Studies on the enzymic degradation of luteinizing hormone releasing hormone by rat pituitary plasma membranes

Isocratic liquid chromatographic assay for monitoring the degradation of luteinizing hormone releasing hormone by extracts from the gastrointestinal tract of possums

A simple HPLC method to separate human luteinizing hormone releasing hormone (LHRH) from its metabolites using an isocratic elution is described. Intact LHRH and five metabolites were separated in 11.4 min. The calibration curve (peak area versus concentration) was linear over the concentration range 1.25-35 microg/ml (r(2)=0.99) with the intercept not significantly different from zero (P>0.05). Intra-day and inter-day variability of the assay was less than 5% for repeat injections of 5, 14.5 and 29 microg/ml. The method was applied to evaluate the susceptibility of LHRH to enzymes present in the lumen and mucosal extracts of the gastrointestinal tract of possums. The major degradation products of LHRH were identified by HPLC separation, amino acid analysis and mass spectrometry as LHRH (1-5), LHRH (1-4), LHRH (1-3) and LHRH (3-4).

Sequential degradation of the neuropeptide gonadotropin-releasing hormone by the 20 S granulosa cell proteasomes

The decapeptide gonadotropin-releasing hormone (GnRH) is degraded by the 20 S multicatalytic proteinase complex (proteasome EC 3.4.99.46), purified from ovarian granulosa cells, at the Tyr5-Gly6 bond and to a lesser extent at the Gly6-Leu7 bond, when incubated for 2 h at 37 degrees C. Further cleavage, at Trp3-Ser4 and Ser4-Tyr5 bonds of the neurohormone occurs only subsequently to the appearance of the initial N-terminal degradation products, (1-5)GnRH and (1-6)GnRH. Our results suggest that the sequential degradation of GnRH can serve as an important mechanism for the rapid termination of its biological activity in target cells.

Facilitation of sexual receptivity in the female rat by C-terminal fragments of LHRH

Ovariectomized female rats implanted with cannulae directed bilaterally towards the ventromedial nucleus of the hypothalamus (VMH) were primed with estrogen (2 micrograms) and tested for sexual receptivity before and after infusion of the C-terminal LHRH fragment, Ac-LHRH5-10. Animals responsive to the Ac-LHRH5-10 fragment were tested at two-week intervals for responsiveness to other LHRH fragments modified in the C-terminal or saline in the following order: LHRH5-10, des-Gly6LHRH, LHRH5-9OH, saline, and LHRH7-9OH. The lordosis-to-mount (L/M) ratio was used as an index of sexual receptivity. The quality of lordotic posturing and the number of proceptive and resistive behaviors were also recorded for each test. A one-way ANOVA performed on the difference in L/M between pre- and postinfusion tests revealed a significant overall treatment effect. The nonacetylated LHRH5-10 fragment and the LHRH5-9OH fragment were effective in enhancing the L/M ratio, whereas des-Gly6LHRH and LHRH7-9OH were not. The enhancement of sexual behavior by C-terminal fragments of LHRH was associated with a lack of proceptive behavior and moderate levels of resistive behavior. The results indicate that several C-terminal fragments of LHRH are capable of elevating the L/M ratio and suggest that amino acids in positions 6 through 9 may be important for this effect.

Studies on in vitro proteolytic sensitivity of peptides inhibiting herpes simplex virus ribonucleotide reductases lead to discovery of a stable and potent inhibitor

The nonapeptide, HSV R2-(329-337), corresponding to the subunit 2 (R2) carboxyl terminus of herpes simplex virus (HSV) ribonucleotide reductases, specifically inhibits this enzyme activity. We report here that under standard reductase assay conditions, this peptide was rapidly degraded by proteases present in the partially purified enzyme extract. The main process of proteolysis involves the successive removal of Tyr329 and Ala330, which corresponds to an aminopeptidase activity. Determination of the proteolytic susceptibility of HSV R2-(329-337) analogs showed that natural modifications which are present in the homologous varicella zoster virus (VZV) nonapeptide decreased its susceptibility to protease action 1.5-fold. Nx-acetylation, a modification known to protect peptides against aminopeptidase attacks, greatly improved the proteolytic resistance of HSV and VZV nonapeptides. Moreover, Ac-VZV R2-(298-306) exhibited a 15-fold higher potency on reductase inhibition than HSV R2-(329-337). The degradation process of HSV R2-(329-337) was partially inhibited by amastatin, bestatin, and leupeptin whereas it was completely abolished by bacitracin, suggesting a combined action of more than one aminopeptidase activity. Moreover, bacitracin protected most of these nonapeptide analogs from proteolysis, although it was less effective in preventing HSV R2-(332-337) degradation. Our results indicate that it is possible to determine, in the presence of bacitracin, the relative inhibitory potencies of HSV R2-(329-337) analogs with minimal error due to proteolytic susceptibility. Moreover, HSV R2-(329-337) modifications that were found to protect the peptide against degradation might be useful to increase its efficacy in vivo.

