The acetoacetyl-coenzyme A thiolases of rat brain and their relative activities during postnatal development.

1. The apparent 3-oxoacyl-CoA thiolase activity of rat brain extracts is due to two different acetoacetyl-CoA thiolases, one cytoplasmic and the other mitochondrial. By the methods developed in the preceding paper (Middleton, 1973), the changes in activities of these two enzymes were determined during postnatal development. 2. Although the total brain acetoacetyl-CoA thiolase activity changes not more than 2-fold from birth to adulthood this masks large changes in the relative proportions of the two types of thiolase present. 3. Cytoplasmic acetoacetyl-CoA thiolase activity declines slowly from 4 units/g fresh wt. at birth to an adult value of 1.3 units/g fresh wt. 4. The mitochondrial acetoacetyl-CoA thiolase (activated by K(+)) rises rapidly in activity from 1 unit/g fresh wt. at birth to a peak value of 5 units/g fresh wt. at 20 days. After weaning the activity declines to 2.3 units/g fresh wt. in the adult. 5. These different developmental patterns are discussed in terms of the probable metabolic roles of the two brain acetoacetyl-CoA thiolases.