Evidence for a mevalonate shunt in a tracheophyte

Abstract

Primary leaf tissue from light and dark grown wheat seedlings incubated with [2- 14 C]acetate and [2- 3 H]mevalonolactone (MVA) synthesized doubly labelled sterols and long chain fatty alcohols (LCFA). While [2- 3 H]MVA was incorporated into LCFA as efficiently as into sterols, [5- 3 H]MVA was metabolized only to sterols. Mevinolin, a specific inhibitor of HMG–CoA reductase, completely inhibited [2- 14 C]acetate incorporation into sterols but it did not completely prevent [2- 3 H]MVA from being incorporated into LCFA. In the presence and absence of mevinolin, 3 H in the purified LCFA was found associated primarily with C 22 , C 24 , and C 26 (components isolated from the subcellular membranes), while 14 C was present additionally in C 28 (the major LCFA isolated from the epicuticular wax). Substantial 14 C and 3 H 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 C 24 . 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.