Carbonic Anhydrase Activity of Intact Erythrocytes from Seven Mammals

Carbonic anhydrase activity of intact erythrocytes from seven mammalian species was determined at 25 degrees C, pH 7.4, by mass spectrometry using the 18O-exchange technique. The seven species were Cavia porcellus, Mustela putorius furo, Felis domesticus, Canis familiaris, Homo sapiens, Equus caballus, and Bos taurus. Carbonic anhydrase activities determined as a function of hemoglobin concentration (std kcat) for intact erythrocytes at pH 7.4 were not significantly different from those determined for lysed erythrocytes at pH 7.20 for each species. The carbonic anhydrase activity of intact erythrocytes was not changed by a concentration of acetazolamide that inhibited it 85% in lysate (10(-7) M) in the 5-10 min needed for the assay. However, ethoxzolamide, another carbonic anhydrase inhibitor, produced the same fractional inhibition of enzyme activity in erythrocyte suspensions as in lysate in 1-2 min. Thus the inhibition constant, Ki, was approximately the same in both intact and lysed cells from each species, and it was possible to measure the apparent molar enzyme concentration inside the erythrocytes from the concentration of bound inhibitor. Intracellular enzyme concentrations were greater in those species with larger cells, but the specific activity of the carbonic anhydrase per molecule was less so that the overall enzyme activity, std kcat, was not related to mean cell volume. The effective permeability of the cells to the self-exchange of bicarbonate ion, P(HCO3-), averaged 2 X 10(-4) cm x s-1 and did not vary among the species.