Isolation of ACTH1-39,ACTH1-38 and CLIP from the calf anterior pituitary

Neuropeptide profiling of the bovine hypothalamus: thermal stabilization is an effective tool in inhibiting post-mortem degradation

The hypothalamus is the central regulatory region of the brain that links the nervous system to the endocrine system via the pituitary gland. It synthesizes and secretes neuropeptide hormones, which in turn act to stimulate or inhibit the secretion of pituitary hormones. We have undertaken a detailed MS investigation of the peptides present in the bovine hypothalamus by adapting a novel heat stabilization methodology, which improved peptide discovery to direct our studies into the molecular mechanisms involved in bovine reproduction. The untreated samples contained large numbers of protein degradation products that interfered with the analysis of the neuropeptides. In the thermally stabilized samples, we were able to identify many more neuropeptides that are known to be expressed in the bovine hypothalamus. Furthermore, we have characterized a range of post-translational modifications that indicate the presence of processed intact mature neuropeptides in the stabilized tissue samples, whereas we detected many trimmed or truncated peptides resulting from post-mortem degradation in the untreated tissue samples. Altogether, using an optimized workflow, we were able to identify 140 candidate neuropeptides. We also nominate six new candidate neuropeptides derived from proSAAS, secretogranin-2 and proTRH.

Beta-endorphin-containing proteins in the human pituitary

Two new proopiomelanocortin (POMC)-derived beta-endorphin (BE)-containing proteins were detected in the human pituitary, using HPLC, trypsin digestion, and a high sensitivity search with liquid secondary ion mass spectrometry (LSIMS) for the protonated molecule ion, (M + H)+, of tryptic peptides that are unique to BE. Proteins were extracted from pituitary tissues and were purified by solid phase extraction (SPE) chromatography and RP-HPLC. Each HPLC fraction was treated with trypsin, and each unseparated peptide mixture was analyzed by LSIMS to detect the two selected marker peptides (BE 20-24 and BE 10-19) that have excellent LSIMS desorption-ionization properties. The detection of both of those peptides indicated the presence of BE-containing proteins in two HPLC fractions (number 47 and 51). Tandem MS determined the amino acid sequence of the marker peptide BE 20-24 (NAIIK), and those sequence data optimized the specificity of the method. The two new BE-containing proteins derive from the C-terminal region of POMC, and were minor components in the two HPLC fractions. The major component in fraction 51 derived from the vasopressin-neurophysin 2-copeptin precursor.

Isolation and full structural characterisation of six adrenocorticotropin-like peptides from porcine pituitary gland. Identification of three novel fragments of adrenocorticotropin and of two forms of a novel adrenocorticotropin-like peptide

A partially purified fraction of extracted porcine pituitary glands which possesses lipolytic and adrenocorticotropic activity has been characterised. It consists of six adrenocorticotropin(ACTH)-like peptides (five of which have not been previously described) which were each purified by sequential reverse-phase (rp) HPLC. Their complete primary structures were determined following amino acid compositional analysis, extensive peptide mapping and partial sequencing. Four of the fragments represent the following ACTH fragments; ACTH(1-31), ACTH(7-34), ACTH(7-36) and ACTH(7-38). By combined analytical rpHPLC and an ACTH radioimmunoassay (with an antiserum exhibiting full cross-reaction with all six ACTH variants isolated here), evidence was obtained from analysis of extracts of whole pituitary that these fragments of ACTH exist in significant amounts relative to intact ACTH(1-39). This suggests that ACTH can undergo more extensive differential proteolytic processing than previously thought. These peptides were found to possess reduced or a complete absence of ACTH-like biological activity. Therefore the biological significance of this processing needs to be resolved. The other two fragments also resembled fragments of ACTH but each possessed the same, single amino acid substitution: a threonine replacing the arginine at the position corresponding to position 8 in the ACTH sequence and had the structures [Thr8]ACTH(1-31) and [Thr8]ACTH(7-31). They possess little ACTH-like biological activity. If these variants are derived from a variant ACTH, this would be a significant finding in view of the site of the amino acid substitution and the highly conserved nature of the ACTH primary structure. The possible physiological and genetic implications are briefly discussed. In this study attempts were also made to identify the DNA coding for the mutant ACTH sequence.

