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The histamine-releasing activity of some linear and cyclic analogues of bradykinin (BK) and kallidin (K) was studied on rat peritoneal mast cells and compared with that of angiotensin (AT) cycloanalogues assayed earlier. Peptide cyclization, irrespective of the main pharmacological effect of the linear precursors (hypotensive for BK and K, hypertensive for AT), considerably enhanced their histamine-releasing activity. The activity of the tested compounds was found to depend on their amphiphilicity and cycle size. Linear AT and BK and their cycloanalogues bind to different receptor structures on rat mast cells. These findings suggest that BK, K and AT cycloanalogues belong to the same group of nonimmunological mast cell activators whose specific mechanism of action is based on the common structural features resulting from cyclization.

The binding of angiotensin II and its analogues (13) to rabbit antibodies and glomerular cell receptors from rat adrenal cortex was studied, using the radioimmunoassay method and radioreceptor analysis. Double modifications introduced into the angiotensin structure were found to increase in an additive fashion its binding to the antibodies and renal cell receptors. The relative binding activity of the analogues carrying a double modification can be assessed if the activities of the analogues with the appropriate single modifications are known. It was concluded that the testing of modifications in the peptide structure for their additivity may provide some insight into the conformational properties of peptides during their binding to the protein.

A comparative study of structural and functional organization of the polypeptides -- ACTH and wasp kinin was made. The effects of fragments Lys 17, 18-ACTH11(-18)-NH2--(I) and WK4(-12)--(II), possessing "common" fragments and a cluster of basic amino-acids, on the lipolytic and steroidogenic effects of ACTH and myotropic effects of bradykinin were studied. Both fragments I and II potentiate ACTH-induced lipolysis and steroidogenesis in isolated rat fat and adrenal cells but suppress the myotropic effect of bradykinin on guinea pig ileum. The similarity of biological effects of ACTH and WK fragments support our supposition on the similarity in structurally functional organization of these peptides.

The steroidogenic and lipolytic activities of ACTH fragments (ACTH11-24--I, ACTH11-19--II, ACTH11-16--III and ACTH 17-24--IV) were studied. Fragments I--IV exert a steroidogenic effect in isolated fasciculata rat adrenal cells at concentrations of 1--500 micrograms/ml. The inner activity (alpha) and concentration at which a half-maximum effect is achieved (EC50) for fragments I and IV are 0.64+/-0.09 and 0.5--2.0 micrograms/ml, for fragment III--0.49+/-0.07 and 0.7 microgram/ml, respectively. Fragments I--IV have no effect on the lipolysis in isolated rat fat cells. The results obtained are indicative of the functional importance of fragment ACTH11-24 in manifestation of steroidogenic action of ACTH and suggest that the second active site of ACTH is enclosed within this amino acid sequence.

The binding of some modified angiotensin (AT) analogs and fragments to isolated rat adrenal glomerular cells was studied by radioreceptor analysis with a view of clarifying the role of C- and N-terminal amino acids in the binding of AT molecules to cell receptors. It was demonstrated that Arg2 and Val3 residues are of great importance for effective binding of the AT molecule to cell receptors. The presence of a free C-terminal carboxylic group in position 8 in the vicinity of the bulky lipophilic residue is a necessary condition for this process. The Asp and Asn residues located in position 1 of the AT molecule are not essential for the binding of the hormone molecule to adrenal cell receptors.

The influence of ACTH fragments, possessing structural elements, common for certain groups of peptide hormones and kinins--"common" fragments and cluster of basic amino-acids--(Lys 17,18-ACTH 11-18-NH2--I; ACTH 11-13-NH2--II; NH2CO-ACTH18-20-NH2--III) on lipolytic effect of ACTH in rat isolated fat cells and on the steroidogenic effect of ACTH in isolated rat adrenal cells was studied. Fragment I exerts a steroidogenic effect (alpha=0,84) at concentrations of 1--100 microng/ml. At low concentrations (10(-8)--10(-3) microng/ml) fragment I potentiates ACTH-induced steroidogenesis. Fragment I has no effect on the lipolysis;however, it potentiates ACTH-induced lipolysis at concentrations of 10--100 microng/ml. The results obtained support our previous supposition that "common" fragments are essential secondary non-specific active sites of hormones.