Specific binding to adrenal particulate fraction of cyclo(histidyl-proline), a TRH metabolite

Histidyl-proline diketopiperazine (cyclo(His-Pro), a metabolite of the neuropeptide thyrotropin releasing hormone, has been shown to possess intrinsic biological activities. The binding of this peptide to various tissue particulate preparations was investigated. While the peptide showed no apparent binding to particulate fractions derived from brain, pituitary, and some other tissues, binding to adrenal and liver was demonstrated. The binding of cyclo(His-Pro) to bovine adrenal cortical particles was further characterized. Binding at equilibrium was greater at 4 degrees C than at 37 degrees C. The binding was dependent on tissue concentration, showed a pH optimum between 7 and 8, and was inactivated by treatment of the particulate fraction with trypsin or by boiling. The interaction of cyclo(His-Pro) with the tissue was not associated with any metabolism of the peptide. Kinetic studies of association of cyclo(His-Pro) with adrenal cortical particles indicated a single class of binding sites with a KD of approximately 900 nM and a maximum number of sites of 92 pmoles/mg protein. The binding was stereospecific and the histidine moiety of the peptide was the major determinant of the binding. A variety of catechols, serotonin and histamine competed with cyclo(His-Pro) for binding with IC50's ranging from 17-450 muM. Cyclo(His-Pro) did not affect monoamine oxidase or adenylate cyclase activity in adrenal cortical particulate preparations.