ATP-DEPENDENT HUMAN ERYTHROCYTE GLUTATHIONE-CONJUGATE TRANSPORTER - I- PURIFICATION, PHOTOAFFINITY-LABELING, AND KINETIC CHARACTERISTICS OF ATPASE ACTIVITY

Dinitrophenyl S-glutathione (DNP-SG) ATPase is a 38 kDa membrane prote in expressed in erythrocytes and other tissues. Although stimulation o f ATP hydrolysis catalyzed by DNP-SG ATPase has been demonstrated in t he presence of several structurally unrelated amphiphilic ions, struct ural and functional properties of this protein have not been well-defi ned. In the present study, we have developed an improved protocol for the purification of DNP-SG ATPase and investigated its kinetic and sub strate-binding properties. The purification procedure was based on hig hly specific elution of the 38 kDa protein from DNP-SG affinity resin in the presence of ATP. The protein could not be eluted using either A DP or adenosine-5'-[beta,gamma-methylene]triphosphate (methylene-ATP), a nonhydrolyzable analogue of ATP. Doxorubicin (DOX), a weakly basic anthracycline chemotherapy agent, was found to be the preferred activa tor for stimulation of ATP hydrolysis by the enzyme. ATP binding to th e enzyme was demonstrated using 8-azido-ATP photoaffinity labeling and binding of trinitrophenyl (TNP)-ATP, a fluorescent analogue of ATP. T he photoaffinity labeling of DNP-SG ATPase (38 kDa) was saturable with respect to 8-azido ATP (K-d = 2 mu M), indicating that the enzyme was capable of specific and saturable binding to ATP. DNP-SG binding was evident from the purification procedure itself and was also demonstrab le by quenching: of tryptophan fluorescence. Results of quenching of t ryptophan fluorescence as well as radioactive isotope-binding studies indicated that DOX was bound to the purified protein as well.