1. The activities in rat tissues of 3-oxo
acid CoA-
transferase (the first
enzyme involved in
acetoacetate utilization) were found to be highest in kidney and heart. In submaxillary and adrenal glands the activities were about one-quarter of those in kidney and heart. In brain it was about one-tenth and was less in lung, spleen, skeletal muscle and epididymal fat. No activity was detectable in liver. 2. The activities of
acetoacetyl-CoA thiolase were found roughly to parallel those of the
transferase except for liver and adrenal glands. The high activity in the latter two tissues may be explained by additional roles of thiolase, namely, the production of
acetyl-CoA from
fatty acids. 3. The activities of the two
enzymes in tissues of mouse, gerbil, golden hamster, guinea pig and sheep were similar to those of rat tissues. The notable exception was the low activity of the
transferase and thiolase in sheep heart and brain. 4. The activities of the
transferase in rat tissues did not change appreciably in
starvation,
alloxan-diabetes or on fat-feeding, where the rates of
ketone-body utilization are increased. Thiolase activity increased in kidney and heart on fat-feeding. 5. The activity of
3-hydroxybutyrate dehydrogenase did not change in rat brain during
starvation. 6. The factors controlling the rate of
ketone-body utilization are discussed. It is concluded that the activities of the relevant
enzymes in the adult rat do not control the variations in the rate of
ketone-body utilization that occur in
starvation or
alloxan-diabetes. The controlling factor in these situations is the concentration of the
ketone bodies in plasma and tissues.