1. The range of fatty acids formed by preparations of ultrasonically ruptured avocado mesocarp plastids was dependent on the substrate. Whereas [1-14C]palmitate and [14C]oleate were the major products obtained from [-14C]acetate and [1-14C]acetyl-CoA, the principal product from [2-14C]malonyl-CoA was [14-C]stearate. 2. Ultracentrifugation of the ruptured plastids at 105000g gave a supernatant that formed mainly stearate from [2-14C]malonyl-CoA and to a lesser extent from [1-14C]acetate. The incorporation of [1-14C]acetate into stearate by this fraction was inhibited by avidin. 3. The 105000g precipitate of the disrupted plastids incorporated [1-14C]acetate into a mixture of fatty acids that contained largely [14C]plamitate and [14C]oleate. The formation of [14C]palmitate and [14C]oleate by disrupted plastids was unaffected by avidin. 4. The soluble fatty acid synthetase was precipitated from the 105000g supernatant in the 35-65%-saturated-(NH4)2SO4 fraction and showed an absolute requirement for acyl-carrier protein. 5. Both fractions synthesized fatty acids de novo.
