Rat kidney mitochondrial carbonic anhydrase

Abstract

Mitochondrial carbonic anhydrase has previously been quantitated in liver mitochondria; it was not detected in guinea pig kidney cortical mitochondria. Evidence of this enzyme in rat kidney cortical mitochondria is reported. Electron microscopy showed that intact mitochondria were free of other intracellular organelles. When intact kidney mitochondria were added to isotonic 3′-( N′-morpholino) propanesulfonic acid buffer with 25 m m KHCO 3 (1% labeled with 18O) the rate of disappearance of C 18O 16O was biphasic; this indicates that there is carbonic anhydrase within the inner mitochondrial membrane. Intact rat kidney mitochondria were assayed for carbonic anhydrase activity at 4 °C by the changing pH technique. The rate of CO 2 hydration in the presence and absence of intact mitochondria was identical; this rate increased when Triton X-100 was added which indicates that all carbonic anhydrase is inside the inner mitochondrial membrane. Carbonic anhydrase activity was quantitated as K enz (units, ml · s −1 mg −1mitochondrial protein) at 37 °C, pH 7.4, in 25 m m NaHCO 3 (1% labeled with 18O) by following the rate of disappearance of C 18O 16O from solutions before and after addition of disrupted mitochondria. Values of K enz for liver and kidney mitochondria from rats given free access to normal rat chow and water at neutral pH were 0.06 and 0.08 (respectively). Values of K enz for liver and kidney mitochondria from rats fed as above and with free access to water adjusted to pH 2.5 with HCl were 0.04 and 0.16, respectively. Values of K enz for rats starved for 48 h were 0.06 and 0.12 (respectively). The values of K enz remained 0.11-0.14 in liver mitochondria from guinea pigs fed normally, given dilute acid, or starved and the value was always at zero in guinea pig kidney mitochondria. Values of K enz were measured with disrupted mitochondria by the 18O technique as a function of pH at 25 °C, 25 to 75 m m NaHCO 3, ionic strength 0.3. From pH 7.0 to 8.0 K enz increased threefold for mitochondria from rat liver, fed rat kidney, and acid rat kidney, and increased eightfold for mitochondria from guinea pig liver. K enz was decreased similarly by increasing HCO − 3 in mitochondria from rat liver, fed kidney, and acid kidney; it is concluded that carbonic anhydrase in rat liver mitochondria is probably the same isozyme as in rat kidney mitochondria. The published observation that rat kidney cortices are up to 10 times as gluconeogenic from pyruvate as guinea pig kidney cortices can be explained by the presence of mitochondrial carbonic anhydrase in rat but not guinea pig mitochondria.