Local Anesthetic Binding to Thylakoid Membranes. Relation to Inhibition of Light-Induced Membrane Energization and Photophosphorylation

The association o f the lipophilic tertiary amine and local anesthetic dibucaine with osmotically shocked chloroplasts o f Spinacia oleracea L. cv. Monatol was investigated. Dibucaine, known as an effective inhibitor o f thylakoid membrane energization and ATP synthesis, ex­ hibited three distinct binding classes with chloroplasts: partitioning in the lipid phase o f the membranes, electrostatic screening o f negative electrical charges on the thylakoid surface and light-induced association o f an as yet unknown nature. Evidence is presented that the mecha­ nism o f inhibition o f the transthylakoid pH gradient, ApH, by dibucaine is distinct from ‘clas­ sical’ amine-type uncoupling: The inhibitory effect o f dibucaine on ApH was independent o f the initial strength o f ApH. Light-induced dibucaine binding was independent o f the volume o f the intrathylakoid space and o f the strength o f ApH as varied by medium pH. Judged from a comparison o f the data on dibucaine binding and on inhibition o f ApH and photophosphory­ lation, dibucaine bound via partitioning in the membrane lipid phase is responsible for the uncoupler-like effects o f the local anesthetic. A mechanism for the inhibition o f thylakoid energi­ zation by local anesthetic amines is discussed.