Synthesis of structural analogs of the 13 – 19 sequence of human growth hormone releasing factor

Development of the principles of synthesis of bioregulators responsible for the secretion of natural hormones in the organism is among currently important tasks. One particular basic problem consists in finding previously unknown peptide agents stimulating the secretion of insulin, a hormone playing a key role in the regulation of metabolic processes in higher mammals and humans. One possible approach to solving this problem is via the synthesis of peptide structures capable of participating in hormone – receptor interactions and enhancing insulin secretion by cells in Largenhans’ islets in the pancreas. A real pathway to such peptide stimulators of insulin secretion can be based on the synthesis of previously unknown structural analogs of the amino end region of somatoliberin, the hypothalamic releasing factor of human growth hormone [1, 2]. Our previous investigations showed that this R-factor is capable of indirectly influencing insulin biosynthesis and secretion by pancreatic islet cells. However, from the standpoint of pharmacology, a significant disadvantage of using natural somatoliberin as a bioregulator of insulin secretion is its relatively low activity and insufficient selectivity. In addition to weakly stimulating the secretion of insulin, natural somatoliberin enhances (to a much greater extent) production of growth hormone and activates the secretion of glucagon. For this reason, much better prospects in solving the problem under consideration can be related to the amino end region of somatoliberin. This conclusion agrees with the results of computer simulation of hormone – receptor interactions involved in the biosynthesis and secretion of insulin. In this context, we have undertaken a special investigation aimed at the development of a method for the synthesis of previously unknown structural analogs of the amino end region of somatoliberin, which can be of interest as potential stimulators of insulin secretion. After a series of preliminary experiments and a comparative analysis of the results, it was concluded that the most promising approach is that based on the use of activated esters of N-protected amino acids. This approach leads to a high yield of the target compounds, excludes undesired side reactions, allows effective control over the racemization processes, and is less tedious than the other possible schemes. As a result, we have developed an effective method for the synthesis of previously unreported structural analogs of the 13 – 19 somatoliberin peptide sequence, which is a part of the amino end of somatoliberin. Distinguishing features of the proposed method are as follows: (i) use of benzyl esters of oligopeptides as amino components; (ii) use of p-nitrophenyl or pentafluorophenyl esters of N-protected amino acids as carboxy components; (iii) use of an acid-labile tert-butyloxycarbonyl protection for masking -amino groups in the carboxy components; (iv) stepwise peptide synthesis in the C N direction. An important advantage of the proposed method is the possibility of using protected derivatives of trifunctional amino acids for the synthesis of hydrophobic analogs of somatoliberin. It was also established that the isolation of target compounds from the reaction mixtures is considerably simplified by conducting the condensation stage in DMF. In this case, the target peptides can be isolated in a crystalline state and purified by recrystallization. The structure of peptides obtained using the proposed scheme is uniquely determined by the scheme of synthesis. The synthetic analogs of the terminal region of somatoliberin can be obtained in an analytically pure form.