Production of immunoreactive adrenocorticotropin and beta-endorphin by hypothalamic and extrahypothalamic brain cells

Despite many in vivo studies, little is known about brain regulation of POMC synthesis or regulation of secretion of POMC-related peptides. To test the hypothesis that dissociated brain cells in culture can produce and release POMC-related peptides, immunoreactive (IR)-adrenocorticotropin (ACTH) and beta-endorphin were measured in cells and media of dissociated cell cultures incubated up to 38 days. Fetal rat hypothalamic and extrahypothalamic forebrain cells were maintained in serum free medium. IR-ACTH and beta-endorphin were measured by radioimmunoassay in concentrated cells and media after various incubation times using two ACTH (mid-portion = R4; carboxy-portion directed = KEND) antisera and a beta-endorphin antiserum. IR-ACTH and IR-beta-endorphin in hypothalamic and extrahypothalamic cells and in media (cumulative) were greater than quantities in cells before culture. Peak hypothalamic cellular content of IR-ACTH (5.3 fmol/10(6) cells-R4; 4.7 fmol/10(6) cells-KEND) and content of IR-beta-endorphin (32.0 fmol/10(6) cells) occurred on days 16, 9 and 23, respectively. Peak extrahypothalamic content of IR-ACTH (2.9 fmol/10(6) cells-R4; 1.0 fmol/10(6) cells-KEND) and content of IR-beta-endorphin (10.8 fmol/10(6) cells) was also seen on different days, was lower than hypothalamic content and was not always concurrent with peak hypothalamic content. Gel filtration chromatography revealed that the predominant forms of IR-ACTH and IR-beta-endorphin in hypothalamic cell extracts co-eluted with synthetic ACTH1-39 and beta-endorphin. Changes in molar ratios of IR-ACTH and IR-beta-endorphin also suggested a differential regulation of different POMC derivatives.

CONCLUSIONS

(1) IR-ACTH and IR-beta-endorphin are produced by hypothalamic and extrahypothalamic forebrain cells in culture: and (2) dissociated brain cell cultures can be used as a potential model for studying regulation of POMC-related peptides in brain.

Effects of ethanol withdrawal on beta-endorphin levels in rat brain and pituitary

Rats which received a liquid diet containing 6.5% (w/v) ethanol for three weeks became tolerant to the hypothermic effect of an acute dose of ethanol. Withdrawal of this diet was followed by loss of the tolerance within 3 days, and by an accompanying pattern of changes in levels of immunoreactive-ir-beta-endorphin in several brain regions. An initial decrease in levels on days 1 and 3 of withdrawal was followed by recovery to control levels on days 8 and 15. This pattern was found in the arcuate nucleus, amygdala, septum, periventricular thalamus and pre-optic periventricular hypothalamus (POPH), but was statistically significant only in the POPH. A different pattern of change in ir-beta-endorphin levels was found in the pituitary. The anterior lobe showed a significant depletion of ir-beta-endorphin levels before alcohol withdrawal, which recovered by day 8 of withdrawal. This depletion was probably not related to the loss of tolerance to ethanol but was a response to a perturbation of the hypothalamic-pituitary axis of hormonal control.

Ontogeny of glucagon-like immunoreactive peptides in rat intestine

The ontogeny of the intestinal glucagon-like peptides was investigated in rats between 16 days of gestation and 4 postnatal days. The intestinal content of glucagon-like immunoreactive (GLI) peptides increased from 0.09 +/- 0.02 pmol/nmol protein at 16-17 days to plateau at 2.8 +/- 0.4 pmol/nmol protein by 20 days of gestation (P less than 0.001). The apparent immunoreactive glucagon (IRGa) content of the gut ranged from 0.03 +/- 0.01 to 0.08 +/- 0.01 pmol/nmol protein. No developmental trends in IRGa peptide content were observed. Following gel filtration of intestines extracted from rats of 18 days of gestation or greater, two main GLI peptides were detected with apparent mol. wts. of 11-12 and 5-6 kDa. Significant peaks of GLI peptides were not detected following gel filtration of intestines extracted from 16- or 17-day fetuses, nor were peaks of IRGa found at any age. In conclusion, the fetal rat intestine undergoes maturational development between 17 and 19 days of gestation to produce the GLI peptides.

