FRI0556 INCREASED RISK OF MORTALITY AND CANCER IN SYSTEMIC LUPUS ERYTHEMATOSUS: RESULTS FROM A LUPUS COHORT STUDY FROM 1998 TO 2015

Background:

Systemic lupus erythematosus (SLE) is a chronic systemic autoimmune inflammatory disease with increased risk for mortality and cancers possibly because of the effects of systemic inflammation and immunodeficiency due to disease itself and/or cytotoxic agents for SLE management. Although there has been improvement in the prognosis of SLE over decades with the treatment advance including standardized treatment strategy for lupus nephritis and less cytotoxic agents, the improvement also is fixed nowadays.

Objectives:

In this study, we we are to investigate the mortality and cancer of SLE patients in a longitudinal SLE cohort and compare the morality ratio and incidence of cancer with general population over time.

Methods:

This study was conducted in Hanyang BAE lupus cohort during the period of 1998 to 2015. Mortality data and malignancy data were derived in connection with data from the Korean National Statistics Office and the Korea Central Incidence Database, respectively. The Standardized Mortality Ratio (SMR) and Standardized Incidence Ratio (SIR) was estimated yearly by dividing the observed number deaths/cancers by the expected number of deaths/cancers of age- and sex- matched general population from matched year.

Results:

Mortality data were available in 1284 patients and total 71 deaths were observed. The most common cause of death was SLE itself (52.1%) followed by infection (18.3%), cerebrovascular disease (8.5%) and suicide (7.0%). The total age and sex adjusted SMR was 3.4 [95% CI (Confidential Interval) 2.6-4.1]. When we conduct subgroup analysis by age, the sex-adjusted SMR was significantly increased in young and middle aged adult patients: the SMR in patients younger than 20 was 12.2, but it was not significant due to the small number of young patients (observed death 3, expected death 0.3, 95% CI 0-26.0). The adjusted SMR in patients aged 20-39, aged 40-59 and aged over 60 were 9.8 (observed death 35, expected death 4.8, 95% CI 6.5-13.0), 3.7 (observed death 24, expected death 11.5, 95% CI 2.2-5.2), and 1.0 (observed death 9, expected death 10.3, 95% CI 0.3-1.6), respectively. Compared with alive patients, died patients had more serositis and more neurologic disorder according to American College of Rheumatology classification criteria for SLE despite the shorter observational period (5.6 years vs. 9.4 years).

Malignancy data were available in 1,020 patients and 56 primary cancers were diagnosed. Solid tumor was developed in 51 patient and hematologic malignancy was developed in 5 patients (3 non-Hodgkin’s lymphoma, 1 solitary plasmacytoma and 1 acute lymphoblastic leukemia). Thyroid cancer was the most common solid cancer (24 patients) followed by colorectal (5 patients), breast (4 patients), cervical cancer (4 patients) and hepatocellular carcinoma (3 patients). The total age and sex adjusted SIR was 1.1 (95% CI 0.8-1.4).

Conclusion:

Patients with SLE had higher risk of mortality than general population and the younger patients had the higher risk of mortality. The leading cause of death was SLE itself followed by infection and cerebrovascular disease. The risk for cancer in patient with SLE was similar with that of general population.

Disclosure of Interests:

None declared

SAT0592 MORTALITY AND CAUSE OF DEATH IN KOREAN PATIENTS WITH RHEUMATOID ARTHRITIS: BASED ON A LARGE COHORT

Background:

Rheumatoid arthritis (RA) is a common chronic inflammatory disease characterized by arthritis of multiple joints. Although the use of corticosteroid and extra-articular complications may lead increased mortality of patients with RA and it have been confirmed by hundreds of studies, the prognosis of RA has improved over the past decades with the introduction of biologics disease-modifying anti-rheumatic drugs and treat-to-target strategy. Along with the increase of overall survival of RA, the needs for re-assessment of actual life expectancy in patients with RA have also been increased.

Objectives:

To investigate the cause and the risk of death of Korean patients with RA in a large RA cohort.

