COMPETITIVE, NONCOMPETITIVE AND COOPERATIVE INTERACTIONS BETWEEN SUBSTRATES OF P-GLYCOPROTEIN AS MEASURED BY ITS ATPASE ACTIVITY

We have studied the interaction between verapamil and other modulators of the P-glycoprotein ATPase from membranes of CR1R12 Chinese hamster ovary cells. Four major categories of interaction were identified. (i ) Non-competitive inhibition of verapamil's stimulation of enzyme acti vity was found with vanadate. (ii) Competitive inhibition of the ATPas e was found for the pair verapamil and cyclosporin A. (iii) Allosteric inhibition with an increase in the Hill number for verapamil was foun d in the cases of daunorubicin, epirubicin, gramicidin S and D, vinbla stine, amiodarone, and colchicine. (iv) Cooperative stimulation of ver apamil-induced ATPase activity was found with progesterone, diltiazem, amitriptyline, and propranolol. At high levels, progesterone and vera pamil mutually enhanced each other's inhibitory action on the ATPase. Our data show that the substrate binding behavior of P-glycoprotein is complex with more than one binding site being present. This informati on could form the basis for the development of improved modulators of P-glycoprotein.