Differential sensitivity to dibucaine of photosynthetic control of electron transport and photophosphorylation in chloroplasts

Abstract

The effect of dibucaine on the photosynthetic control of electron transport, as related to acidification of the thylakoid lumen, and photophosphorylation in isolated chloroplasts of Spinacia oleracea was studied and compared to that of a typical uncoupler (nigericin) and energy transfer inhibitor (3′- O-napthyl-ADP). In some respects, dibucaine resembled an uncoupler: it inhibited photophosphorylation and decreased quenching of 9-aminoacridine fluorescence as well as uptake of [ 14C]methylamine and inhibited the intrathylakoid accumulation of H +, as measured by a glass electrode. The preactivated thylakoid ATPase was stimulated by dibucaine, although this stimulatory effect was to some extent superimposed by an inhibition of the enzyme. However, as opposed to what is expected for an uncoupler, dibucaine did not release the pH-dependent control of electron transport. In addition, pH-dependent high-energy quenching of chlorophyll fluorescence, normally inhibited by uncouplers, was not affected by dibucaine. It is concluded that dibucaine selectively reduces the driving force of photophosphorylation, while pH-dependent control of electron transport remains largely unaffected. The data are discussed regarding current models on coupling of photophosphorylation and on regulation of electron transport.