NATURE OF LIGAND AFFINITY AND DIMERIZATION OF CORTICOTROPIN-RELEASINGFACTOR-BINDING PROTEIN MAY BE DETECTED BY CIRCULAR-DICHROISM

As the association of corticotrophin-releasing factor (CRF) with its b inding protein (BP) to form a dimer complex (CRF(2)/BP2) appears to be dependent on the nature of the ligand we have compared the circular d ichroism difference spectra after association of the BP with ovine (o) CRF, human (h) CRF and the a-helical CRF(9-41) antagonist. All three ligands caused a negative change in molar ellipticity above 210 nm, wi th oCRF having the least and hCRF the greatest effect. Below 210 nm th ere was a marked divergence of difference spectra, with the reaction w ith the natural peptides, hCRF and oCRF, resulting in a positive chang e in ellipticity, whilst that with the antagonist produced a negative change. In view of the BP spectrum indicating predominantly beta-sheet and the peptides showing mainly alpha-helix these results were interp reted as the changes above 210 nm being due to dimerization and below 210 nm to a change in the conformation of ligand on binding. The oppos ite change in alpha-helicity of the antagonist observed on binding com pared with the two natural CRF peptides could have fundamental pharmac ological implications.