Co-operative binding sites for transported substrates in the multiple drug resistance transporter Mdr1

Suramin, a known inhibitor of ATP binding enzymes with six negatively charged sulfonic acid groups, stimulated the ATPase activity of the multiple drug resistance transporter Mdr1 in low concentrations by acting as a substrate and by increasing the affinity for both verapamil and ATP. At higher concentrations suramin inhibited the ATPase activity competitively with respect to ATP and noncompetitively with respect to verapamil. This indicates an interaction of suramin with the ATP site. Verapamil itself activated the ATPase activity of Mdr1 only at moderate concentrations, but showed substrate inhibition at higher concentrations. This was also observed for progesterone, which decreased the K-i of Mdr1 for verapamil but increased the K-m. Additionally, verapamil increased the Hill coefficient of Mdr1 for progesterone from 1.1 to 3.2. These results indicate the existence of multiple binding sites (at least four for progesterone) for transported substrate in Mdr1 and a complicated mode of interactions between them.