Studies on carbonic anhydrase (CA) of light microsomal membranes isolated from bovine and pig gastric mucosa.

1. The occurrence and characteristics of carbonic anhydrase (CA) activity were studied in light microsomal membranes (LMM) purified from bovine gastric mucosa. 2. Bovine gastric LMM contained a high activity of CA ranging from 170 to 400 mumol.H+/min/mg protein when assayed at 0 degree C by pH-stat technique. 3. The addition of 2mM EDTA to the assay mixture increased the enzyme activity. Lower concentrations (0.5-1 mM) had no effect. 4. The enzyme activity was dose-dependently inhibited by acetazolamide and furosemide (I50: 5 x 10(-10) M and 4.8 x 10(-7) M, respectively) and by chloride ion (Ki 85 mM) and appeared to be quite stable to treatment with alkaline Triton X-100. 5. Most of the CA activity is loosely associated with the LMM since it was removed by different washing treatments. Nevertheless, after extensive washes, gastric LMM still contained CA activity suggesting the existence of a firmly membrane-associated form of CA. 6. Values of CA activity higher than those reported previously were found in pig gastric LMM. Furthermore, the washing treatments described in this work were more effective in washing CA activity off pig gastric LMM than procedures described previously.