Rat liver peroxisomes. I. New peroxisome population induced by thyroid hormones in the liver of male rats.

The paper presents results that a hyperthyroid state for 5 to 10 days induces a new peroxisome population in the liver of male rats. This assumption is based on the following observations: 1. Treatment of male rats for five consecutive days with 25 micrograms thyroxine per day resulted in an increase of catalase and D-amino acid oxidase activities by 32 and 61%, respectively. On the other hand, urate oxidase activity declined by 20%. Prolonged treatment for ten days raised catalase activity by 68% and lowered urate oxidase activity by 35%. 2. Morphological examinations on the light and electron microscopic level revealed that the increase in peroxisomal enzyme activities was accompanied by an augmentation of peroxisomal profiles. "Microperoxisomes" and peroxisomes lacking the electron dense core were frequently observed. 3. The peroxisomes of the livers of thyroxine-treated animals exhibited an altered centrifugal behaviour in sucrose density gradients. The mean equilibrium density of normal liver peroxisomes was 1.23, whereas peroxisomes of thyroxine-treated livers equilibrated at the density of 1.20. It is discussed that peroxisomal metabolism may contribute considerably to the late biochemical effects of thyroid hormones, that means enhancement of lipid oxidation and heat production. Up to now these late effects were attributed to a stimulated mitochondrial metabolism only.