Methods:

We analyzed patients in Hanyang BAE RA cohort who fulfilled the American College of Rheumatology criteria. A total of 2,355 patients were enrolled from October 2001 to December 2015. Mortality data were derived by linking with data from the Korean National Statistical Office and date and cause death were identified. Standardized Mortality Ratio (SMR) was estimated by dividing the observed deaths by the expected number of deaths of age- and sex- matched general population. Confidence intervals were calculated based on the Poisson distribution.

Table 1.

Comparison of demographic and clinical characteristics of analyzed patients

TotalDied patientsSurvivorsP value(n=2,355)(n=225)(n=2,130)Age at onset41.8 ± 12.849.7 ± 12.741.0 ± 12.5<0.001Age at enrollment50.8 ± 12.363.3 ± 9.449.5 ± 11.8<0.001Male265 (11.3)42 (18.7)223 (10.5)<0.001Disease duration18.1 ± 10.420.7 ± 10.917.8 ± 10.3<0.001Observation period9.0 ± 4.47.0 ± 3.69.2 ± 4.5<0.001Smoking<0.001 Never smoker1,953 (83.9)161 (74.9)1,792 (84.8) Former smoker212 (9.1)36 (16.7)176 (8.3) Current smoker164 (7.0)18 (8.4)146 (6.9)Comorbidity Hypertension330 (14.0)63 (28.0)267 (12.5)<0.001 Diabetes mellitus106 (4.5)24 (10.7)82 (3.9)<0.001

*Values are number (%) or mean ± standard deviation. ** Missing data were excluded from analysis.

Results:

Over the observation period, 225 deaths were reported. The age at enrollment was 50.8 ± 12.3 years and disease duration was 18.1 ± 10.4 years. The most common cause of death was malignancy (40 cases) followed by respiratory disease (38 cases), cardiovascular disease (32 cases) and musculoskeletal disease (21 cases). Total SMR was increased [1.7, 95% CI 1.5-2.0] but age- and sex- adjusted SMR was not increased [SMR 1.0, (95% CI 0.9-1.1)]. When we classify patients by age conduct subgroup analysis, sex-adjusted SMR was also similar with that of general population by all group: the adjusted SMR in patients aged 15-39, aged 40-59 and aged over 60 were 0.7 (observed death 1, expected death 1.5, 95% CI 0-2.0), 0.9 (observed death 29, expected death 32 95% CI 0.6-1.3), and 0.9 (observed death 195, expected death 221.0, 95% CI 0.8-1.0), respectively. Compared with survivors, patients who died were more likely to be male (18.7% vs. 10.5%, p<0.001) and they had an older age of onset of RA (49.7 years vs. 41.0 years, p<0.001). Died patients were more likely to be current smoker or ever smoker (25.1% vs. 15.2%, p<0.001) and accompanied by more comorbidities including hypertension (28.0% vs. 12.5%, p<0.001) and diabetes mellitus (10.7% vs. 3.9%, p<0.001).

Conclusion:

The overall age- and sex- matched SMR of patients with RA was similar with that of general population. However, compared with survivors, patients who died were more likely to be male, diagnosed with RA at older age, more likely to be smoker, and they have more hypertension and diabetes mellitus. Therefore, attention should be paid not only to RA itself but also managing comorbidities to improve the survival of patients with RA.

Disclosure of Interests:

None declared

Biomedical applications of nisin

Nisin is a bacteriocin produced by a group of Gram-positive bacteria that belongs to Lactococcus and Streptococcus species. Nisin is classified as a Type A (I) lantibiotic that is synthesized from mRNA and the translated peptide contains several unusual amino acids due to post-translational modifications. Over the past few decades, nisin has been used widely as a food biopreservative. Since then, many natural and genetically modified variants of nisin have been identified and studied for their unique antimicrobial properties. Nisin is FDA approved and generally regarded as a safe peptide with recognized potential for clinical use. Over the past two decades the application of nisin has been extended to biomedical fields. Studies have reported that nisin can prevent the growth of drug-resistant bacterial strains, such as methicillin-resistant Staphylococcus aureus, Streptococcus pneumoniae, Enterococci and Clostridium difficile. Nisin has now been shown to have antimicrobial activity against both Gram-positive and Gram-negative disease-associated pathogens. Nisin has been reported to have anti-biofilm properties and can work synergistically in combination with conventional therapeutic drugs. In addition, like host-defence peptides, nisin may activate the adaptive immune response and have an immunomodulatory role. Increasing evidence indicates that nisin can influence the growth of tumours and exhibit selective cytotoxicity towards cancer cells. Collectively, the application of nisin has advanced beyond its role as a food biopreservative. Thus, this review will describe and compare studies on nisin and provide insight into its future biomedical applications.

