Ca 2+ and calmodulin antagonists inhibit the proton gradients associated with non-cyclic and cylic photophosphorylation in spinach chloroplasts

Abstract

The oxygen evolution associated with electron transport in spinach chloroplasts can be stimulated by low concentrations (<10 μM) of phenothiazines and pimozide, known as calmodulin antagonists, which in this case act as uncouplers of electron transport. Higher concentrations of phenothiazines and pimozide (<10 μM) inhibit electron transport in the plastoquinone region, presumably between the B protein and the plastoquinone pool. Electron transport in chloroplasts can also be inhibited by calcium antagonists (La Cl 3) or by Ca 2+ chelators [8-(diethylamino) octyl 3,4,5-trimethoxy benzoate HCl]. The formation of proton gradients associated with non-cyclic or cyclic photophosphorylation can also be inhibited by calmodulin antagonists or by calcium chelators in concentrations below 10μM, but not by LaCl 3. Bovine brain calmodulin also inhibits the non-cyclic proton gradient in presence of inorganic phosphate and adenosine diphosphate. Fifty percent inhibition is given by the addition of 20 μg bovine brain calmodulin/ml.