GRF analogs and fragments: correlation between receptor binding, activity and structure.

GH-releasing activity in vitro was directly correlated with GRF receptor binding affinity for all hGRF analogs examined. hGRF(1-29)-NH2 analogs with Ala15-substitution (for Gly15) displayed 4-5 times higher affinity for the GRF receptor relative to hGRF(1-44)-NH2. Replacement of Gly15 with Sar15 resulted in a dramatic loss of activity and receptor binding. The present data supports the proposal that Ala15-substitution increases receptor affinity, and hence potency, due to increased amphiphilic alpha-helical interactions. Fragments of hGRF, representative of DPP-IV and trypsin-like cleavage, are inactive as a consequence of greatly diminished GRF receptor binding. These results provide a comprehensive analysis of the structural features required for both GRF receptor binding and activation.