INHIBITION OF THE PLASTIDIC PYRUVATE-DEHYDROGENASE COMPLEX IN ISOLATED PLASTIDS OF OAT

The activity of the plastidic pyruvate dehydrogenase complex (pPDHC) i s one source of acetyl-CoA in plastids of higher plants needed for de novo fatty acid biosynthesis. This plastidic enzyme reaction is specif ically inhibited by acetylmethylphosphinate (AMPI), a compound which h ad hitherto been known only as an inhibitor of the mitochondrial pyruv ate dehydrogenase complex (mPDHC). In the test system of isolated inta ct oat plastids (Avena sativa) [2-C-14]pyruvate was used for de novo f atty acid biosynthesis. The incorporation of label from [2-C-14]pyruva te in fatty acids was inhibited by AMPI in a dose-dependent manner. Th e inhibition rose with increasing preincubation time of plastids with the inhibitor. I-50 values for the inhibition of de novo fatty acid bi osynthesis from [2-C-14]pyruvate by AMPI for isolated etioplasts and c hloroplasts were 4.5 and 80 mu M, respectively. The activity of the pP DHC decreased during greening of oat seedlings, as is seen from the de creasing incorporation of [2-C-14]pyruvate into fatty acids during the light-induced transformation of etioplasts into chloroplasts. In cont rast to the decreasing pPDHC activity, the activity of the plastidic a cetyl-CoA synthetase (ACS), which transfers acetate to acetyl-CoA, ros e parallel to the transformation of etioplasts into chloroplasts. Duri ng the assay time of 20 min we could not detect an incorporation of ra diolabel from pyruvate or acetate into beta-carotene or any other caro tenoid.