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

The Cell Adhesion Activity of the Joining Peptide of Proopiomelanocortin

Proopiomelanocortin (POMC) is a precursor protein of several peptide hormones, such as ACTH and β-endorphin. Almost all of the peptide hormones in POMC have been drastically investigated in terms of their biological activities. However, the biological activity of the joining peptide region (JP) in POMC is unknown. Therefore, to explore the biological activity of JP, sequence analyses of mammalian POMC were performed. We found an -Arg-Gly-Asp- (RGD) motif in several mammalian species, such as porcine, suggesting that JP has cell adhesion activity. To validate this hypothesis, the cell adhesion activities of the synthetic porcine JP peptides were examined using 293T cells. Cell adhesions were observed in a concentration-dependent manner of the JP peptides. In addition, the JP peptide competitively inhibited cell adhesion to the POMC-coated plates. Moreover, the cell adhesion activity of the joining peptide was inhibited by the addition of EDTA, indicating that the JP peptide mediates the cell adhesion activity via a receptor protein, integrin. Interestingly, a human JP peptide, which possesses an -Arg-Ser-Asp- (RSD) sequence in place of the RGD sequence, exhibited a higher ability in the cell adhesion activity than that of the porcine JP peptide, suggesting that the cell adhesion activity of the joining peptide is developed during the molecular evolution of POMC. In conclusion, our results reveal that the joining peptide in POMC plays an important role during cell adhesion and provide useful information related to signal transduction of nerve peptide hormones derived from POMC.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides

Pro-opiomelanocortin (POMC) is a prohormone that encodes multiple smaller peptide hormones within its structure. These peptide hormones can be generated by cleavage of POMC at basic residue cleavage sites by prohormone-converting enzymes in the regulated secretory pathway (RSP) of POMC-synthesizing endocrine cells and neurons. The peptides are stored inside the cells in dense-core secretory granules until released in a stimulus-dependent manner. The complexity of the regulation of the biosynthesis, trafficking, and secretion of POMC and its peptides reflects an impressive level of control over many factors involved in the ultimate role of POMC-expressing cells, that is, to produce a range of different biologically active peptide hormones ready for action when signaled by the body. From the discovery of POMC as the precursor to adrenocorticotropic hormone (ACTH) and β-lipotropin in the late 1970s to our current knowledge, the understanding of POMC physiology remains a monumental body of work that has provided insight into many aspects of molecular endocrinology. In this article, we describe the intracellular trafficking of POMC in endocrine cells, its sorting into dense-core secretory granules and transport of these granules to the RSP. Additionally, we review the enzymes involved in the maturation of POMC to its various peptides and the mechanisms involved in the differential processing of POMC in different cell types. Finally, we highlight studies pertaining to the regulation of ACTH secretion in the anterior and intermediate pituitary and POMC neurons of the hypothalamus.

60 YEARS OF POMC: N-terminal POMC peptides and adrenal growth

The peptide hormones contained within the sequence of proopiomelanocortin (POMC) have diverse roles ranging from pigmentation to regulation of adrenal function to control of our appetite. It is generally acknowledged to be the archetypal hormone precursor, and as its biology has been unravelled, so too have many of the basic principles of hormone biosynthesis and processing. This short review focuses on one group of its peptide products, namely, those derived from the N-terminal of POMC and their role in the regulation of adrenal growth. From a historical and a personal perspective, it describes how their role in regulating proliferation of the adrenal cortex was identified and also highlights the key questions that remain to be answered.

Production of recombinant rat proopiomelanocortin1-74 and characterization of its mitogenic action on pituitary lactotrophs