The continuing development of gastric acid pump inhibitors

The synthesis and action of H2-receptor antagonists changed the understanding of gastric acid secretion as well as changing medical therapy for peptic ulcer disease. It is now known that peripheral regulation of gastric acid secretion depends largely, but not entirely, on histamine release from the enterochromaffin-like cell. There is, therefore, no final common pathway for stimulation of the parietal cell. In contrast, all stimuli converge to activate the acid pump, the H+,K(+)-ATPase. Inhibition of this pump by clinically useful drugs was achieved by developing derivatives of timoprazole, pyridyl-2-methylsulfinyl benzimidazole. Two of these derivatives, omeprazole and lansoprazole, have shown superiority in acid control and therefore in therapy for peptic ulcer disease compared to the available H2-receptor antagonists.

Soraprazan: setting new standards in inhibition of gastric acid secretion

After treatment of millions of patients suffering from gastroesophageal reflux disease (GERD) and other acid-related ailments with proton pump inhibitors, there are still unmet medical needs such as rapid and reliable pain relief, especially for nocturnal acid breakthrough. In this work, we introduce and characterize the biochemistry and pharmacology of the potassium-competitive acid blocker (P-CAB) soraprazan, a novel, reversible, and fast-acting inhibitor of gastric H,K-ATPase. Inhibitory and binding properties of soraprazan were analyzed together with its mode of action, its selectivity, and its in vivo potency. This P-CAB has an IC(50) of 0.1 microM if measured with ion leaky vesicles and of 0.19 microM in isolated gastric glands. With a K(i) of 6.4 nM, a K(d) of 26.4 nM, and a B(max) of 2.89 nmol/mg, this compound is a highly potent and reversible inhibitor of the H,K-ATPase. Soraprazan shows immediate inhibition of acid secretion in various in vitro models and in vivo and was found to be more than 2000-fold selective for H,K-ATPase over Na,K- and Ca-ATPases. Soraprazan is superior to esomeprazole in terms of onset of action and the extent and duration of pH elevation in vivo in the dog. Rapid and consistent inhibition of acid secretion by soraprazan renders the P-CABs a promising group of compounds for therapy of GERD.

Review article: the clinical pharmacology of proton pump inhibitors

Proton pump inhibitors inhibit the gastric H+/K+-ATPase via covalent binding to cysteine residues of the proton pump. All proton pump inhibitors must undergo acid accumulation in the parietal cell through protonation, followed by activation mediated by a second protonation at the active secretory canaliculus of the parietal cell. The relative ease with which these steps occur with different proton pump inhibitors underlies differences in their rates of activation, which in turn influence the location of covalent binding and the stability of inhibition. Slow activation is associated with binding to a cysteine residue involved in proton transport that is located deep in the membrane. However, this is inaccessible to the endogenous reducing agents responsible for restoring H+/K+-ATPase activity, favouring a longer duration of gastric acid inhibition. Pantoprazole and tenatoprazole, a novel proton pump inhibitor which has an imidazopyridine ring in place of the benzimidazole moiety found in other proton pump inhibitors, are activated more slowly than other proton pump inhibitors but their inhibition is resistant to reversal. In addition, tenatoprazole has a greatly extended plasma half-life in comparison with all other proton pump inhibitors. The chemical and pharmacological characteristics of tenatoprazole give it theoretical advantages over benzimidazole-based proton pump inhibitors that should translate into improved acid control, particularly during the night.

