Mitochondrial carbonic anhydrase.

Abstract

Carbonic anhydrase activity (EC 4.2.1.1) and bicarbonate permeability were assayed in suspensions of broken and intact guinea pig mitochondria by monitoring the disappearance of C16O18O. Significant activity was found in preparations from liver and skeletal muscle, but not in preparations from heart muscle, brain and kidney. Intact mitochondria containing carbonic anhydrase produce a 2-phase acceleration of the disappearance of the labeled CO2 than to HCO3-. Acetazolamide inhibits the enzyme activity instantly in broken mitochondria after a delay in intact mitochondria, indicating that the enzyme is in a region not immediately accessible to the inhibitor. Sonication of mitochondria containing carbonic anhydrase activity releases the enzyme, which remains in the supernatant after sedimentation of the submitochondrial particles. Mitochondrial carbonic anhydrase is in the matrix compartment and not in, or bound to, the inner membrane. The activity of the enzyme increases markedly with increasing pH. The enzyme activity of intact mitochondria is greater than that of the broken mitochondria at the same pH of the suspending fluid, corresponding to an intramitochondrial pH that is 0.2-0.5 unit more alkaline.