The role of citrate in lipid synthesis in developing soybean cotyledons 1

The role of citrate and the citrate cleavage enzyme in lipid synthesis in developing soybean cotyledons (Glycine max L. Merr. var. Harosoy 63) was investigated. The activity of the enzyme was inhibited by (—) hydroxycitrate, which is a specific inhibitor of citrate cleavage by this enzyme. Incorporation of label from citrate-1-14C and -5-14C indicated that the citrate molecule is cleaved between carbons 3 and 4. Acetyl CoA-14C and oxaloacetate-14C phenylhydrazone were isolated as products of the citrate cleavage reaction. The production of oxaloacetate-14C-phenylhdrazone from citrate-6-14C was carried out using a nucleotide free enzyme preparation and did not require the addition of ATP or CoA. Therefore it would appear that the citrate cleavage reaction is not CoA dependent in developing soybean seeds. Incorporation of pyruvate-2-14C into the crude lipid fraction was shown to require both the particulate and soluble fractions. Apparently in soybeans, as in animal systems, pyruvate is oxidized by the pyruvate dehydrogenase complex and the acetyl CoA formed condenses with oxaloacetate to produce citrate in the mitochondria. Citrate is then transported out of the mitochondria to the cytosol where it is cleaved to form acetyl CoA for lipid synthesis.