THE ACTIVITY OF ACETYL-COA CARBOXYLASE IS NOT CORRELATED WITH THE RATE OF LIPID-SYNTHESIS DURING DEVELOPMENT OF OILSEED RAPE (BRASSICA-NAPUS L) EMBRYOS

Acetyl-CoA carboxylase (ACCase; EC 6.4.1.2) activity has been determin ed in seed tissues of oilseed rape (Brassica napus L.), pea (Pisum sat ivum L.) and castor bean (Ricinus communis L.). A new method is descri bed which leads to significantly higher measurable activities of the e nzyme in tissue homogenates than previously reported. This method does not involve either Triton X-100 or centrifugation treatments which ha ve been used previously in the study of the enzyme. In the case of oil seed rape the activity was also increased by removal of the testa from the seed. The activity of ACCase was determined throughout the develo pment of oilseed rape embryos. Enzyme activity increased 3.5-fold as t he embryo fresh weight increased from 0.3 to 2.0 mg and then reached a plateau at 1.1 nmol malonyl-CoA.min-1.embryo-1. The main period of li pid accumulation commenced at an embryo fresh weight of 2.3 mg, which was after the plateau in ACCase activity had been reached. Activity of the enzyme declined after an embryo fresh weight of 3.5 mg, which was before lipid accumulation in the embryo had been completed. Compariso n of the activity of ACCase and the apparent in-vivo rate of lipid syn thesis on an embryo-fresh-weight basis (i.e. nmol malonyl-CoA formed o r utilized.min-1.mg-1 fresh weight) revealed that ACCase activity decl ines relative to the rate of lipid synthesis throughout development. T he negative correlation between these two rates is discussed in relati on to the role of ACCase in the regulation of accumulation of storage lipid during embryo development.