Edman Degradation Reveals Unequivocal Analysis of the Disulfide Connectivity in Peptides and Proteins

Disulfide bridges in peptides and proteins play an essential role in maintaining their conformation, structural integrity, and consequently function. Despite ongoing efforts, it is still not possible to detect disulfide bonds and the connectivity of multiply bridged peptides directly through a simple and sufficiently validated protein sequencing or peptide mapping method. Partial or complete reduction and chemical cysteine modification are required as initial steps, followed by the application of a proper detection method. Edman degradation (ED) has been used for primary sequence determination but is largely neglected since the establishment of mass spectrometry (MS)-based protein sequencing. Here, we evaluated and thoroughly characterized the phenyl thiohydantoin (PTH) cysteine derivatives PTH-S-methyl cysteine and PTH-S-carbamidomethyl cysteine as bioanalytical standards for cysteine detection and quantification as well as for the elucidation of the disulfide connectivity in peptides by ED. Validation of the established derivatives was performed according to the guidelines of the International Committee of Harmonization on bioanalytical method validation, and their analytical properties were confirmed as reference standards. A series of model peptides was sequenced to test the usability of the PTH-Cys-derivatives as standards, whereas the native disulfide-bonded peptides CCAP-vil, μ-conotoxin KIIIA, and human insulin were used as case studies to determine their disulfide bond connectivity completely independent of MS analysis.

Self-emulsifying drug delivery systems (SEDDS): Proof-of-concept how to make them mucoadhesive

AIM

The objective of this study was to provide a proof-of-concept that self-emulsifying drug delivery systems can be made mucoadhesive by the incorporation of hydrophobic mucoadhesive polymers.

METHODS

In order to obtain such a hydrophobic mucoadhesive polymer, Eudragit® S100 was thiolated by covalent attachment of cysteamine. After determination of the thiol group content, in vitro mucoadhesion studies (rotating cylinder and rheological measurements) were performed. Then, synthesized conjugate was incorporated into self-emulsifying drug delivery systems (SEDDS) and their toxic potential as well as that of unmodified and thiolated Eudragit® S100 was examined on Caco-2 cell line. Lastly, the mucoadhesiveness of developed SEDDS on porcine intestinal mucosa was determined.

RESULTS

Generated thiolated Eudragit® S100 displaying 235±14μmol of free thiol groups and 878±101μmol of disulfide bonds per gram polymer showed a great improvement in both: dynamic viscosity with mucus and adhesion time on mucosal tissue compared to the unmodified polymer. Resazurin assay revealed that unmodified and thiolated polymers and also SEDDS dispersions were non-toxic over Caco-2 cells. Furthermore, the incorporation of 1.5% (w/w) of such thiomer into SEDDS led to remarkably improved mucoadhesiveness. Blank SEDDS were completely removed from the mucosa within 15min, whereas >60% of SEDDS containing thiolated Eudragit® S100 were still attached to it.

CONCLUSION

These results provide evidence that SEDDS can be made mucoadhesive by the incorporation of hydrophobic mucoadhesive polymers.

Lipase activation by nonionic detergents. The crystal structure of the porcine lipase-colipase-tetraethylene glycol monooctyl ether complex

The crystal structure of the ternary porcine lipase-colipase-tetra ethylene glycol monooctyl ether (TGME) complex has been determined at 2.8 A resolution. The crystals belong to the cubic space group F23 with a = 289.1 A and display a strong pseudo-symmetry corresponding to a P23 lattice. Unexpectedly, the crystalline two-domain lipase is found in its open configuration. This indicates that in the presence of colipase, pure micelles of the nonionic detergent TGME are able to activate the enzyme; a process that includes the movement of an N-terminal domain loop (the flap). The effects of TGME and colipase have been confirmed by chemical modification of the active site serine residue using diisopropyl p-nitrophenylphosphate (E600). In addition, the presence of a TGME molecule tightly bound to the active site pocket shows that TGME acts as a substrate analog, thus possibly explaining the inhibitory effect of this nonionic detergent on emulsified substrate hydrolysis at submicellar concentrations. A comparison of the lipase-colipase interactions between our porcine complex and the human-porcine complex (van Tilbeurgh, H., Egloff, M.-P., Martinez, C., Rugani, N., Verger, R., and Cambillau, C.(1993) Nature 362, 814-820) indicates that except for one salt bridge interaction, they are conserved. Analysis of the superimposed complexes shows a 5.4 degrees rotation on the relative position of the N-terminal domains excepting the flap that moves in a concerted fashion with the C-terminal domain. This flexibility may be important for the binding of the complex to the water-lipid interface.

