P‐glycoprotein transports corticosterone and is photoaffinity‐labeled by the steroid

Multi-drug-resistant cells overproduce a 130-180-kDa integral membrane phosphoglycoprotein known as P-glycoprotein which acts as an energy-dependent drug efflux pump. While P-glycoprotein has been shown to transport hydrophobic anti-tumor drugs out of multi-drug-resistant cells in tissue culture, its endogenous substrates remain unknown. This report shows that 3H-corticosterone can specifically photoaffinity label P-glycoprotein. Furthermore, corticosterone is effluxed from multi-drug-resistant cells by P-glycoprotein. These data suggest that corticosterone may be an endogenous substrate for P-glycoprotein.