2Institute of Molecular Genetics, Russian Academy of Sciences, pl. Kurchatova 2, 123182 Moscow, Russia; fax: (495) 196-0221
* To whom correspondence should be addressed.
Received May 17, 2012; Revision received June 1, 2012
The synthetic peptide octarphin (TPLVTLFK, fragment 12-19 of β-endorphin), a selective agonist of the non-opioid β-endorphin receptor, was labeled with tritium yielding specific activity of 28 Ci/mmol. The binding of [3H]octarphin to rat adrenal cortex membranes was studied under normal conditions as well as after cold and heat shocks. It was found that under normal conditions [3H]octarphin specifically binds to the membranes with high affinity: Kd1 = 36.3 ± 2.5 nM, Bmax1 = 41.0 ± 3.8 pmol/mg protein. The specific binding of [3H]octarphin to the membranes was inhibited by unlabeled β-endorphin (Ki = 33.9 ± 3.6 nM) and the agonist of the non-opioid receptor decapeptide immunorphin (Ki = 36.8 ± 3.3 nM). Unlabeled naloxone, [Leu5]- and [Met5]enkephalins, a- and γ-endorphins, and corticotropin were inactive (Ki > 1 µM). Both cold and heat shocks decreased the binding affinity: Kd2 = 55.6 ± 4.2 nM and Kd3 = 122.7 ± 5.6 nM, respectively. In both cases, the maximal binding capacity of the receptor did not change. Thus, even a short-term thermal shock significantly affects the sensitivity of the non-opioid β-endorphin receptor of adrenal cortex membranes.
KEY WORDS: β-endorphin, naloxone, peptides, receptors, adrenal cortex