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Metabolism and Enzymology

Lidocaine and ATPase Inhibitor Interaction with the Chloroplast Envelope ¹

Horticulture Department, Cook College, Rutgers-The State University of New Jersey, New Brunswick, New Jersey 08903

Photosynthetic capacity of isolated intact chloroplasts is known to be sensitive to K⁺ fluxes across the chloroplast envelope. However, little is known about the system of chloroplast envelope proteins that regulate this K⁺ movement. The research described in this report focused on characterizing some of the components of this transport system by examining inhibitor effects on chloroplast metabolism. Digitoxin, an inhibitor of membrane-bound Na⁺/K⁺ ATPases, was found to reduce stromal K⁺ at a range of external K⁺ and inhibit photosynthesis. Scatchard plot analysis revealed a specific protein receptor site with a K_m for digitoxin binding of 13 nanomolar. Studies suggested that the receptor site was on the interior of the envelope. The effect of a class of amine anesthetics that are known to be K⁺ channel blockers on chloroplast metabolism was also studied. Under conditions that facilitate low stromal pH and concomitant photosynthetic inhibition, the anesthetic, lidocaine, was found to stimulate photosynthesis. This stimulation was associated with the maintenance of higher stromal K⁺. Comparison of the effects on photosynthesis of lidocaine analogs which varied in lipophilicity suggested a lipophilic pathway for anesthetic action. The results of experiments with lidocaine and digitoxin were consistent with the hypothesis that a K⁺ channel and a K⁺-pumping envelope ATPase contribute to overall K⁺ flux across the chloroplast envelope. Under appropriate assay conditions, photosynthetic capacity of isolated chloroplasts was shown to be much affected by the activity of these putative envelope proteins.

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