Role of fertilization promoting peptide (FPP) in modulating mammalian sperm function

Gonadins, a novel family of glutamyl-tripeptide amides present in the testis with activity in the hypophyseal–gonadal axis

Here we present a new family of endogenous peptides identified in rat testis with structure of glutamyl-tripeptide amides which are also present in plasma. These peptides have different activities in the hypophyseal-gonadal axis. Evidences showing the endocrine activities of some of the peptides are presented. In this communication we demonstrate the presence of peptides with a common structure Glu-X-Pro amide, where X can be one of the following amino acids: glutamic acid, glutamine, aspartic acid, asparagine, phenylalanine or tyrosine. These peptides have been identified by a series of chromatographies and by mass spectrometry. Some of the peptides where tested for its biological activity observing that subcutaneous administration of the peptides Glu-Glu-Pro amide, Glu-Gln-Pro amide and Glu-Phe-Pro amide were able to reduce plasma levels of testosterone and luteinizing hormone (LH) without modification of the levels of follicle stimulating hormone (FSH). The peptide Glu-Asp-Pro amide, however, produced an increase in the levels of testosterone without modifying LH or FSH levels. It is proposed that the glutamyl-tripeptide amides that reduce the levels of testosterone and LH are released from the testis and act in the pituitary via circulation in an endocrine manner. The specific inhibition of LH release is similar to that produced by inhibin on FSH release. On the other hand the peptide that increases the levels of testosterone is produced in the testis and seems to act directly in the testis in a paracrine or autocrine manner. It is proposed here a new mechanism of regulation of hypophyseal-gonadal axis, a negative feedback exerted by the glutamyl-tripeptide amides in the pituitary. Also it is proposed the generic name of gonadins for the novel family of glutamyl-tripeptide amides. We suggest that gonadins could be used in the future as drugs for treatment of different endocrine disorders, hormone-dependent cancer and as contraceptives.

Characterization of a circulating N-extended form of the thyrotropin-releasing hormone-like peptide pGlu-Glu-Pro amide in human plasma

The TRH-like peptides pGlu-Glu-Pro amide, pGlu-Phe-Pro amide, and pGlu-Gln-Pro amide were isolated and identified some years ago, and these peptides have been proven to be present in many tissues and fluids. The presence of TRH-like immunoreactivity distinct from TRH in blood has been observed previously. In the present study, the presence of N-extended forms of TRH-like peptides in plasma has been investigated. Peripheral blood samples of human, rat, and rabbit were obtained and plasma was extracted. The peptides were separated in several steps of chromatography, including gel filtration, cation and anion exchange, and HPLC. The concentrations of the TRH-like peptides in the column fractions were measured by RIA with TRH antibody. The N-extended forms of TRH-like peptides were determined by RIA after trypsin digestion. In human plasma it was observed an N-extended form of TRH-like peptides in substantial concentration. After trypsin and heating, the N-extended forms of TRH-like peptides were rechromatographed on Sephadex G-50. This showed that the TRH-like peptides released have a similar size to TRH. The peptides were then separated by cation exchange chromatography, and the major fraction was unretained, indicating a neutral or acidic nature. Part of the unretained fraction was then chromatographed on anion exchange column in which the major fraction was retained, demonstrating the acidic nature of the peptides. Similar results have been observed in rat and rabbit. The other part of the unretained fraction from cation exchange chromatography of human plasma was purified on HPLC. The results demonstrated that the major component observed by HPLC cochromatographed with synthetic pGlu-Glu-Pro amide. This study represents the first demonstration of a circulating N-extended form of any TRH-like peptide.