Simulataneous and independent versus antagonistic inhibition of muscle carbonic anhydrase (CA III) by acetazolamide and cyanate

The inhibition by cyanate and acetazolamide of pig muscle carbonic anhydrase III (CA III) CO₂ hydratase activity was studied in order to explore mechanistic features possibly unique to the muscle isoenzyme. The turnover number for CO₂ hydration was found to be 6000 sec^?1 with a K_m of 83 mM for CO₂. Cyanate inhibition (K_i, 3 μM) and acetazolamide inhibition (K_i, 44 μM) were both found to be noncompetitive with respect to CO₂. Significantly, acetazolamide and cyanate displayed non-exclusive binding to pig muscle carbonic anhydrase. The similarity of mode and degree of inhibition of muscle carbonic anhydrase by cyanate as compared with the inhibition of the erythrocyte isoenzymes suggests the existence of a similar metal environment. However, the observation that cyanate and acetazolamide bind simultaneously to CA III and the comparatively large K_i for acetazolamide per se appear to be more compatible with a different mode of coordination of the zinc with the sulfonamide, thus supporting a five-coordinant zinc in the catalytic mechanism of CO₂ hydration for CA III.