Characterization of progesterone membrane binding sites from porcine liver probed with a novel azido-progesterone radioligand.

A new derivative of progesterone was synthesized for photoaffinity labelling of specific binding sites in porcine liver microsomes. Using progesterone-3-(O-carboxymethyl)-oximino-(3-125I-iodo-4-azidosa licylamidobutylamine) as a photoactivatable radioligand, selective covalent labelling of proteins was detected in porcine liver microsomes at apparent molecular weights of 90-100 kDa and 60-65 kDa by SDS-PAGE and subsequent radioautography. These proteins showed different ligand specifity for various steroids tested. On blue native polyacrylamide-gels three selectively labelled proteins were found corresponding to apparent molecular weights of approximately 310 kDa, approximately 215 kDa and approximately 75 kDa, respectively. Using two-dimensional electrophoresis for the analysis of these protein complexes, the 215 kDa-band could be correlated to the 90-100 kDa-band, while the 75 kDa-band may correspond to the 60-65 kDa-band at one-dimensional SDS-PAGE, respectively. One or more of these proteins may be involved in the rapid progesterone-induced increase of intracellular Ca2+ described previously in cultured hepatic cells.