Degradation of gonadotropin-releasing hormones in the gilthead seabream, Sparus aurata. I. Cleavage of native salmon GnRH and mammalian LHRH in the pituitary

The pattern and kinetics of degradation of native salmon gonadotropin-releasing hormone (sGnRH) and mammalian leuteinizing hormone-releasing hormone (LHRH) by pituitary bound enzymes were studied in the gilthead seabream, Sparus aurata. sGnRH and LHRH were incubated for different periods of time with membrane or cytosolic fractions of pituitary homogenates. At the end of the incubation, the degradation mixture was fractionated on reverse-phase high-pressure liquid chromatography. The degradation products were identified by comparing their retention times to those of synthesized GnRH fragments and by analyzing their amino acid composition. The main GnRH degradative activity resides in the cytosolic fraction of the pituitary homogenate. Both sGnRH and LHRH are rapidly degraded by pituitary cytosol, with 78.3 and 87.7% of the peptides, respectively, cleaved after 3 hr of incubation. Maximal degradation of sGnRH occurred at a pH range of 7 to 8. The main initial products of degradation of sGnRH and LHRH are the 1-5, 6-10, and 1-9 fragments. This suggests the involvement of two site-specific peptidases, a Tyr5-Gly6 endopeptidase and a Pro9-Gly10NH2 peptidase or postproline cleaving enzyme. While the 1-6 and 1-9 fragments undergo rapid secondary degradation, the 1-5 is relatively stable. Competition experiments suggest that the endopeptidase cleaving the sGnRH at the Tyr5-Gly6 bond is not specific to the neuropeptide and is probably a general proteolitic enzyme. However, the cleavage at the 9-10 bond has a high degree of specificity to the Pro9-Gly10NH2 sequence found in sGnRH. The two proposed pituitary peptidases of S. aurata have some characteristics similar to those of rat hypophyseal and hypothalamic GnRH cleaving enzymes. No differences are found in hypophyseal GnRH degradative activity between females with occytes undergoing previtellogenesis or advanced stages of vitellogenesis.

Degradation of thyrotropin-releasing hormone and luteinising hormone-releasing hormone by enzymes of brain tissue

In this article, the enzymes of brain and associated tissues that can degrade thyrotropin-releasing hormone (TRH) and luteinising hormone-releasing hormone (LH-RH) are reviewed. As both TRH and LH-RH are considered to act as neurotransmitters or neuromodulators in the CNS, attention is paid to the subcellular location of the enzymes described and how their topographies and substrate specificities fit them to playing roles as inactivating agents for TRH and LH-RH or as regulators of intracellular concentrations of TRH and LH-RH. Consideration is also given to enzymes involved in biotransformation of TRH to secondary metabolites that exhibit biological activity and to enzymes involved in the metabolism of secondary metabolites.

Endopeptidase-24.15 is the primary enzyme that degrades luteinizing hormone releasing hormone both in vitro and in vivo

