The Sensitivity of Acetyl-coenzyme A Carboxylase to Citrate Stimulation in a Homogenate of Rat Liver Containing Subcellular Particles

1. Although citrate is known to activate purified preparations of acetyl-CoA carboxylase, it had no stimulatory effect on the incorporation of [(14)C]acetate into long-chain fatty acids in a whole homogenate of rat liver (S(0.7)) under conditions in which the activity of acetyl-CoA carboxylase was rate-limiting for fatty acid synthesis. 2. The rate of incorporation of acetyl carbon into fatty acids was estimated in S(0.7) preparations incubated with [(14)C]acetate, by measuring the specific radioactivity of the acetyl carbon of acetyl-CoA and the incorporation of (14)C into fatty acids. These estimates were compared with estimates of acetyl-CoA carboxylase activity in the S(0.7) preparation obtained by direct assay in conditions in which the enzyme was in the fully activated state. 3. In the absence of citrate, incorporation of acetyl carbon into fatty acids was about 75% of the value expected if the acetyl-CoA carboxylase in the S(0.7) preparation were in the fully activated state. 4. Incorporation of acetyl carbon into fatty acids in the S(0.7) preparation was stimulated by citrate, but the effect was many times less than the stimulation of [(14)C]acetate incorporation by citrate in particle-free preparations. 5. When the mitochondria and microsomes were removed from the S(0.7) preparation, [(14)C]acetate incorporation into fatty acids fell to a negligible value and the preparation became highly sensitive to stimulation by citrate. 6. It is suggested that in the presence of mitochondria and microsomes, and in the intact liver cell, the degree of activation of acetyl-CoA carboxylase is such that citrate activation may not be of physiological significance.