Abstract
Kinetic and equilibrium studies are presented for the reversible binding of [(3)H]dexamethasone by "specific" macromolecular receptors in the cytoplasmic fraction of cultured rat hepatoma cells. As in the case of the nuclear receptors in the same cells, the binding affinities of various steroids for the cytoplasmic receptors are closely correlated with the activities of these compounds as inducers of both tyrosine aminotransferase (EC 2.6.1.5) and cell adhesiveness. This suggests that the binding reaction is important for the biological effects of the hormones. Steroid-binding activity is inhibited by various proteases, mercurials, and 1 M KCl, but not by DNase or RNase. The receptors sediment in sucrose gradients in 0.5 M KCl near 4S, and at lower ionic strength near 7S; some of their physical properties are altered upon binding steroid. Bound dexamethasone can be recovered from the receptors as the unaltered steroid.