Inactivation of pancreatic lipases by amphiphilic reagents 5-(dodecyldithio)-2-nitrobenzoic acid and tetrahydrolipstatin. Dependence upon partitioning between micellar and oil phases

We have reported previously that Cys103 (SHII) of human pancreatic lipase (HPL), unlike the nonessential Cys181 (SHI), was buried and inaccessible to classical water-soluble sulfhydryl reagents. The lipolytic activity of HPL was lost after the labeling of the above two SH groups with the amphiphilic sulfhydryl reagent, 5-(dodecyldithio)-2-nitrobenzoic acid (C12-TNB), suggesting that the SHII residue may play an important role in the hydrolytic process [Gargouri, Y., Cudrey, C., Medjoub, H., & Verger, R. (1992) Eur. J. Biochem. 204, 1063-1067]. For the present experiments, we selected dog pancreatic lipase (DPL), purifying it for the first time, and recombinant guinea pig pancreatic lipase (r-GPL), which both contain a buried SHII group but no accessible SHI group. The single SHII of DPL and r-GPL reacted only with the amphiphilic SH reagent (C12-TNB), and its labeling was correlated with a rapid lipase inactivation. Although it is spatially remote from the catalytic triad, the SHII group of pancreatic lipases, when chemically labeled, was found to be responsible for the loss of their lipolytic activity. The presence of a bulky dodecyl chain, linked by a disulfide bond to the SHII, may have prevented the critical beta-5 loop (residues 76-85) movement by steric hindrance and consequently disturbed the formation of the oxyanion hole. Thus, pancreatic lipase inactivation by the amphiphilic sulfhydryl reagent can be said to be due to the prevention of a productive induced fit. Tetrahydrolipstatin (THL) is an amphiphilic inactivator reacting with the essential serine of the lipase active site.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence of horse pancreatic lipase as determined by protein and cDNA sequencing. Implications for p-nitrophenyl acetate hydrolysis by pancreatic lipases

The complete sequence of the horse pancreatic lipase was elucidated by combining polypeptide chain and cDNA sequencing. Among the structural features of horse lipase, it is worth mentioning that Lys373 is not conserved. This residue, which is present in human, porcine and canine lipases, has been assumed to be involved in p-nitrophenyl acetate hydrolysis by pancreatic lipases. Kinetic investigation of the p-nitrophenyl acetate hydrolysis by the various pancreatic lipases and by the C-terminal domain (336-449) of human lipase reveals that this hydrolysis is the result of the superimposition of independent events; a specific linear hydrolysis occurring at the active site of lipase, a fast acylation depending on the presence of Lys373 and a non-specific hydrolysis most likely occurring in the C-terminal domain of the enzyme. This finding definitely proves that pancreatic lipase bears only one active site and raises the question of a covalent catalysis by pancreatic lipases. Moreover, based on sequence comparison with the above-mentioned pancreatic lipases, three residues located in the C-terminal domain, Lys349, Lys398 and Lys419, are proposed as possible candidates for lipase/colipase binding.

Inactivation of human pancreatic lipase by 5-dodecyldithio-2-nitrobenzoic acid

Both thiol groups of native human pancreatic lipase can react with the new hydrophobic sulfhydryl reagent 5-dodecyldithio-2-nitrobenzoic acid (Dod-S-NbS) in the absence of a denaturing agent. Here we describe for the first time the covalent and stoichiometric modification of the inaccessible SHII group of native pancreatic lipase, using a 16-fold molar excess of this hydrophobic sulfhydryl reagent. A direct correlation was found to exist between the covalent modification of this SHII group and the loss of lipase activity. The question has not yet been answered, however, as to how Dod-S-NbS reaches the SHII-containing residue, whereas classical hydrophilic sulfhydryl reagents are unable to do so. This difference in reactivity may be attributable to the hydrophobic character of Dod-S-NbS and its potential capacity to form aggregates inducing a conformational change in the lipase molecule.