Synthesis or rupture: duration of acid inhibition by proton pump inhibitors

Insight has been gained into the relationship between the structure of proton pump inhibitors (PPIs), their binding, and their suppression of acid secretion. PPIs accumulate in the acidic space of the secreting parietal cell, where then their active forms create disulfide bonds with key cysteines of the H(+), K(+)-ATPase. Studies in humans on the half-lives of recovery of acid secretion have found that while lansoprazole showed a half-life of less than 15 h, and both omeprazole and rabeprazole showed one of less than 30 h, for pantoprazole the half-life was approximately 46 h. This difference in duration of inhibition with PPIs may be related to variations in proton pump inhibitor dwell time. A study in rats suggests that the recovery of gastric pump activity after treatment with omeprazole, esomeprazole, lansoprazole and rabeprazole is likely due to both reversal of binding by disulfide-reducing agents and to pump synthesis. However, for pantoprazole, reversal of acid inhibition is probably due mainly to de novo pump synthesis and not loss of binding. This profile is likely related to the unique binding of pantoprazole to cysteine 822, a binding site which is buried deep within the membrane domain of the pump and may therefore be inaccessible to reducing agents. Although clinical data supporting these findings are limited, prolonged binding of pantoprazole may confer a longer duration of action in comparison with other PPIs.

Selective Fe2+-catalyzed oxidative cleavage of gastric H+,K+-ATPase: implications for the energy transduction mechanism of P-type cation pumps

In the presence of ascorbate/H(2)O(2), Fe(2+) ions or the ATP-Fe(2+) complex catalyze selective cleavage of the alpha subunit of gastric H(+),K(+)-ATPase. The electrophoretic mobilities of the fragments and dependence of the cleavage patterns on E(1) and E(2) conformational states are essentially identical to those described previously for renal Na(+),K(+)-ATPase. The cleavage pattern of H(+),K(+)-ATPase by Fe(2+) ions is consistent with the existence of two Fe(2+) sites: site 1 within highly conserved sequences in the P and A domains, and site 2 at the cytoplasmic entrance to trans-membrane segments M3 and M1. The change in the pattern of cleavage catalyzed by Fe(2+) or the ATP-Fe(2+) complex induced by different ligands provides evidence for large conformational movements of the N, P, and A cytoplasmic domains of the enzyme. The results are consistent with the Ca(2+)-ATPase crystal structure (Protein Data Bank identification code; Toyoshima, C., Nakasako, M., Nomura, H., and Ogawa, H. (2000) Nature 405, 647-655), an E(1)Ca(2+) conformation, and a theoretical model of Ca(2+)-ATPase in an E(2) conformation (Protein Data Bank identification code ). Thus, it can be presumed that the movements of N, P, and A cytoplasmic domains, associated with the E(1) <--> E(2) transitions, are similar in all P-type ATPases. Fe(2+)-catalyzed cleavage patterns also reveal sequences involved in phosphate, Mg(2+), and ATP binding, which have not yet been shown in crystal structures, as well as changes which occur in E(1) <--> E(2) transitions, and subconformations induced by H(+),K(+)-ATPase-specific ligands such as SCH28080.

Sequence analysis of hemagglutinin and nucleoprotein genes of measles viruses isolated in Korea during the 2000 epidemic

To characterize the genetic properties of currently circulating measles viruses in Korea, the complete nucleotide sequences of hemagglutinin (H) protein and nucleoprotein (N) genes of Korean viruses were analyzed. The entire genes of H and N were directly amplified by RT-PCR from each clinical specimen and sequenced. Sequence analyses of H and N genes indicated that all Korean viruses had a high degree of homology (>99.8%) when compared with each other. The Korean viruses differed from other wild-type viruses by as much as 6.8% in the H gene and 6.5% in the N gene at the nucleotide level. The deduced amino acid variability was up to 6.4% for the H protein and up to 6.5% for the N protein. Phylogenetic analyses of nucleotide sequences and deduced amino acid sequences of the H and N genes revealed that all Korean viruses were grouped into the clade H1.