The concentration of luteinizing hormone releasing hormone (LHRH) (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2), which reaches the anterior pituitary via the hypothalamo-hypophyseal portal system, appears to be controlled in part by the rate of LHRH degradation within the hypothalamus and/or pituitary. Specific, active site-directed endopeptidase inhibitors synthesized in our laboratory were used to identify the enzyme(s) involved in LHRH degradation by hypothalamic and pituitary membrane preparations, and by an intact anterior pituitary tumor cell line (AtT20). Incubation of LHRH with pituitary and hypothalamic membrane preparations led to the formation of pGlu-His-Trp (LHRH1-3) as the main reaction product. Under the same conditions, addition to the incubation mixtures of captopril, an inhibitor of the angiotensin converting enzyme, led to accumulation of pGlu-His-Trp-Ser-Tyr (LHRH1-5) and, to a lesser extent, pGlu-His-Trp-Ser-Tyr (LHRH1-6). The degradation of LHRH and the formation of the N-terminal tri- and pentapeptides was blocked by N-[1-(R,S)-carboxy-3-phenylpropyl]-Ala-Ala-Phe-p-aminobenzoate (cFP-AAF-pAB), a specific, active site directed inhibitor of endopeptidase-24.15. Some inhibition of LHRH degradation and formation of the N-terminal hexapeptide was also obtained in the presence of N-[1-carboxy-2-phenylethyl]-Phe-p-aminobenzoate (cFE-F-pAB), an inhibitor of endopeptidase-24.11. Similar results were obtained with AtT20 cell membranes and with intact AtT20 cells in monolayer culture. Following cleavage by endopeptidases the C-terminal part of LHRH was rapidly degraded by aminopeptidases. Superactive analogs of LHRH in which Gly6 was replaced by a D-amino acid are resistant to degradation by both endopeptidase-24.11 and -24.15. In vivo, when LHRH was injected directly into the third ventricle of rats, the presence of cFP-AAF-pAB inhibited LHRH degradation. It is concluded that LHRH degradation is primarily initiated by the membrane-bound form of endopeptidase-24.15 to yield pGlu-His-Trp-Ser-Tyr and to a lesser extent by endopeptidase-24.11 to yield pGlu-His-Trp-Ser-Tyr-Gly.

Proteolytic inactivation of luteinizing hormone-releasing hormone (LHRH) by the whole rat ovary in vitro

Using 3H-labeled luteinizing hormone-releasing hormone (LHRH) at low concentrations, the in vitro proteolytic inactivation of the peptide hormone by whole rat ovaries was studied and compared with that by the soluble and particulate rat ovarian fraction. Whole rat ovaries were found to express the three proteolytic activities that were, according to their properties, also observed in rat ovarian homogenates: (1) soluble intracellular activity which was released into the medium, (2) released activity of membrane-bound origin, and (3) firmly membrane-bound activity. It is suggested that in vivo LHRH is largely inactivated extracellularly at least by enzymes that are located in the plasma membrane although the membrane-bound activity comprises only about 1% of the whole LHRH-inactivating capacity of the ovary.

Endopeptidase-24.11 in the adenohypophysis of the pig is localized in the gonadotrophic cells

Endopeptidase-24.11, an ectoenzyme with a key role in metabolizing peptides at cell surfaces, is present in the adenohypophysis. A specific polyclonal antibody to the endopeptidase has been used to explore its localization in cryostat sections of pig pituitary glands by an immunoperoxidase method. Immunoreactivity was symmetrically but not uniformly distributed over the anterior lobe, with the highest intensity a zone just beneath the capsule along the anterior surface. In detail, the staining was observed to be in the cell membrane, but in some cells a small area of intense paranuclear staining was also observed. Serial 5 micron sections were immunostained alternately for endopeptidase-24.11 and for pituitary proteohormone. Luteinizing hormone (LH), follicular stimulating hormone (FSH), thyrotropin, adrenocorticotropin, prolactin and growth hormone were studied in this way. It was possible to identify groups of cells in adjacent sections and a good correlation was observed for endopeptidase-24.11-immunoreactivity with that for LH and FSH. The association of the endopeptidase with gonadotrophs was confirmed by double labelling. No evidence of colocalization was observed with the other proteohormone antibodies. We conclude that among the cells of the adenohypophysis only the gonadotrophs express endopeptidase-24.11 and discuss the possible significance of this observation in regard to the termination of peptide signals, such as that of luteinizing hormone-releasing hormone (LHRH) acting at this site.

