Long chain fatty acid synthesis in developing castor bean seeds IV. The synthetic system in proplastids

The proplastid fraction containing no cytosol and mitochondrionwas isolated from developing castor bean endosperm by stepwisesucrose density centrifugation. This fraction possesses thecapacity to synthesize LFAs from u-14C]sucrose, u-¹⁴C]-glucose,u-¹⁴C]G-1-P, u-¹⁴C]G-6-P, 2-¹⁴C]pyruvate and 1-¹⁴C]acetate.Little was incorporated from 1-¹⁴C]pyruvate into LFAs, butmuch into ¹⁴CO_a. Addition of cytosol to the proplastid fractiondid not enhance the LFA synthesis. From these data, the wholepath from sucrose to LFAs through glycolytic path and pyruvatedecarboxylation seems to be located within the proplastid indeveloping castor bean endosperm. The difference in utilizationof substrates indicates that the rate of LFA synthesis in castorbean proplastids is limited at a step between sucrose and hexosephosphate. In addition, experiments with CO₂ output and LFAsynthesis from 1-¹⁴C]glucose, 6-¹⁴C]glucose and u-¹⁴C]G-6-Pstrongly suggest that the path flow branches actively throughG-6-P to the pentose phosphate path and little through acetylCoAto the TCA cycle. (Received May 12, 1975; )