Long chain fatty acid synthesis in developing castor bean seeds II. Operation of the path from sucrose to long chain fatty acids in a subcellular particulate system

Particles spun down at 10,000 × g from developing castor bean seeds were capable of synthesizing LFAs from sucrose, a physiological precursor transferred from leaves as a photosynthetic product. Tracer experiments, in combination with inhibitor effects, intermediate dilutions and cofactor requirements, indicated the operation of the following path: sucrose→UDPG→G-1-P→G-6-P→GAP→pyruvate→acetyl-CoA→malonyl-CoA→LFA. The whole path appears to be associated with 10,000 × g particles, because repeated washings were unsuccessful in dissociating a partial path from the particles, depsite of disorganizing the structure of the particles. Based on the occurrence of free hexose(s) and the utilization of UDPG similar to that of sucrose or G-1-P in this reaction, it is probable that hexose phosphate is formed from sucrose via UDPG and fructose, although the conversion of sucrose to hexose phosphates via glucose and furctose is not ruled out. Inhibitor experiments showed that ATP is self-supporting over the whole path, the ATP formed in the glycolytic path being consumed in a acetyl-CoA carboxylation step. Since oxidized pyridine nucleotides are as available as reduced ones for LFA synthesis, they seem to shuttle between the reduction in the conversion of sucrose to acetyl-CoA and the oxidation in LFA synthesis from acetyl-CoA. From the pattern of the LFAs synthesized, NAD+ is available for the synthesis of saturated LFAs (18: 0, 16: 0). whereas NADP+ is available for that of unsaturated LFAs (18: 1, 16: 1).