Review article: the control of gastric acid and Helicobacter pylori eradication

This review focuses on the gastric acid pump as a therapeutic target for the control of acid secretion in peptic ulcer and gastro-oesophageal reflux disease. The mechanism of the proton pump inhibitors is discussed as well as their clinical use. The biology of Helicobacter pylori as a gastric denizen is then discussed, with special regard to its mechanisms of acid resistance. Here the properties of the products of the urease gene clusters, ureA, B and ureI, E, F, G and H are explored in order to explain the unique location of this pathogen. The dominant requirement for acid resistance is the presence of a proton gated urea transporter, UreI, which increases access of gastric juice urea to the intrabacterial urease 300-fold. This enables rapid and continuous buffering of the bacterial periplasm to approximately pH 6.0, allowing acid resistance and growth at acidic pH in the presence of 1 mM urea. A hypothesis for the basis of combination therapy for eradication is also presented.

Analysis of the membrane domain of the gastric H(+)/K(+)-ATPase

A structure of the catalytic or alpha subunit of the H(+)/K(+)-ATPase, with ten transmembrane segments, and of the beta subunit, with a single such segment, was established using a combination of tryptic cleavage and peptide sequencing and in vitro translation. Sites at which covalent ligands bind to external surfaces were also defined by cleavage, separation and sequencing. Cys813 was found to be the common covalent binding site for all the substituted pyridyl methylsulfinyl benzimidazoles. The binding region of a K(+)-competitive reagent, the 1,2 α -imidazo-pyridine SCH 28080, was defined by the kinetic effects of site-specific mutations. Amino acids substitutions in membrane-spanning segments M1, M3, M4 and M6 were found to influence the apparent inhibitor constant, K(i), to varying degrees, some having a large effect, some a moderate effect and some a slight effect, whereas some mutations had no effect. We interpret changes in K(i) without effects on the apparent Michaelis constant, K(m), as affecting SCH 28080 binding only. Mutation of Cys813 significantly affected the K(i) for SCH 28080, explaining the prevention of benzimidazole inhibition by the imidazo-pyridine.A model of the α subunit was constructed with a vestibule on the luminal surface of the pump bounded by M1-M6 and containing the SCH 28080 binding region. The cation binding site is suggested to be more towards the cytoplasmic face of the enzyme's membrane domain. This model predicts the membrane peptide associations for the catalytic subunit. Biochemical and yeast two-hybrid methods place the beta subunit in association with M8, whereas similar methods place M5/6 in proximity to M9/10. These results, when combined with analysis of the two-dimensional crystals of the sarcoplasmic reticular Ca(2+) and Neurospora crassa H(+)-ATPases, provide the basis for a tentative model of the arrangement of the six core segments of the gastric H(+)/K(+)-ATPase.

Stabilization of the H,K-ATPase M5M6 membrane hairpin by K+ ions. Mechanistic significance for p2-type atpases

The integral membrane protein, the gastric H,K-ATPase, is an alpha-beta heterodimer, with 10 putative transmembrane segments in the alpha-subunit and one such segment in the beta-subunit. All transmembrane segments remain within the membrane domain following trypsinization of the intact gastric H,K-ATPase in the presence of K+ ions, identified as M1M2, M3M4, M5M6, and M7, M8, M9, and M10. Removal of K+ ions from this digested preparation results in the selective loss of the M5M6 hairpin from the membrane. The release of the M5M6 fragment is directed to the extracellular phase as evidenced by the accumulation of the released M5M6 hairpin inside the sealed inside out vesicles. The stabilization of the M5M6 hairpin in the membrane phase by the transported cation as well as loss to the aqueous phase in the absence of the transported cation has been previously observed for another P2-type ATPase, the Na, K-ATPase (Lutsenko, S., Anderko, R., and Kaplan, J. H. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7936-7940). Thus, the effects of the counter-transported cation on retention of the M5M6 segment in the membrane as compared with the other membrane pairs may be a general feature of P2-ATPase ion pumps, reflecting a flexibility of this region that relates to the mechanism of transport.

