Symposium: Binding of upids by proteins conducted Ly The American Oil Chemists舗 Society at its 37tn pall meeting, Minneapolis, Minnesota September 30舑Octoter 2, 1963

Abstract

Steroid compounds form dissociable complexes of low binding energy with numerous proteins of different origin as can be demonstrated by various physicochemical procedures. This interac‐tion has definite physiological consequences in case of the steroid hormones. The sites of attach‐ment between ख⁴‐3‐ketosteroids and human serum albumin appear to be located at the alpha side of the steroid molecule. The affinity of inter‐action with serum albumins is increased by en‐trance of electron‐repelling groups (alkyl) into the steroid, and decreased by electron‐attracting groups (‐OH; =O; halogen) (舠polarity rule舡). This rule is reversed in interactions with certain proteins which have a higher content of aliphatic hydroxyl groups. It was concluded from compe‐tition studies with higher fatty acids that the attachment of ख₄‐3‐ketosteroids to serum albumin does not take place at the anion‐binding sites. The SH‐group of serum albumin is not involved in the interaction with testosterone. The ॅ₁‐acid glycoprotein (orosomucoid) from human serum was found to have a particularly high binding affinity for progesterone. Removal of sialic acid results in a decrease of this binding affinity. Complex formation with the orosomucoid leads to physiological inactivation of progester‐one. A highly specific interaction occurs between the adrenocorticoid hormones and the corticosteroid‐binding globulin (transcortin) in serum of hu‐man and other species. For a quantitative test, the endogenous corticosteroids have to be removed by dialysis at 37C. Cortisol, corticosterone and related hormones are bound by transcortin; aldo‐sterone interacts with serum and transcortin‐containing fractions more strongly than with albumin. The 舠transcortin舡 activity of rat se‐rum increases after adrenalectomy and hypo‐physectomy; injection of corticosterone into adre‐nalectomized rats reverses this effect. The general increase of the total ॅ‐globulin fraction in adrenalectomized rats (15‐16%) is smaller than the increase in 舠transcortin舡 activity (100%). The corticosteroid‐binding serum pro‐teins of different mammalian species (rat, rab‐bit, steer, horse) were found to be ॅ‐globulins. Their binding affinities towards different cortico‐steroids will be discussed.