High and unique carbonic anhydrase activity of Photosystem II from Pisum sativum: Measurements by a new and very...

Carbonic anhydrase (CA) activity, associated with Photosystem II (PSII), has been shown to enhance water oxidation. However, CA activity was thought to be a side effect or even a "contamination" of other CAs because of the relatively low rates of CA reactions in PSII measured previously. Here, by using 8-hydroxy-pyrene-1,3,6-trisulfonate (pyranine), a fluorescent dye, as a pH indicator, we show that PSII preparations (∗∗BBYs) from Pisum sativum have a high CA activity (as measured by HCO₃^- dehydration), which is close to that of highly active CAs. This fluorescence method is new for BBYs giving at least ten times higher activity than the other methods used earlier, as well as being highly sensitive and, thus, more convenient to use for BBYs than any other approach. We show here that the pH range of 5.0-7.5 is optimum for the pyranine measuring system, in general, and this pH range is suitable not only for the CA in BBYs but also for other CAs. Further, the CA activity of BBYs has the following unique properties: (1) low sensitivity to some known, and otherwise, effective CA inhibitors; (2) an opposite pH profile of HCO₃^- dehydration than observed in other known CAs. These findings indicate that the high CA activity, we have observed, belongs to BBYs, i.e., free of other CAs. At pH 6.5, CA activity of BBYs is shown to be directly correlated with that of photosynthetic O₂ evolution. We propose that the CA activity may accelerate the removal of H⁺s during water oxidation. # Celebrating 80th birthday of Alan James Stemler, a pioneer on the role of bicarbonate on the electron donor side of Photosystem II. S.G. Vaklinova & associates (1982), and A.J. Stemler (1986) were the first who have measured carbonic anhydrase activity in Photosystem II preparations. ∗∗ BBYs stand for Photosystem II samples made by the procedure of Berthold (B), Babcock (B) and Yocum (Y); see Berthold et al. (1981).