We report the production of biologically active recombinant rat Gly-2-Ser-1-POMC1-74 (rrPOMC1-74) in a prokaryotic expression system. The polypeptide was produced as a fusion protein with glutathione-S-transferase (GST), using the pGEX-4T-1 vector and subsequently cleaved by thrombin. Amino acid sequencing, up to residue 45, showed a correct primary structure including the two additional amino acids at the N-terminus, Gly and Ser, derived from the thrombin cleavage site. Electrospray ionization mass spectrometry showed a Mr of 8358.5 Da which was 14-16 Da heavier (oxidation or methylation) than the calculated mass. Combined digestion with trypsin and endoproteinase Glu-C followed by MALDI-TOF mass spectrometry and N-terminal sequencing of the separated fragments showed a correct disulphide bridge configuration. In reaggregate cell cultures of immature rat pituitary, rrPOMC1-74 displayed biological activity similar to that of natural human (h) POMC1-76 or rat POMC1-74: it stimulated DNA replication in lactotrophs but not in other pituitary cell types. However, its efficacy was significantly lower than that of the natural product. Gamma3-MSH, a peptide that can be generated from POMC1-74 and a typical ligand of the melanocortin-3 (MC-3) receptor, also stimulated DNA replication in lactotrophs and, in contrast to rrPOMC1-74, also in somatotrophs and thyrotrophs. rrPOMC1-74 increased cAMP levels in 293HEK cells stably transfected with the MC-3 receptor with an intrinsic activity and potency similar to that of gamma3-MSH. However, natural hPOMC1-76 was inactive in the latter test system. These data show that rrPOMC1-74 mimics the selective mitogenic action of natural POMC1-74 on lactotrophs. Since natural POMC1-74 is N- and O-glycosylated and rrPOMC1-74 is not, glycosylation does not seem to determine the selectivity for lactotrophs. In spite of the feature that rrPOMC1-74 is an agonist at the MC-3 receptor and the reported evidence that the MC-3 receptor is expressed in the anterior pituitary, the mitogenic action of rrPOMC1-74 on lactotrophs does not seem to be mediated by the MC-3 receptor.

Molecular cloning and characterization of the chicken pro-opiomelanocortin (POMC) gene

The gene for pro-opiomelanocortin (POMC), a common precursor of melanocortins, lipotropins and beta-endorphin, was isolated in the chicken first among avian species. The chicken POMC gene was found to be a single copy gene and appeared to show the same structural organization as that of other species of different classes. The predicted POMC displayed the highest identity to Xenopus POMC(A) (60. 1%), and consisted of 251 amino acid residues with nine proteolytic cleavage sites, suggesting that it could be processed to give rise to all members of the melanocortin family, including adrenocorticotropic hormone and alpha-, beta- and gamma-melanocyte-stimulating hormones, as well as the other POMC-derived peptides. RT-PCR analysis detected the POMC mRNA in the brain, adrenal gland, gonads, kidney, uropygial gland and adipose tissues, each of which has been demonstrated to express melanocortin receptors. These results suggest that melanocortins act in a paracrine and/or autocrine manner to control a variety of functions both in the brain and in the peripheral tissues in the chicken.

Bilateral adrenal enucleation-induced changes in adenohypophyseal pro-opiomelanocortin (POMC)-related peptides synthesis and secretion: a comparative study with adrenalectomized rats

The aim of the present study was to elucidate the modulatory effect of transient changes in endogenous glucocorticoids, occurring after bilateral adrenal enucleation (ENUC), on anterior pituitary (AP) proopiomelanocortin (POMC)-derived peptides synthesis and output in rats. For this purpose, adult female rats were either bilaterally ENUC, adrenalectomized (ADX), or sham-operated (SHAM) and killed by decapitation 2, 7, 14, and 21 days after surgery. Trunk blood was collected for measurements of ACTH, beta-endorphin (beta-END) and corticosterone (B) concentrations; APs were quickly dissected for the determination of ACTH, beta-endorphin (beta-END)-like (beta-END-LI) and gamma 3-MSH contents and adrenal glands were removed and submitted to histological study. The results indicate that ENUC and ADX increased AP POMC-related peptides synthesis and release in association with changes in the AP processing of peptides belonging to the N-terminal (gamma 3-MSH), mid (ACTH) and C-terminal (beta-LPH/ENDs) portions of POMC. While ADX abolished plasma B levels, ENUC induced a transient (day 2) decrease in plasma B concentrations which returned to SHAM levels at 7 days after surgery. These data tallied with the histological observations carried out, indicating a time-dependent regenerative process of the adrenal which was completed by three weeks after ENUC. There was a different pattern in plasma ACTH and beta-END levels between ENUC and ADX; maximal plasma peptide levels were found 7-14 days after ENUC, then falling down to SHAM values at 21 days post ENUC. Conversely, there was a constant increment in plasma peptide levels up to 21 days after ADX. At 2 days after both ENUC and ADX all peptides measured in the AP were lower than SHAM values, thus reflecting a rapid corticotrope secretion. Thereafter, 7 or more days after surgery, AP peptide content in ADX rats increased, in a time-related fashion, up to 21 days after surgery. Only beta-END-LI showed a similar AP content to that of the SHAM group, thereafter indicating a preferential cleavage of POMC to beta-END long after ADX (21 days). ENUC rats showed increased AP POMC peptides content throughout the whole time, and it was significantly different from SHAM and ADX values 14 days post-surgery. Interestingly, we found an increment in AP gamma 3-MSH, a peptide which is preferentially synthesized in the intermediate lobe of the rat pituitary, in both ENUC and ADX situations. Our results further indicate that: 1) glucocorticoids, from regenerating adrenal origin, induce a fast negative feedback mechanism on AP secretion, and 2) there might be a delayed inhibitory action of newly synthesized corticosteroids on higher levels of the central nervous system. The lack of glucocorticoids (ADX) clearly corroborates a persistent enhancement of AP POMC-related peptides synthesis and secretion. The differences in AP processing of POMC between ENUC and ADX might be due to qualitative/quantitative changes in hypothalamic ACTH secretagogues output.