Structure and bioactivity of the amino-terminal fragment of pro-opiomelanocortin

The primary sequence of the amino-terminal or 16 K fragment (16 K) of pro-opiomelanocortin (POMC) is highly conserved throughout the mammals. This suggests an important biological role for this peptide. We have performed studies to determine the structure, biosynthetic origin and bioactivity of this pituitary peptide. A comprehensive study of all the biosynthetic derivatives of POMC in the neurointermediate lobe of the rat and mouse pituitary was undertaken. Inspection of the amino acid composition of these peptides indicated that cleavage at all available dibasic processing sites within POMC is essentially complete except for Arg49-Lys50 within the 1-74 16 K fragment (16 K1-74). Only about 50% of 16 K1-74 was found to be processed to give rise to the extreme amino-terminal 1 to 49 sequence (16 K1-49) and the carboxyl-terminal 50 to 74 sequence (Lys1 gamma 3 melanotropin). Sufficient 16 K1-77 and 16 K1-49 was purified from bovine posterior pituitaries in order to determine if there are any structural features controlling the limited degree of processing of 16 K within the intermediate lobe. Both bovine 16 K1-77 and 16 K1-49 were found to have cystine bridges linking cystine residues 2 and 24 and linking cystine residues 8 and 20. While 16 K1-77 was found to be O-glycosylated at threonine45 and N-glycosylated as asparagine65, 16 K1-49 was found to have no carbohydrate content. Thus the presence of O-glycosylation at threonine45 apparently inhibits cleavage at -Arg49-Lys50-. Lys1 gamma 3 MSH 16 K1-74 and 16 K1-49 purified from rat neurointermediate pituitaries were tested for their ability to potentiate the action of corticotropin (ACTH) in an isolated rat adrenal cell bioassay. None of the 16 K-related peptides showed any intrinsic steroidogenic activity. Experiments were performed in which dispersed adrenal cells were incubated with serial dilutions of ACTH. Constant amounts of test peptides were added in concentrations ranging from 10 pM to 5 nM. Lys1 gamma 3 MSH potentiated the steroidogenic activity of ACTH by up to 2-fold with an ED50 of approx 0.5 nM. 16 K1-49 showed no ability to potentiate the action of ACTH. In contrast the most highly glycosylated form of 16 K1-74 potentiated the action of ACTH by up to 6-fold.

Use of Pico-Tag methodology in the chemical analysis of peptides with carboxyl-terminal amides

A chemical method has been established for the detection of carboxyl-terminally amidated peptides in tissue extracts. Tissue was homogenized in an acidic medium designed to solubilize peptides while precipitating high-molecular-weight protein. The homogenate supernatant was in turn subjected to reversed-phase extraction with C18 Sep-Pak cartridges. The eluates were fractionated by reversed-phase high-performance liquid chromatography (RP-HPLC). Individual fractions were exhaustively digested with thermolysin, derivatized with phenylisothiocyanate (PITC), and then subjected to ethyl acetate extraction under basic conditions. The phenylthiocarbamyl (PTC)-amino acid amide derivatives were selectively taken up into the organic phase, while the other digestion products remained in the aqueous phase. The organic phase was analyzed by RP-HPLC on a Pico-Tag amino acid analysis column, monitoring eluates at 254 nm. PTC-amino acid amides were identified and quantitated by comparing their elution positions and peak areas, respectively, with those of standards. Their identities were confirmed by amino acid analysis, following hydrolysis with hydriodic acid. The technique was applied to extracts of bovine posterior pituitaries and a human medullary thyroid carcinoma. Vasopressin (-Leu-Gly-amide), oxytocin (-Gly-amide), Lys1 gamma 1-melanotropin (-Phe-amide), and various acetylated and non-acetylated forms of alpha-melanotropin (-Val-amide) were identified in the posterior pituitary extract. Various forms of calcitonin (-Val-Gly-Ala-Pro-amide) were detected in the tumour extract. For vasopressin and calcitonin the thermolytic digest resulted in di- and tetra-peptides, respectively, reflecting thermolytic cleavage at more favoured sites.

