1. An improved method was developed for the assay of plant holo-(acyl carrier protein) synthase activity, using Escherichia coli acyl-(acyl carrier protein) synthetase as a coupling enzyme. 2. Holo-(acyl carrier protein) synthase was partially purified from spinach (Spinacia oleracea) leaves by a combination of (NH4)2SO4 fractionation and anion-exchange and gel-permeation chromatography. 3. The partially purified enzyme had a pH optimum of 8.2 and Km values of 2 microM, 72 microM and 3 mM for apo-(acyl carrier protein), CoA and Mg2+ respectively. Synthase activity was inhibited in vitro by the reaction product 3′,5′-ADP. 4. Results from the fractionation of spinach leaf and developing castor-oil-seed (Ricinus communis) endosperm cells were consistent with a cytosolic localization of holo-(acyl carrier protein) synthase activity in plant cells.