Primary structure of rat pancreatic lipase mRNA

The sequence of rat pancreatic lipase mRNA was determined. The data have been assigned the following accession number, X61925, in the EMBL data library. The total length of the messenger is 1531 nucleotides, plus a poly(A) stretch of about 60 nucleotides. A 72-nucleotides 5'-noncoding region is followed by a 1419-nucleotides open reading frame which encodes a protein of 473 amino acids, including the 17 amino acid signal peptide. The mature enzyme (456 residues) has 6 additional C-terminal amino acids, as compared with the amino acid sequence of pig (direct amino acid sequence), dog, man and rat isoenzyme from Genbank, M58369 (all deduced from the nucleotide sequence). A higher degree of homology exists between the amino acid sequence of rat mature enzyme with those of dog (88%), pig (75%) and man (75%) than with that of rat isolipase (74%).

Characterization of cDNA encoding the mouse hepatic triglyceride lipase and expression by in vitro translation

A cDNA coding for the mouse hepatic triglyceride lipase has been isolated from a mouse liver cDNA library. The nucleotide sequence of the cDNA shows an open reading frame encoding a polypeptide of 510 amino acids that is 91.5% and 86% homologous to rat and human hepatic lipase, respectively. The most drastic protein sequence divergence is found at the carboxyterminal end which was speculated to harbour one heparin-binding site. By in vitro translation of cRNA in the presence of pancreatic membranes the hepatic lipase was shown to be glycosylated and to have an electrophoretic mobility of 53 kDa.

The histidines reacting with ethoxyformic anhydride in porcine pancreatic lipase: their relationships with enzyme activity

The activities of porcine pancreatic lipase (449 amino acid residues) toward two different substrates, p-nitrophenylacetate and tributyrylglycerol, and their dependence on histidine ethoxyformylation were studied. In parallel, the ethoxyformylation of the lipase fragment constituting the C-terminal sequence of lipase (residues 336 to 449) was also investigated. This fragment was found to have retained the ability of lipase to catalyse p-nitrophenylacetate hydrolysis. The first histidine to react either in lipase or in the lipase fragment was His-354. The activities of the two compounds toward p-nitrophenyl-acetate were lost but that of the enzyme toward tributyrylglycerol was almost entirely retained. When a larger excess of ethoxyformic anhydride was used for the lipase reaction, 2.8 histidine residues were ethoxyformylated and characterised as His-354, His-156 and His-75, which resulted in an 85% inhibition of the tributyrylglycerol hydrolysis by the enzyme. Hydroxylamine treatment reactivated most of the lipase and lipase fragment. This is the first demonstration that the two lipase activities are not associated with the same active site. The loss of activity toward triacylglycerol hydrolysis suggests that His-156 and/or His-75 belong(s) to the active site or that a conformational change resulting from the ethoxyformylation renders the lipase inactive.

Minireview on pancreatic lipase and colipase

By hydrolyzing the dietary triacylglycerols, pancreatic lipase causes catalysis in heterogeneous medium. In vivo, lipase action cannot take place without colipase due to the presence of bile salts. The cofactor enables lipase anchoring to the water-lipid interface. The lipase-colipase system furnishes an excellent example of specific interactions (protein-protein and protein-lipid). The studies of lipase catalytic properties brought to light the importance of certain parameters related to the 'quality of the interface'. The structure-function relationship analyses revealed a certain number of functional amino acid residues in lipase and colipase involved either in the catalytic site of the enzyme or in the recognition sites (lipase-colipase and protein-interface). Comparisons of the sequences of lipases derived from different sources display interesting similarities in certain cases.

