosphate C-atoms away from their inclusion into the sterol pathway. Photosynthesis had no apparent effect on shunt activity.