Transbilayer movement of phosphatidylserine in erythrocytes. Inhibitors of aminophospholipid transport block the association of photolabeled lipid to its transporter

The ability to cross-link [¹²⁵I]iodo-azido-phosphatidylserine (¹²⁵IN₃-PS) to the putative 32-kDa aminophospholipid transporter of human red blood cells (RBC) has been examined by SDS-PAGE. In the absence of transport inhibitors, ¹²⁵I-N₃-PS preferentially labeled the 32-kDa polypeptide, whereas treatment of the cells with pyridyldithioethylamine (PDA), a potent inhibitor of the aminophospholipid translocate, abrogated the association of the probe to this protein. ATP-depletion, low temperature, and diamide or 5,5′-dithiobis(2-nitrobenzoic acid), inhibitors that oxidize an endofacial sulfhydryl distinct from the PDA-sensitive site (Connor, J. and Schroit, A.J. (1990) Biochemistry 29, 37-43), also blocked association of the PS analogue to the protein. One ¹²⁵I-N₃-PS became associated with the transporter, however, only PDA was able to partially displace it. These data suggest that sulfhydryl reactive reagents inhibit PS transport by blocking the association of PS with its transporter, a process that is also ATP- and temperature-dependent.