Effect of disulfide-bond reducing agents on the specific binding of growth hormone to microsomal membrane preparations from rabbit liver.

The effect of the disulfide-bond reducing agents, mercaptoethanol (ME) and dithiothreitol (DTT), on the specific binding of 125I-labeled human growth hormone (hGH) to microsomal membrane preparations from rabbit liver was investigated. The presence of ME or DTT caused a time- and dose-related inhibition of [125I]hGH binding to both particulate and solubilized somatotrophic receptors of rabbit liver membranes. Maximum inhibition was 20-30%. Disulfide bond reduction also caused a marked increase in the extent of reversibility of [125I]hGH binding. These effects were not due to effects on the GH, itself, but appeared to be directed at the receptor. Scatchard analysis showed that ME and DTT caused a change in the nature of the binding interaction, with at least partial conversion of receptors into sites with reduced affinity. These data together suggest that the partial effect of reducing agents on somatotrophic receptors of rabbit liver may reflect two types of receptors within the microsomal membrane preparations--one (approximately 30% of total receptors) involving disulfide bonding and the remaining type (70%) being independent of disulfide bonds--or, alternatively, that the state of reduction affects a single class of receptors in a rather more complex manner. These studies provide further intriguing insights into the overall mechanism(s) involved in the interaction of GH with its target cell receptors.