Finding inhibitors and new activators for the biliverdin transporter ABCB10.

Abstract

The absence of the mitochondrial ATP binding cassette (ABC)-transporter ABCB10 in mice is embryonic lethal, with fetuses presenting severe anemia and oxidative damage. Initially, ABCB10 was thought to be a heme transporter, but recent evidence from our group and collaborators suggests instead that ABCB10 transports biliverdin, a heme degradation product with antioxidant properties. Since some ABC transporters are promiscuous, interacting and/or transporting a large variety of drugs, it is important to determine the specificity of ABCB10 for its substrate. Commonly, ABC transporters present a basal ATPase activity that is increased by substrate binding. As expected, the substrate biliverdin significantly increases the basal ATPase activity of the transporter. Now, we are studying the effect of related compounds on the ATPase activity of purified ABCB10 reconstituted in lipid nanodiscs. We have found inhibition with pyrroles closely related to biliverdin, suggesting that specific substrate-protein interactions are required for the biliverdin-induced activation. We have also found additional compounds (pyrroles and porphyrins) that stimulate the ATPase activity of the transporter, although at a lower level than biliverdin does. Competition assays are underway to determine if these compounds bind to a single site or if there is evidence of the coexistence of multiple drug binding sites on the transporter. Finding compounds that can modulate or inhibit the activity of this essential transporter brings additional tools for needed biochemical and structural studies and opens the door for possible therapeutic purposes. Funding: 1R01GM145938-01 to MEZ.