An investigation of the interaction of steroids with human serum albumin by the spin label method

The interaction of steroid hormones with human serum albumin (HSA) has been studied by the spin label method. Preparations of HSA spin labelled with radicals I and II (HSA, RI and HSA RII) bind steroid molecules, and at pH 7.0 and 25° C this binding is accompanied by loosening of the structure of the protein in the region of addition of the labels. The rate constant of the exchange reaction K as a result of collision with paramagnetic ions for radicals I and II bound to HSA in the presence of the steroid varies only slightly. The addition of 6 methylprednisolone to the HSA RI or HSA RII system lowers the temperature sensitivity of the spin labels I and II bound to the protein; in these circumstances, the curve of log v(eff) as a function of 1/T is displaced, which corresponds to a change in the effective entropy of activation, Δs(eff) with practically no change in the effective energy of activation, E(eff). The competing nature of the binding by the HSA molecule of the hydrophobic spin label III and a steroid has been shown by the method of double paramagnetic labels. The distance between the hydrophobic label III and RI and RII covalently attached to a protein is more than 18 Å, which excludes the possibility of the direct (isosteric) interaction of the radical. The hypothesis has been put forward that the change in the conformation of HSA in the binding of steroids has an allosteric nature.