Degradation of luteinizing hormone-releasing hormone by neuroblastoma cells and their membrane: evidence for the involvement of a thiol protease and angiotensin-converting enzyme

Luteinizing hormone-releasing hormone was degraded by cells of the N-18 line of mouse neuroblastoma and their membrane. Cleavage products were separated by HPLC and identified by amino acid analysis. Fragments (1-3), (4-5), and (6-10) were major cleavage products. All the products increased in level as a function of time except for fragment (1-5), which increased in amount only during a short incubation time and then decreased. The accumulation of fragment (1-5) was increased in the presence of captopril or EDTA, whereas that of fragments (1-3) and (4-5) decreased inversely. On the other hand, the generation of either fragment (1-3) or (4-5) was stimulated by the presence of Cl-. The results suggest that the conversion of fragment (1-5) into fragments (1-3) and (4-5) is catalyzed by angiotensin-converting enzyme. p-Chloromercuribenzoate inhibited the formation of fragment (1-5), a result suggesting the involvement of a thiol protease in this formation. Thus, the degradation of luteinizing hormone-releasing hormone by neuroblastoma cells and their membrane seems to take place mainly through the cleavage of the Tyr5-Gly6 bond by a thiol protease, followed by the release of the dipeptide Ser-Tyr from the liberated fragment (1-5) by angiotensin-converting enzyme. It is further suggested that the thiol protease and angiotensin-converting enzyme are also responsible for the initial minor cleavages of the Gly6-Leu7 bond and the Trp3-Ser4 bond, respectively.

CCK26-33 degrading activity in brain and nonneural tissue: a metalloendopeptidase

Cholecystokinin octapeptide (CCK26-33) is metabolized by neural membranes with an initial cleavage to CCK29-33 and subsequent breakdown to CCK31-33 and CCK32-33; this pattern of proteolysis occurs on incubation with either P2 or purified lysed synaptosomal membranes. To determine whether the pattern of CCK26-33 proteolysis is unique to the brain and whether regional brain differences in its pathway or rate exist, we analyzed the proteolysis of CCK by synaptic membranes of various brain areas and cellular membranes of peripheral tissue. The pattern of degradation in brain did not differ among the regions studied. The overall proteolysis rate, as measured by the formation of tryptophan, was higher in the striatum than in the cortex, although CCK29-33 was formed at the same rate in both areas. In nonneural tissue, the rate of degradation was highest in liver membranes and lowest in pancreatic acinar cell preparations. Thus, it appears that degradative peptidases are not necessarily colocalized with CCK receptors. The pattern of product formation is the same in peripheral compared with CNS membranes; thus, the degradative pathway does not appear to be unique to brain tissue. The enzyme present in synaptic membranes that is responsible for CCK29-33 formation requires a metal ion and sulfydryl groups for the catalysis and thus is a metalloendopeptidase. Furthermore, its activity is inhibited by Ac-Gly-Phe-Nle-al, a peptide aldehyde whose sequence bears some homology to the amino acid sequence in the region of CCK26-33 that is cleaved by this enzyme.

GnRH secreting cultured fetal rat hypothalamic cells do not degrade GnRH

The possible degradation of GnRH in the hypothalamus was investigated. Rat fetal hypothalamic cells were kept in culture for two weeks and basal and stimulated GnRH release was measured by highly sensitive RIA. These intact hypothalamic cells did not degrade GnRH during 4 hours of incubation, but a 50% degradation occurred after 24 hours incubation followed by HPLC using specifically tritium labeled GnRH or RIA. The rate of degradation or the kinetic of degradation did not change by increasing GnRH concentration in the medium or by mechanical instead of enzymatic dispersion of the cells. For comparison GnRH degradation in homogenized hypothalamic tissue and in synaptosomal preparation was measured and rapid degradation was found. Our results suggest that intact hypothalamic cells under physiological circumstances do not degrade extracellular GnRH.

Subcellular distribution of particle-bound neutral peptidases capable of hydrolyzing gonadoliberin, thyroliberin, enkephalin and substance P

Subcellular fractions from rat anterior pituitary homogenates were obtained by differential and gradient centrifugation, identified with the help of marker enzymes and screened for peptidases capable of hydrolyzing gonadoliberin, thyroliberin, enkephalin and substance P. Since each neuropeptide is susceptible to cleavage by more than one enzyme, specific substrates or inhibitors have been used for the selective determination of the individual peptidasic activities. Among the various enzymes tested, the angiotensin-converting enzyme, the thermolysin-like metalloendopeptidase ('enkephalinase'), a thyroliberin-degrading enzyme and some aminopeptidasic activities were found to be associated with the plasma membrane. Other aminopeptidases, a gonadoliberin-degrading and a substance-P-degrading enzyme are associated with the mitochondria and thus are most likely not involved in the biological inactivation of neuropeptides.