Glucocorticoid effects on the molecular forms of adrenocorticotrophic hormone secreted by cultures of rat anterior pituitary cell monolayers

The effects of glucocorticoids and corticotrophin-releasing factor (CRF) on the release of various molecular forms of adrenocorticotrophic hormone (ACTH) have been investigated in primary cultures of rat anterior pituitary. The rat cells responded to a 30 min challenge with CRF by secreting increased amounts of ACTH, as assessed both by bioassay, using rat adrenocortical cells, and by radioimmunoassay. Inclusion of a synthetic glucocorticoid, such as dexamethasone (DEX), in the incubation for 5 min prior to, and during the CRF challenge, had no effect on the response as measured by radioimmunoassay. Bioassay, however, indicated profound suppression of the response to CRF. This discrepancy between ACTH immuno- and bioactivity was investigated by fractionating the immunoreactive ACTH species using high-performance liquid chromatography. The lower molecular weight (<15kd) forms (ACTH(1-39) , phosphorylated ACTH(1-39) and glycosylated ACTH(1±39) ) were separated from higher molecular weight (>15kd) forms (i.e. ACTH biosynthetic intermediate and proopiomelanocortin) using C-18 Sep-Pak. The lower molecular weight molecules were further resolved into glycosylated and non-glycosylated ACTH, using an acetonitrile gradient high-performance liquid chromatography with trifluoroacetic acid as an ion-pairer. Neither the proportion of low molecular weight forms of ACTH, nor that of glycosylated ACTH(1-39) secreted in response to CRF, were affected by DEX. Further fractionation of non-glycosylated ACTH, also using acetonitrile gradient high-performance liquid chromatography but with heptafluorobutyric acid as the ion-pairer, yielded peaks corresponding to phosphorylated and non-phosphorylated ACTH(1-39) . DEX significantly increased the proportion of phosphorylated ACTH secreted in response to CRF by 18%. An additional effect of DEX was revealed when Sep-Pak extracts were treated with alkaline phosphatase, prior to analysis. After dephosphorylation, it became clear that the peptides released by CRF-stimulated cells were different if DEX was present in the medium. The peptide released in the presence of DEX (ACTH-S) had a slightly, but consistently, different retention time on high-performance liquid chromatography and very little biological activity. Antibody cross-reactivity studies suggested that ACTH-S was modified in the 1-24 region of the peptide. It is concluded that challenge of anterior pituitary cells in primary culture with CRF, following 5 min previous exposure to DEX, results in a molecular change. The consequence of this is that ACTH immunoreactivity is released, but the molecule has reduced biological activity. This may be part of the mechanism by which fast feedback inhibition of ACTH secretion is effected.

