Purification and characterization of the stearoyl-acyl carrier protein desaturase and the acyl-acyl carrier protein thioesterase from maturing seeds of safflower

Two enzymes involved in oleic acid biosynthesis have been purified from immature safflower seed. The stearoyl-acyl carrier protein (ACP) desaturase which catalyzes the formation of the double bond of oleate has been purified 200-fold and is a dimer with a molecular weight of 68,000. The enzyme shows strong preference for stearoyl-ACP as substrate; by comparison of its activity with stearoyl-CoA and palmitoyl-ACP as substrates, it appears that the ACP moiety is primarily important for binding of substrate and the chain length is important for catalytic activity. The desaturase requires 56 microM oxygen for half-maximal activity, 400 microM oxygen for maximal activity, and is stimulated severalfold by catalase. The acyl-ACP thioesterase has been purified 700-fold and is also a dimer of molecular weight 74,000. It shows a 5-fold preference for oleoyl-ACP versus stearoyl-ACP and is relatively inactive with corresponding acyl-CoAs.