Heterogeneous origin of carbonic anhydrase activity of thylakoid membranes

Carbonic anhydrase activities of pea thylakoids as well as thylakoid fragments enriched either in Photosystem 1 (PS1-membranes) or Photosystem 2 (PS2-membranes) were studied. The activity of PS1-membranes if calculated on chlorophyll basis was much higher than the activity of PS2-membranes. Acetazolamide, a non-permeable inhibitor of carbonic anhydrases, increased carbonic anhydrase activity of PS2-membranes at concentrations lower than 10(-6) M and suppressed this activity only at higher concentrations. A lipophilic inhibitor of carbonic anhydrases, ethoxyzolamide, effectively suppressed the carbonic anhydrase activity of PS2-membranes (I50 = 10(-9) M). Carbonic anhydrase activity of PS1-membranes was suppressed alike by both inhibitors (I50 = 10(-6) M). In the course of the electrophoresis of PS2-membranes treated with n-dodecyl-beta-maltoside "high-molecular-mass" carbonic anhydrase activity was revealed in the region corresponding to core-complex of this photosystem. Besides, carbonic anhydrase activity in the region of low-molecular-mass proteins was discovered in the course of such an electrophoresis of both PS2- and PS1-membranes. These low-molecular-mass carbonic anhydrases eluted from corresponding gels differed in sensitivity to specific carbonic anhydrase inhibitors just the same as PS1-membranes versus PS2-membranes. The results are considered as evidence for the presence in the thylakoid membranes of three carriers of carbonic anhydrase activity.