Effect of calcium ions on the processing of pro-opiomelanocortin by bovine intermediate lobe pro-opiomelanocortin-converting enzyme

The effect of Ca2+ on the extent and pattern of processing of pro-opiomelanocortin and an N-terminal fragment by a purified pituitary secretory vesicle, soluble aspartic endoprotease, was studied. Ca2+ stimulated the first cleavage of pro-opiomelanocortin by pro-opiomelanocortin-converting enzyme to yield 21-23 kDa adrenocorticotropin and beta-lipotropin, but its effect was minimal. The production of adrenocorticotropin from the 21-23 kDa intermediate was stimulated approximately 2.3-fold in the presence of 10 mM Ca2+, and processing of beta-lipotropin to beta-endorphin was stimulated about 1.3-1.4-fold by 5-10 mM Ca2+. The production of gamma-melanotropin-immunoreactive material from bovine N-pro-opiomelanocortin(1-77) was stimulated approximately 1.3-fold at both 100 microM and 1.5-2.0 mM Ca2+. Further characterization of the gamma-melanotropin-immunoreactive material by HPLC demonstrated that the major products were gamma 3-[Lys]melanotropin and gamma 3-melanotropin at both Ca2+ concentrations. These results indicate that pro-opiomelanocortin-converting enzyme is stimulated by Ca2+.

Differential glycosylation of N-POMC1-77 regulates the production of gamma 3-MSH by purified pro-opiomelanocortin converting enzyme. A possible mechanism for tissue-specific processing

The amino terminus of bovine pro-opiomelanocortin (N-POMC1-77) is partially processed in the intermediate lobe of the pituitary to N-POMC1-49 and lys-gamma 3-melanotropin. Two pools of N-POMC1-77 were isolated which were differentially glycosylated at threonine45, while N-POMC1-49 isolated from bovine intermediate lobe extracts existed in a non-glycosylated form. This suggested that differential O-linked glycosylation of N-POMC1-77 may regulate cleavage at the Arg49-Lys50 processing site. We tested this hypothesis by incubating N-POMC1-77 glycoforms with purified proopiomelanocortin converting enzyme. Only non-O-glycosylated N-POMC1-77 and O-glycosylated N-POMC1-77 with truncated oligosaccharide sidechains were sensitive to cleavage and generated predominantly lys-gamma 3-melanotropin, identified by high-performance liquid chromatography. These data provide the first functional evidence to support a role for differential O-linked glycosylation in the regulation of the processing of the N-terminus of bovine POMC.

Mass spectrometric charting of bovine posterior/intermediate pituitary peptides

The feasibility for charting neuropeptides in neuroendocrine tissues on the basis of the universal property and inherent specificity of their molecular weights was explored. As a model, a comprehensive MS analysis of extractable peptides from bovine posterior/intermediate pituitary was performed. Two suitable MS techniques--namely, plasma-desorption time-of-flight and fast atom bombardment MS--were evaluated, and each method could identify more than 20 peptides, including N-terminally acetylated and C-terminally amidated species. In toto these peptides account for almost the entire lengths of propressophysin, prooxyphysin, and proopiomelanocortin. Some of the experimentally determined molecular weights did not match any known peptides. Three of these species were identified as acidic joining peptide (4-24) [proopiomelanocortin(83-103)], C-terminal glycopeptide(22-39) [propressophysin(130-147)], and glycosylated C-terminal glycopeptide(1-19) [propressophysin(109-127)] by conventional sequence analysis.

A trace-enrichment technique for the loading of gel-permeation high-performance liquid chromatography columns

Gel-permeation high-performance liquid chromatography (GP-HPLC) columns provide rapid high-resolution separations but are frequently limited to analytical tasks because the injection volumes must be small. The reduction of volume required for the loading of solutes can often be impractical and lead to poor recoveries. We have developed a trace-enrichment technique to circumvent this problem. By placing a Waters Guard Pak within the loop of a Valco injector and connecting a pump to the injection port it is possible to concentrate proteins and peptides onto the guard column from relatively large volumes. Enrichment onto a reversed-phase guard column insert is achieved by loading solutes in an aqueous solution or one of low organic solvent concentration. Provided that the GP-HPLC is mean-while equilibrated with a solvent system of sufficiently high organic solvent concentration (i.e., 40% acetonitrile containing 0.1% trifluoroacetic acid) it is possible to elute material that has been loaded in this manner by simply placing the injection loop in line with the column. The solvent strength abruptly increases and the peptide or protein sample is loaded onto the column in a very small volume. We have applied this loading principle to both analytical and semipreparative problems. The amino-terminal fragment of pro-opiomelanocortin (POMC) has been extracted from a single human fetal pituitary (18 weeks gestation) and characterized in terms of its molecular weight. This study indicated that no proteolytic processing of the amino-terminal fragment of POMC takes place at this stage in development. In a larger scale application the amino-terminal fragment of POMC was purified from bovine anterior pituitaries.(ABSTRACT TRUNCATED AT 250 WORDS)

