THE EFFECTS OF EXCESS EXOGENOUS MEVALONIC ACID ON STEROL AND STERYL ESTER BIOSYNTHESIS IN CELERY (APIUM-GRAVEOLENS) CELL-SUSPENSION CULTURES

Cell suspension cultures of celery (Apium graveolens) have been used to investigate the fate of excess added mevalonic acid and the effects on the amounts of free sterols and steryl esters in the cells. Radiolabelling studies using [2-C-14] MVA showed that the main effect of excess MVA (5 mM) was an increase in the synthesis of steryl esters. Further analysis revealed that esterified 4,4-dimethylsterols were more highly labelled than 4-mono- and 4-desmethylsterols in MVA-treated cells, suggesting a restriction in the flow of carbon downstream from cycloartenol and 24-methylenecycloartanol, and thus indicating the presence of a post-mevalonic acid control point in the sterol biosynthesis pathway. The total fatty acids recovered from the celery cell lipids showed a very low incorporation of radioactivity from the [2-C-14] mevalonate, indicating that a limited breakdown of excess mevalonate to acetate may occur in these cells, possibly by the operation of the mevalonate shunt. However, this does not appear to be a major route for the disposal of excess mevalonate. Analysis of the free sterols and steryl esters showed that the administration of excess exogenous MVA (5 mM) to Apium graveolens cells caused a two-fold increase in the steryl ester content when compared with control cells, although there was little effect on the free sterol content or composition. When excess MVA was administered in conjunction with the sterol biosynthesis inhibitor paclobutrazol (50 mu M), the steryl ester content increased almost three-fold. It is possible that, as well as providing a means of storage of excess sterols, the esterification of sterols may have a regulatory role in sterol biosynthesis, by controlling the rate of C-4 demethylation of precurser 4,4-dimethylsterols