FORMATION OF CHLOROPLAST ISOPRENOIDS FROM PYRUVATE AND ACETATE BY CHLOROPLASTS FROM YOUNG SPINACH PLANTS - EVIDENCE FOR A MEVALONATE PATHWAY IN IMMATURE CHLOROPLASTS

The C-14-incorporation of [2-C-14]pyruvate and [2-C-14]acetate into ch loroplast (chip) isoprenoids and (free and esterified) fatty acids by purified chloroplasts isolated from young spinach (Spinacia oleracea) plants was studied. The chloroplasts, though intact, were sujected to a slight hypotonic treatment (0.165 M sorbitol) in order to facilitate pyruvate transfer across envelope membranes. [2-C-14]Acetate was inco rporated into chip isoprenoids at a rate about five times higher than found for [2-C-14]pyruvate. For both C-14-substrates the ratios of C-1 4 incorporated into chip isoprenoids compared to that into fatty acids did not differ significantly. Considering the situation in immature c hloroplasts, these results and findings from other investigations (e.g . chloroplast mevalonate kinases) indicate a synthesis of isopentenyl diphosphate (IPP) via mevalonate rather than the novel pathway of IPP synthesis in several bacteria found by Rohmer and coworkers, which ori ginates from pyruvate and not from acetyl-CoA.