The specific activity of ATP citrate lyase is two to four times as great in livers of mice with hereditary obesity as in livers of their non-obese siblings. The enzyme activity in both types of mice can be reduced by starvation, and can be increased by refeeding starved animals. The specific activity of acetyl-CoA synthetase is approximately the same in both types of mice. 2. ATP citrate lyase of mammary gland of the rat undergoes large increases in activity after the onset of lactation. It declines rapidly upon weaning. 3. The changes in activity of ATP citrate lyase in obesity and lactation are consistent with the hypothesis that the enzyme supplies extramitochondrial acetyl-CoA for fatty acid synthesis.
