Carbonic anhydrases in the C 3 -plant leaf cell

Carbonic anhydrase (CA) activities of the mesophyll protoplasts from Pisum sativum L. leaves (plasmalemma CA), the supernatant after thylakoid precipitation (soluble CA), and the washed thylakoids (thylakoid CA) were studied. It was found that the K m (CO2) were 104 mM for the plasmalemma CA, 20 mM for the soluble CA, and 9 mM for the thylakoid CA. The activity of the last differed from the first two in response to the inhibitor acetazolamide by increasing at submicromolar concentrations of inhibitor. pH-dependencies of K m (HCO3-) for soluble CA and thylakoid CA differed in that the former increased as pH increasd from 7.0 to 8.0, while the latter slightly decreased over this pH change. A comparison of the pH-dependencies of the soluble CA and thylakoid CA dehydrase activities expressed in Wilbur-Andersen units demonstrated that, while the activity of soluble CA was unchanged over pH range 6–8, the activity of thylakoid CA had a distinct pH optimum at pH 6.8–7.0. The possible functions of the three forms of CA in the leaf cell of C3 -plants are briefly discussed.