Substrate Flow from Photosynthetic Carbon Metabolism to Chloroplast Isoprenoid Synthesis in Spinach Evidence for a Plastidic Phosphoglycerate Mutase

The carbon flow from 3-phosphoglycerate to pyruvate and acetyl-CoA within the chloroplast as well as the pathway for the formation of ß-carotene, plastoquinone-9 etc. as plastidic isoprenoids from photosynthetically fixed C 0 2 hitherto remained unclear because the presence of the phos­ phoglycerate mutase in chloroplasts had not unequivocally been proven. To clarify this question, the incorporation o f 14C 0 2, [2-14C]acetate, [2-14C]pyruvate, [2-14C]glycine, [3-14C]serine and [2-14C]mevalonate into ß-carotene, plastoquinone-9, sterols and fatty acids was studied using spinach and barley protoplasts and barley seedlings. In protoplasts as well as in seedlings under conditions of C 0 2 fixation, the largest portion of the acetyl-CoA derived compounds formed was ß-carotene and plastoquinone-9 rather than fatty acids and sterols. High rates o f fatty acid synthesis were obtained by supplying acetate as well. Mevalonate was incorporated into sterols but not into ß-carotene and plastoquinone-9. Direct evidence was obtained for the hitherto questioned plastidic phosphoglycerate mutase. Low activities were found in spinach chloroplasts which were substantiated by the criteria of latency method. Substrate flow from 3-phosphoglycerate to pyruvate via 2-phosphoglycerate and phosphoenolpyruvate was shown by applying [1-14C]glycerate and following its incorporation. From this the following conclusions were drawn: (i) The plastidic IPP synthesizing system is strongly separated from the cytosolic-ER one. (ii) As the formation of ß-carotene and plastoquinone-9 is favoured under conditions o f photosynthetic C 0 2 fixation, a direct carbon flow from 3-phosphoglycerate to isoprenoids via acetyl-CoA by a lowcapacity pathway, which shows high affinities for the substrates, is suggested.