Molecular Pharmacology, Vol 5, 605-614, Copyright © 1969 by the American Society for Pharmacology and Experimental Therapeutics

The Interaction between Chlorpromazine Free Radical and Microsomal Sodium- and Potassium-Activated Adenosine Triphosphatase from Rat Brain

¹ Department of Pharmacology, Michigan State University, East Lansing, Michigan 48823

The inhibitory effect of chlorpromazine free radical on (Na⁺ + K⁺)-ATPase [Mg²⁺-dependent, (Na⁺ + K⁺)-activated ATP phosphohydrolase] activity was studied by means of ultraviolet irradiation to generate the free radical, with a deoxycholate- and NaI-treated microsomal fraction obtained from rat brain as the enzyme source. Treatment with NaCl, cysteine, or dithiothreitol restored the activity of the enzyme preparation inhibited by chlorpromazine free radical. Washing of the inhibited enzyme preparation with Tris-HCl buffer or addition of sodium metabisulfite or of ascorbic acid to the incubation mixture did not restore the activity of the inhibited enzyme preparation. It appeared that the inhibition by chlorpromazine free radical was of either the "pseudo-irreversible" or the irreversible type. Inhibition of the enzyme activity by chlorpromazine free radical and p-hydroxymercuribenzoate was reduced, while inhibition by ouabain was enhanced, at low KCl concentrations. These data indicate that the mechanism by which chlorpromazine free radical and p-hydroxymercuribenzoate inhibit the (Na⁺ + K⁺-ATPase system is different from that of ouabain. K⁺-dependent p-nitrophenyl phosphatase activity was as sensitive to the inhibitory effect of chlorpromazine free radical as (Na⁺ + K⁺)-ATPase activity, while Mg²⁺-dependent ATPase activity in the same preparation was significantly less sensitive.

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ACKNOWLEDGMENT We thank Mrs. Lillian Edney for expert technical assistance.