Characterization of an Acyl-Coenzyme A Thioesterase Associated with the Envelope of Spinach Chloroplasts

The enzymic hydrolysis of acyl-coenzyme A occurs in intact and purified chloroplasts. The different components of spinach chloroplasts were separated after a slight osmotic shock and the purified envelope membranes were shown to be the site of very active acyl-CoA thioesterase activity (EC 3.1.2.2.). The enzyme, which had a pH optimum of 9.0, was not affected by sulfhydryl reagents or by serine esterase inhibitors. However, the acyl-CoA thioesterase was strongly inhibited by unsaturated fatty acids, especially oleic acid, at concentrations above 100 micromolar. In marked contrast, saturated fatty acids had only a slight effect on the thioesterase activity. Substrate specificities showed that the velocity of the reaction increased with the chain length of the substrate from decanoyl-CoA to myristoyl-CoA and then decreased with the chain length from myristoyl-CoA to stearoyl-CoA. Interestingly, oleoyl-CoA was only slowly hydrolyzed. These results suggest that the envelope acyl-CoA thioesterase coupled with an envelope acyl-CoA synthetase may be involved in a switching system which indirectly allows acyl transfer from acyl carrier protein derivatives to unsaturated acyl-CoA derivatives and ensures the predominance of unsaturated 18 carbon fatty acids in plants. Furthermore, the position of both acyl-CoA thioesterase and synthetase in the envelope membranes suggest that these two enzymes may be involved in the transport of oleic acid from the stroma phase to the cytosol compartment of the leaf cell.