Use of reversed-phase and ion-exchange batch extraction in the purification of bovine pituitary peptides

Bovine posterior pituitaries were extracted with an acidic medium designed to maximize solubilization of peptides while precipitating high-molecular-weight protein. The supernatant was then extracted with C18 reversed-phase cartridges to generate a peptide-enriched fraction. Cartridge eluates were subjected to ion-exchange extraction, using a batch procedure which fractionated the peptides into basic, acidic, and neutral pools. Amino-terminal fragments of bovine pro-opiomelanocortin were found to be resolved into separate pools by this method. The 1 to 49 fragment was eluted in the acidic pool while the 1 to 77 fragment was eluted in the basic pool. The 1 to 77 fragment was purified by reversed-phase high-performance liquid chromatography. Amino acid analysis of the fragments, generated from trypsin and V8 protease digestion of the 1 to 77 fragment, permitted assignment of cystine bridges between residues 2 and 24 and between residues 8 and 20. Results from amino sugar analysis were consistent with the presence of an O-linked oligosaccharide at threonine45 and an N-linked oligosaccharide at asparagine65.

Isolation and characterization of the 1 to 49 amino-terminal sequence of pro-opiomelanocortin from bovine posterior pituitaries

An amino-terminal fragment of pro-opiomelanocortin (POMC) has been isolated from bovine intermediate pituitaries by reversed-phase high-performance liquid chromatography. Peptide mapping and amino acid analysis indicated that the primary sequence corresponds exactly to that predicted by cDNA techniques for the first 49 residues of POMC including the presence of four cysteine residues. This peptide is almost certainly generated together with the gamma-melanotropins during the biosynthetic processing of the 16K amino-terminal fragment of bovine POMC. Careful analysis of the fragments resulting from V8 protease and trypsin digestion has permitted assignment of cystine bridges between residues 2 and 24 and between residues 8 and 20.

Novel variants of adrenocorticotrophic hormone in porcine anterior pituitary

Extracts of porcine anterior pituitary contain several corticotrophic variants of ACTH 1-39. They were isolated by adsorption chromatography, ion-exchange chromatography and reverse-phase high-performance liquid chromatography. Four variants were then identified as starting and ending at positions corresponding to ACTH 1-38, 1-37, 7-39 and 7-38. Several of these fragments were recovered in chromatographically multiple forms. Although all fragments isolated had corticotrophic activity (measured on isolated rat adrenal cells) those with a shortened N-terminal region had a lower potency than those with an intact N-terminal region. Corticotrophic activity of porcine ACTH 7-38 was detected in a preparation with a beta-aspartyl shift at position 25. However, in agreement with previous studies, synthetic human ACTH 7-38, in which an intact Asn-Gly bond was structurally proven, possessed no such activity. The results indicate that position 25 as well as positions 1-6 are important for corticotrophic activity, and that a deamidative beta-aspartyl shift at position 25 can influence the activity.

Adrenocorticotropin: ACTH1-38 is a major product of biotransformation by brain synaptic membranes

Since adrenocorticotropic hormone is found in the brain, and several of its fragments affect adaptive behavior, the formation of fragments of ACTH1-39 by a rat brain synaptic membrane fraction was investigated. Following the incubations at physiological pH conditions, the digests were fractionated by HPLC to quantitate the amounts of ACTH1-39 remaining and products formed. Time- and enzyme-dependent disappearance of ACTH1-39 was accompanied by the accumulation of a major peptide metabolite (product B). Amino acid analysis and NH2-terminal end-group determination revealed that product B was identical to ACTH1-38. These results indicate the predominance of carboxypeptidase activity in the degradation of ACTH1-39 by brain synaptic membranes.

Biosynthesis of phosphorylated forms of corticotropin-related peptides

Phosphorylated forms of corticotropin[ACTH (1-39)], corticotropin-like intermediary lobe peptide[CLIP, ACTH (18-39)], and the common precursor for ACTH and beta-lipotropin (beta-LPH) have been identified in extracts of rat pituitaries, 32P-Labeled inorganic phosphate was successfully incorporated into ACTH (1-39), CLIP, and the ACTH/beta-LPH precursor in rat neurointermediary lobe explants and into ACTH (1-39) in isolated rat anterior pituitary cells. After peptidase digestion of the labeled CLIP and ACTH, the radioactive phosphate was recoverable as O-phosphoserine. The serine residue at position 31 was the only amino acid found to be phosphorylated in CLIP and ACTH (1-39). The unphosphorylated forms of both peptides were also synthesized. The demonstration of he incorporation of [32P]phosphate into CLIP, ACTH (1-39), and the ACTH/beta-LPH precursor is consistent with the hypothesis that, within the rat intermediary lobe, phosphorylated CLIP is derived from a phosphorylated form of the common precursor, with phosphorylated ACTH (1-39) acting as a biosynthetic intermediate.