Proteolytic degradation of gonadotropin-releasing hormone (GnRH) by rat ovarian fractions in vitro

GnRH in physiological concentrations is highly degradable by both soluble and particulate fractions of rat ovarian homogenate in vitro. The two proteolytic enzyme activities differ strongly by the soluble activity showing a dithiothreitol optimum, high inhibition by diisopropyl fluorophospate (ki = 0.7 microM), and a relatively high affinity (Km = 1.1 microM) as opposed to the particulate fraction (Ki = 3.5 mM and Km = 150 microM, respectively). The results of this study show that the rat ovary is differently endowed with GnRH-degrading activity at different sites. The involvement of these in terminating the biological activity of the hormone on the ovary may possibly depend on its exact pathway in this GnRH-target organ.

Gonadotropin releasing hormone (GnRH) is not degraded by intact pituitary tissue in vitro

The possible degradation of GnRH by intact pituitary tissue was investigated. Trypsin-, collagenase- and mechanically dispersed pituitary cells in culture and fresh pituitaries cut into eight segments were incubated with specifically tritiated GnRH or unlabeled GnRH which were detected by HPLC and RIA in incubation media. Degradation of GnRH could not be detected by either method during incubations with any of the pituitary cell cultures or fresh tissue. The results suggest that pituitary degradation is not involved in the regulation of circulating levels of GnRH.

Synthesis and biological activity of [D-Trp6] chicken luteinizing hormone-releasing hormone

An agonist of chicken hypothalamic luteinizing hormone-releasing hormone (cLH-RH), [D-Trp6] cLH-RH, was synthesized and tested for luteinizing hormone (LH)-releasing activity using dispersed chicken anterior pituitary cells, as well as for binding to rat anterior pituitary membrane receptors. cLH-RH and mammalian LH-RH (mLH-RH) gave identical dose-response curves in stimulating chicken LH release (ED50 = 1.6 and 1.8 X 10(-9) M respectively) and similar estimates of potency. The [D-Trp6] analogs of cLH-RH and mLH-RH stimulated LH release at lower doses (ED50 = 7.0 and approximately 7.0 X 10(-11) M respectively) and were approximately 20-fold more potent. In contrast to the activity in the chicken bioassay, cLH-RH bound to rat anterior pituitary membrane receptors with a much lower affinity than did mLH-RH and had a relative potency of 2%. [D-Trp6] cLH-RH was approximately 100-fold more potent than cLH-RH in the rat receptor assay while [D-Trp6] mLH-RH was 28-fold more active than mLH-RH. These data demonstrate that substitution of Gly6 of LH-RH with D-Trp enhances the LH release from chicken pituitary cells to a similar extent to that observed in mammals, and indicate that the approaches used to produce active LH-RH analogs in mammals are likely to be applicable to birds.

The regulation of LHRH action at the pituitary and gonadal receptor level: a review

Luteinizing hormone releasing-hormone (LHRH) and its highly active agonists are under clinical investigation for the control of reproductive function and for suppression of hormone dependent tumours. The regulation of LHRH action by pituitary receptors and expression of the biological LHRH effect by gonadotropin release and activation of steroid biosynthesis are discussed in this context. Pituitary LHRH receptors are controlled by autoregulation via endogenous LHRH secretion. The gonadal response to LHRH stimulation is regulated by LH action on receptors for LH, prolactin and FSH. Pituitary and gonadal inhibition are achieved by different mechanisms. Continuous exposure to LHRH blocks gonadotropin release and reduces pituitary LH/FSH content, whereas inhibition of steroid biosynthesis requires daily LH release to maintain receptor down-regulation. Pituitary enzymes involved in LHRH degradation at the receptor site are required for terminating hormone action, but their role in modulating hormonal responsiveness is secondary to receptor regulation. Direct gonadal effects of LHRH are exerted in the presence of gonadotropins by modulating the gonadotropin effect, e.g. in hypophysectomized animals. The presence of specific receptors for LHRH agonists in ovarian and testicular tissue suggests local control mechanisms for gonadotropin activation of steroid biosynthesis.