Inhibition of energy-transducing functions of chloroplasts by spegazzinine

Abstract

The effects of spegazzinine, a dihydroindole alkaloid, on various energy-transducing functions of chloroplasts were studied. The following observations were made, (i) Spegazzinine inhibited both cyclic and noncyclic photophosphorylation in isolated spinach chloroplast. The I 50 value was about 80 μ m. Over a concentration range which gave marked inhibition of ÀTP synthesis, there was no effect on basal or uncoupled electron flow or light-induced proton accumulation by isolated thylakoids, while the fraction of electron transport stimulated by coupled phosphorylation was reduced to the basal level by spegazzinine. (ii) The regulatory effect of low concentrations of ATP on proton movements and electron transport was diminished by the alkaloid, (iii) Spegazzinine also inhibited with similar efficiency the ATPase activities of membrane-bound coupling factor 1 (CF 1) and of purified CF 1. One mole of spegazzinine per mole of CF 1 seemed to be required to inhibit the ATPase activity, (iv) The allosteric effect of ADP on ATPase activity was not affected by spegazzinine. (v) On the basis of these results it is concluded that spegazzinine acts as an energy transfer inhibitor of hotophosphorylation and that its site of action may be at or near the catalytic site of ATPase.