The Contribution of Light-Dependent Bicarbonate Uptake in Thylakoid Membrane Energization

Abstract

Illumination of the well stirred suspension of isolated spinach chloroplasts induced CO₂ uptake (up to 320 nmol CO₂/mg chlorophyll) from air phase over suspension. The process started immediately after onset of illumination, developed during 15–20 sec and completely reversed for the same time after switching off the light. Uncouplers (gramicidine D, NH₄Cl) inhibited the light-induced CO₂ uptake.

The value of light-induced CO₂ uptake was dependent on carbonic anhydrase (CA) activity and inhibited by CA inhibitors — lipophylic ethoxyzolamide (EZ) and water-soluble acetazolamide. The effect of exogenic bicarbonate and inhibitors of carbonic anhydrase on the rate of photophosphorylation was examined in the pH range 7.0–8.2. It was shown that exogenic bicarbonate (3–6 mmol) effectively stimulated photophosphorylation. The bicarbonate-induced enhancement of photophosphorylation showed a marked pH dependence, with the greatest response occurring at pH near 7.0. Both EZ and AZ reduced the stimulating effect of HCO₃ ⁻ on the rate of photophosphorylation. It is concluded that light-induced ATP synthesis depends not only on exogenic bicarbonate, but also on the activity of carbonic anhydrase that rapidly converts the forms of carbonic acid thereby facilitating protons removal from sites of their evolution.