Biosynthesis of pro-opiomelanocortin is increased in the rat intermediate pituitary following denervation

In order to study the effect of pituitary intermediate lobe denervation on pro-opiomelanocortin (POMC) biosynthesis and processing, radioactive amino acids were incorporated in vitro into whole neurointermediate lobe (NIL) explants obtained from hypothalamic lesioned rats and control animals. The lesion in the basal hypothalamus removed the neural input to the intermediate pituitary and cut the neurohypophyseal neurons. One week after the lesion, approximately a 3-fold increase in the rate of synthesis of POMC peptides was found in the NIL. The content of POMC peptides was decreased. The results imply that denervation increases the rate of POMC synthesis and release, without altering the pattern of proteolytic processing.

Novel N-terminal fragments of pro-gamma-melanocyte-stimulating hormone isolated from pig pituitary

Two novel N-terminal fragment pairs of porcine pro-gamma-melanocyte-stimulating hormone (MSH)-(1-103), viz. pro-gamma-MSH-(1-30)/(2-30) and pro-gamma-MSH-(1-67)/(2-67) were characterized. A third pair of peptides of still larger size was also detected. The two characterized peptide pairs terminate at sites different from the dibasic sequences typical of prohormone cleavage. This suggests either a different processing event or proteolysis during purification; in both cases cleavages are selective since the two peptides end at distinct positions. Unlike most previously described pro-gamma-MSH forms, which begin with Trp at position 1 in pro-opiomelanocortin (position -105 in relation to the start of ACTH), the novel peptide pairs show N-terminal heterogeneity with one of the components beginning with Trp as in other forms, and the second component (present in relative amounts of 10-70%) beginning with Cys at position 2 (corresponding to position -104).

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.

Biosynthetic fate of the amino-terminal fragment of pro-opiomelanocortin within the intermediate lobe of the mouse pituitary

All the biosynthetic derivatives of pro-opiomelanocortin (POMC) were purified from an extract of 300 mouse neurointermediate pituitaries. Inspection of the amino acid composition of these peptides indicated that cleavage at all available dibasic processing sites within POMC was essentially complete except for -Arg49-Lys50- within the 1 to 74 amino-terminal sequence. Only about 50% of the 1 to 74 fragment was processed to the 1 to 49 sequence and Lys1 gamma 3MSH (i.e., the 50 to 74 sequence). The existence of these derivatives of the 1 to 74 fragment was confirmed by pulse-labelling explant cultures of mouse neurointermediate pituitaries with tritiated amino acids. Pulse/chase biosynthetic experiments indicated that the cleavage of the 1 to 74 sequence takes place 3 to 6 hours post-translation. This time course of biosynthesis suggests that the cleavage of the 1 to 74 sequence is a secretory granule event. Time course studies revealed that the minimum time required for newly synthesized derivatives of POMC to emerge from the intermediate lobe tissue was approximately 3 hours.

Reinvestigation of the disulfide bridge arrangement in human pro-opiomelanocortin N-terminal segment (hNT 1-76)

The cystine bridge structure of the amino-terminal fragment of human pro-opiomelanocortin has been reinvestigated. Highly purified amino-terminal fragment 1-76 was rapidly isolated from human pituitaries using only reverse-phase liquid chromatography (RP-HPLC). This peptide was then subjected to trypsin and V8-protease digestion and the products separated by RP-HPLC and subjected to amino acid and microsequence analysis. The results show that disulfide bridges link Cys-2 to Cys-24 and Cys-8 to Cys-20. Amino acid analysis and amino sugar determination confirm (i) the previously proposed sequence and (ii) the suggestion of the presence of two glycosylation sites in this molecule. These are most probably located at Thr-45 (O-glycosylation) and at Asn-65 (N-glycosylation).

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.