Heterogenetiy of glucocorticoid binders: a high affinity triamcinolone acetonide binder in bovine serum.

While investigating glucocorticoid-binding proteins in bovine tissues, a new binder was found in fresh bovine serum which exhibited high affinity for certain synthetic glucocorticoids. This serum binder was characterized using [3H]triamcinolone acetonide (TA) as the ligand. On sucrose gradients, the [3H]TA peak sedimented at 8S, which was easily distinguishable from the [3H]cortisol-transcortin peak at 4S. Unlike the tissue receptor, which showed ionically dependent transformation from 8S in equilibrium or formed from 4S, the serum binder sedimented at 8S in both hypo- and hypertonic gradients. Binding properties were evaluated employing sephadex G-50 chromatography to separate bound from free steroid. Scatchard analysis of specific [3H]TA binding data revealed a straight line. The apparent equilibrium dissociation constant (Kd) was 7.8 +/- 0.7 X 10(-8) M, and the binding capacity was 772 +/- 70 fmol/mg serum protein. Hormonal specificity was determined by a competitive binding assay and revealed the following sequence: TA (100%) > betamethasone (47%) > triamcinolone (33%) > dexamethasone (2%) = cortisol = progesterone; aldosterone, estradiol, and testosterone exhibited negligible competitive activity. The serum binder was very stable, withstanding heating to 37 C for 60 min and long term storage in the frozen state. However, binding was significantly destroyed by trypsin. The binder was absent from fresh samples of chicken, mouse, rat, rabbit, dog, monkey, and human sera and frozen horse and porcine sera, but was clearly present in commercially available frozen calf, fetal calf, and lamb sera. At this time, we are unable to define the function of this binder, although its existence in both ovine and bovine sera suggests a possible role in ruminants. However, since bovine serum is routinely employed in tissue culture studies, the presence of this glucocorticoid binder might significantly influence many experiments.