β-Carotene Synthesis in Isolated Spinach Chloroplasts

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Abstract
Carefully isolated intact spinach chloroplasts virtually free of contamination of other organelles effectively form β-carotene from NaH14CO3 or [U-14C]-3-phosphoglycerate (PGA) under photosynthetic conditions. The photosynthate pool formed in chloroplasts from 1 to 2 millimolar [U-14C]-3-PGA or 3 to 6 millimolar NaH14CO3 was fully sufficient to supply β-carotene synthesis with intermediates for about 1 hour at maximal rates of about 20 nanomoles 14C incorporated per milligram chlorophyll per hour. Fatty acid synthesis remains, under these circumstances, in linear dependence to substrate concentrations with far lower activity. Isotopic dilution of the β-carotene synthesis by adding unlabeled glyceraldehyde 3-phosphate, dihydroxyacetone-P, 3-PGA, 2-PGA, phosphoenolpyruvate, pyruvate, respectively, may be interpreted as a direct substrate flow from photosynthetically fixed CO2 to isopentenyl pyrophosphate synthesizing system. Unlabeled acetate did not dilute β-carotene synthesis. Fatty acid synthesis acted similarly with unlabeled substrates; but it also was diluted by unlabeled acetate. These results indicate a tight linkage of photosynthetic carbon fixation and plastid isoprenoid synthesis.