Azide has no effect on photophosphorylation at normal light intensities, inhibits acid-base ATP synthesis in the dark weakly, and is a moderately strong inhibitor of the ATP-Piexchange reaction occurring in the dark after activation of spinach thylakoids by light. However, azide is a potent inhibitor of the methanol-activated ATPase of thylakoids (which is not associated with proton pumping). When ATPase has been activated by light plus dithiothreitol it pumps protons during ATP hydrolysis, so the thylakoid high-energy state is maintained in darkness. Azide has no effect on this reaction. When the light + dithiothreitol-activated ATPase is maximally stimulated by the uncoupler, NH4Cl, azide becomes a strong inhibitor. We suggest that inhibition of ATPase by azide occurs only when the net thylakoid protonmotive force differential is less than maximal. © 1988.
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