Fixation of carbon dioxide by the testes of rabbit and fish

Abstract 1. Incubation studies on the testicular tissue of rabbit and cod, in the presence of 14 CO 2 indicated incorporation of label into the supernate as well as the residue of the incubation mixture; of the total 14 C-label incorporated, the supernatant fluid had the highest amount, 92·7 and 99 per cent for rabbit and fish respectively. 2. Radiochromatography of the supernate demonstrated the presence of radioactivity in keto, amino and other organic acids. 3. Although less 14 C was recovered from the residue of the incubation mixture, it was found in lipids, nucleic acids and proteins. 4. In the testicular tissue of the two species examined, the addition of pyruvate to the incubation medium enhanced the fixation of CO 2 , whereas the addition of malonate depressed it. 5. A search for carbon dioxide fixing enzymes, in the testicular extracts of rabbit and cod, revealed the presence of malic enzyme, phosphoenolpyruvate car☐ykinase and propionyl CoA car☐ylase. 6. Although the testicular tissue from the two groups of animals reacted essentially the same in the ability to fix CO 2 , certain differences were noted in the pattern of incorporation into the different fractions, and also in the activity of the CO 2 -fixing enzymes.