Equilenin: a specific fluorescent probe for steroid-protein interactions in sex steroid-binding protein

Equilenin, a naturally fluorescent steroid, has high binding affinity for human sex steroid-binding protein (SBP). At 4 degrees C the equilibrium association constant is approximately 6 X 10(7) M-1. The fluorescence excitation and emission spectra of the steroid-protein complex indicate that both hydrophobic interactions and hydrogen bonding of the 3'-hydroxyl group of the estrogen are important in its binding to the protein. Equilenin has a substantially different 3-dimensional spatial configuration compared with the normally bound androgens, and yet exhibits very tight binding to SBP. This suggests that SBP undergoes a conformational change to accomodate equilenin.