Specific, high affinity relaxin-like factor receptors

The relaxin-like factor (RLF) is a circulating hormone that binds to specif ic membrane-bound uterine receptors in the mouse. Mono-iodinated RLF tracer s were produced and characterized specifically to study the properties of t he RLF receptor. The tracers bound to the RLF receptor in uterine crude mem brane preparations with high affinity (73 nM for I-125-Tyr(A9) RLF and 90 n M for I-125-Tyr(A26) RLF) as determined by Scatchard analysis. The specific ity of binding was confirmed by chemical cross-linking experiments. Binding of 125I-Tyr(A9) RLF to the putative receptor was inhibited in the presence of a 640-fold excess of unlabeled human RLF but not by the same excess of human relaxin. SDS-gel electrophoresis of the RLF-receptor complex revealed a molecular mass of >200 kDa, which remained unchanged upon reduction. The size and the lack of subunit structure of the receptor is similar to the f eatures reported for the relaxin receptor. In this regard both, the RLF and the relaxin receptor are different from the insulin- and the insulin-like growth factor-type 1 receptors. This observation supports the relaxin-liken ess of this new factor not only toward potential target tissues but also as regards receptor features.