Characterization of the adenosine triphosphatase activity of Chinese hamster P-glycoprotein.

A multidrug-resistant Chinese hamster ovary cell line (CR1R12) was obtained which constitutively expresses P-glycoprotein, up to 32% by weight of plasma membrane protein. CR1R12 plasma membranes had high, drug-activated ATPase activity referable to P-glycoprotein. The specific ATPase activity in the presence of verapamil was calculated to be approximately 9 mumol/min/mg (identical to 21 s-1) at 37 degrees C, pH 7.4. KM ATP was 1.4 mM, and ADP and 5'-adenylyl imidodiphosphate were competitive inhibitors with Ki values 0.35 and 0.44 mM, respectively. 2'-dATP was a good substrate, GTP and ITP were real but poor substrates, and ADP and AMP were not hydrolyzed. Optimal pH for ATP hydrolysis was 7.3. MgATP was the preferred substrate, and CaATP was hydrolyzed very weakly. 7-Chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) covalently labeled the P-glycoprotein, and incorporation of 1.1 mol of NBD-Cl/mol of P-glycoprotein gave 100% inactivation. ATP protected against NBD-Cl inactivation. N-Ethylmaleimide was a potent inhibitor in the absence of ATP, and in its presence significant protection from inhibition could be achieved. Vanadate and fluoroaluminate were also strong inhibitors. The plasma membranes from CR1R12 cells should provide material for purification and reconstitution of P-glycoprotein and for screening of potential "multidrug-reversal" reagents by enzymic assay.