Variations of 3-hydroxybutyrate dehydrogenase activity in brain and liver mitochondria of the developing chick.

1. The 3-hydroxybutyrate dehydrogenase activity was estimated in the crude mitochondrial fraction isolated from the cerebral hemispheres, the optic lobes, the cerebellum and the liver of the chick between the 20th day of embryonic life and the 30th day of postnatal maturation. 2. The optimal conditions of liberation and of determination of 3-hydroxybutyrate dehydrogenase activity were studied in the mitochondrial fraction isolated from chick cerebral hemispheres and liver. 3. The subcellular distribution of the enzyme in the chick brain and liver is very different from that in the rat. 3-Hydroxybutyrate dehydrogenase is completely mitochondrial in the rat brain and liver whereas in the chick brain and liver, it is located in mitochondrial and microsomal fractions; moreover, a third component can even be found in the soluble fraction of chick liver. 4. The 3-hydroxybutyrate dehydrogenase activity reaches the same value in the three areas of 20-day-old chick embryo brain. Between this stage and the 4th day after hatching, it increases to reach the same peak in the three areas. This peak however, appears at different stages according to the considered brain area. At 30 days after hatching, the enzyme activity is higher in the cerebellum than in the cerebral hemispheres and optic lobes. 5. The activity of hepatic 3-hydroxybutyrate dehydrogenase is 10 to 20 times lower than in the brain. It does not significantly change between 1 day before and 4 days after hatching and increases 2-fold between 4 and 30 days after hatching. 6. The variations of 3-hydroxybutyrate dehydrogenase activity in chick brain indicate correlations of this enzyme activity with development, particularly related to the nutritional state of the chicks. The fairly important differences in the activity of 3-hydroxybutyrate dehydrogenase in the liver of the chick and the rat enable us to come to a better understanding of the regulation of the concentration of the different ketone bodies in the blood of the chick and the rat. Moreover, the presence of the microsomal component of 3-hydroxybutyrate dehydrogenase in chick brain probably originates in the low concentration of acetoacetate in chick blood.