Characterization of the hydrophobic interaction of steroids with endoplasmic reticulum membranes by quenching of 6,8(14)-bisdehydro-17α-hydroxyprogesterone fluorescence

Abstract

The fluorescence behaviour of 4,6,8(14)-trien-3-one steroids, which exhibit fluorescence in protic media but no fluorescence in hydrophobic environments, was used to characterize the molecular nature and temperature-sensitivity of steroid hormone—biomembrane interactions. Since 17α-hydroxyprogesterone as the key intermediate is known to accumulate in smooth endoplasmic reticulum membranes in the course of adrenal and testicular steroid hormone biosynthesis, its fluorescent analogue, 6,8(14)-bisdehydro-17α-hydroxyprogesterone (BDHP), was used as the probe molecule. With rat testis microsomal membranes and liposomes, fluorescence quenching in the presence of membranes (related to fluorescence in aqueous solution) was independent on steroid concentration but was dependent on membrane lipid concentration in terms of a hyperbolic function. Complete fluorescence loss occurred at infinite lipid concentration at 20°C, indicating complete insertion of the steroid probe into the hydrophobic portion of the membrane compartment. The partition coefficient K p increased with increasing temperature as a consequence of increased membrane fluidity. The result that BDHP fluorescence decreased considerably with elevated temperature in both the aqueous and the membrane milieu was interpreted as the consequence of increasing molecular mobility; this effect was much more pronounced in the aqueous than in the membrane environment. On the basis of local BDHP concentrations within the membrane phase (calculated from K p), relative fluorescene quenching was over-proportional at low temperatures; under that condition, hydrophobic interactions with rigid membrane lipid domains are obviously favoured.