Primary structure of the glycans of porcine pancreatic lipase

The glycan primary structure of the main glycopeptide fraction obtained by pronase and carboxypeptidase A digestions of porcine pancreatic lipase has been investigated by 500-MHz 1H-NMR spectroscopy and methylation analysis. The results demonstrate that the glycopeptide fraction was a mixture containing the following structures: (formula; see text)

Human hepatic triglyceride lipase: cDNA cloning, amino acid sequence and expression in a cultured cell line

By immunoscreening of a human cDNA expression library and hybridization of colonies, four partially overlapping cDNA clones of human hepatic triglyceride lipase (HTGL) mRNA were isolated. The clones included the complete coding sequence, the 3'- and at least part of the 5'-untranslated region. The length of the composite HTGL cDNA segment (1.7 kb) was consistent with the size of the mRNA identified in an established human hepatoma cell line. DNA-sequence analysis of cDNAs of partially unspliced mRNAs, and of cloned genomic DNA indicated that the HTGL coding sequence comprises at least six exons. As predicted from the cDNA, the unprocessed HTGL protein has a molecular weight of 56, three potential glycosylation sites, and a signal peptide of 23 amino acids. Sequence comparison with cDNA of other lipases, including rat hepatic lipase, revealed 30%-75% protein-sequence homology. The data establish that HTGL is a secretory protein produced in the hepatocyte, and that its synthesis can be continued in permanent cell lines of hepatoma origin. Our studies also showed that HTGL is another member of a lipase gene family which has interfacial binding sites and possibly other functional domains in common.

Cleavage of the Arg-Ile bond in the native polypeptide chain of human pancreatic stone protein

The pancreatic stone protein (PSP) isolated from calculi (Mr 14,000) and the 5 protein forms (PSP S1-5) detected in pancreatic juice (Mr 14,000-19,000) derive from the same source differing seemingly in their carbohydrate contents or/and in their polypeptidic chain lengths. This kind of protein would inhibit in vivo CaCO3-crystal growth in pancreatic juice. PSP and PSP S1 N-terminal sequences are identical (NH2Ile-). This report demonstrates that: in PSP S2-5 the amino-terminal is blocked; the C-terminus is alike in every form; the single polypeptide chain of PSP S2-5 is converted into that of PSP S1 or PSP by the specific trypsin cleavage of the Arg-Ile bond.

Human gastric lipase. The effect of amphiphiles

Human gastric lipase (HGL) activity on tributyrin emulsion was detected only in the presence of amphiphiles such as bile salts, proteins (serum albumin, beta-lactoglobulin or ovalbumin) or phosphatidylcholine. These findings are contrary to the strong inhibitory effect of amphiphiles observed on pure pancreatic lipase. To reveal HGL activity, amphiphiles should be added prior to HGL. This may prevent irreversible interfacial denaturation. HGL activity was found to be restricted to a triacylglycerol/water surface tension ranging from 8 dyn/cm to 13 dyn/cm. All amphiphiles, which decrease the interfacial tension below 8 dyn/cm, act as irreversible inhibitors of HGL in the absence and in the presence of bile salts. Our results confirm that HGL is capable of hydrolysing triacylglycerol in the presence of the physiological concentration of bile salts prevailing in the upper small intestine and in the presence of alimentary proteins. These observations could explain the high dietary lipid absorption observed under pancreatic lipase deficiency.

Limited proteolysis of porcine pancreatic lipase. Lability of the Phe 335-Ala 336 bond towards chymotrypsin

Mild chymotrypsin digestion of native lipase (449 amino acids) preferentially cleaved the Phe 335-Ala 336 bond. On SDS-gel electrophoresis, 3 major bands were observed: band 1 (52 kDa) representing native lipase, bands 2 and 3 (40 and 12 kDa) representing the two lipase fragments A and B. Fragment A does not retain lipase activity but maintains its ability to adsorb to interfaces. Fragment B was identified with the lipase C-terminal region (336-449). It does not exhibit any activity towards tributyrylglycerol emulsions and any ability to adsorb to interfaces.