Carbonic anhydrase activity in mitochondria from rat liver.

An 18O exchange method has been used to determine the location of carbonic anhydrase in mitochondria from rat liver and to examine the role of this enzyme in the kinetics of CO2 in resting and respiring mitochondria. Using digitonin subfractionation, we have determined that a substantial fraction, 40 to 60%, of the carbonic anhydrase activity in the mitochondrion from rat liver is located in the space between the inner and outer membranes; the remaining activity was found in the matrix with no detectable activity in the sedimented membranes. The total catalytic CO2 hydration activity measured in intact mitochondria from rat liver was about 1% of that found in an equal volume of rat erythrocytes. The apparent permeability constant representing the barrier for the diffusion of HCO3(-) from external solution to intramitochondrial carbonic anhydrase, 9 X 10(-5) cm s-1, is near in magnitude to the permeability constant for the diffusion of HCO3(-) across the rat erythrocyte membrane, 4 X 10(-4) cm s-2. Calcium-induced respiratory jumps were shown to cause changes in the rate of 18O exchange between CO2 and H2O that were consistent with a net uptake of CO2 by the mitochondria.