Structural aspects of the gastric H,K ATPase: the M5/M6 domain and alpha beta association

This review summarizes some of the structural information that has been obtained on the gastric H,K ATPase. Methods such as tryptic digestion, site specific labeling and in vitro translation combine to provide a ten membrane segment model with however reservations as to the full transmembrane nature of M5 or M6. Labeling this region with the thiophilic luminal face reagent omeprazole provided cogent evidence that cys 813 but not cys 822 was labeled. On the other hand, cysteine mutagenesis provided evidence that removal of cys 813 did not affect inhibition of Rb transport by omeprazole whereas removal of cys 822 although not affecting ATPase activity abolished omeprazole inhibition of transport. A model to reconcile these data is presented where M5 and M6 although intramembranal are not transmembrane hairpin structures. Analysis of the region of alpha beta interaction by tryptic digestion and WGA chromatography to define those fragments of alpha that remain beta associated shows that leu 853 to arg 922 in the TM7-loop are a major region of association with the beta subunit. Yeast two hybrid analysis, when combined with these data and those from a chimeric construct, indicates that the sequence Q 907 to R 922 is the important element of interaction in the alpha subunit and no other extracytoplasmic domain was found to interact. Two regions of the beta subunit interact with this region of the alpha subunit between Q64 and N130 as well as A156 and R188. Apparently the beta subunit is folded around a small region of the large extracytoplasmic loop between TM7 and TM8, closer to TM8.

Regions of association between the alpha and the beta subunit of the gastric H,K-ATPase

A binding and a yeast two-hybrid analysis were carried out on the gastric H,K-ATPase to determine interactive regions of the extracytoplasmic domains of the alpha and beta subunits of this P type ATPase. Wheat germ agglutinin fractionation of fluorescein 5-maleimide-labeled tryptic fragments of detergent-solubilized H, K-ATPase showed that a fragment Leu855 to Arg922 of the alpha subunit was bound to the beta subunit. The yeast two-hybrid system showed that the region containing only a part of the seventh transmembrane segment, the loop, and part of the eighth transmembrane segment was capable of giving positive interaction signals with the ectodomain of the beta subunit. The sequence in the extracytoplasmic loop close to the eighth transmembrane segment, namely Arg898 to Thr928, was identified as being the site of interaction using this method. We deduced that the sequence Arg898 to Arg922 in the alpha subunit has strong interaction with the extracytoplasmic domain of the beta subunit. Again, using yeast two-hybrid analysis, two different sequences in the beta subunit Gln64 to Asn130 and Ala156 to Arg188 were identified as association domains in the extracytoplasmic sequence of the beta subunit. These data enable identification of major associative regions of the alpha-beta subunits of the H,K-ATPase.

The Na(+)-K(+)-ATPase beta 1 subunit is associated with the HK alpha 2 protein in the rat kidney

The Na-K-ATPase beta 1 subunit acts as the beta subunit for the HK alpha 2 protein in the rat kidney. The colonic H(+)-K(+)-ATPase is a member of the P-type ATPases, and has been shown to contribute to potassium transport by the mammalian kidney and colon. The P-type ATPases often consist of an alpha subunit that contains the catalytic site and a beta subunit that participates in regulation of enzyme activity and targeting of the enzyme to the plasma membrane. The cDNA of the alpha subunit (HK alpha 2) has been cloned and the HK alpha 2 protein has been isolated from the rat kidney and colon. However, a unique beta subunit for the colonic H(+)-K(+)-ATPase has not been described. To determine if one of the known beta subunits present in the kidney might act as the beta subunit for the colonic H(+)-K(+)-ATPase, microsomes enriched in the colonic H(+)-K(+)-ATPase were isolated using an HK alpha 2-specific antibody (AS 31.7) and the Minimac magnetic separation system. Immunoblots of rat kidney microsomal protein isolated with antibody AS 31.7 were probed with antibodies directed against the gastric HK beta subunit, Na(+)-K(+)-ATPase alpha 1, and Na(+)-K(+)-ATPase beta 1 subunits. A band of the appropriate size was detected with Na(+)-K(+)-ATPase beta 1-specific antibodies, but not those directed against HK beta 1. These data suggest that Na(+)-K(+)-ATPase beta 1 could be the beta subunit for the colonic H(+)-K(+)-ATPase in the kidney.

Human insulin production from a novel mini-proinsulin which has high receptor-binding activity

To increase the folding efficiency of the insulin precursor and the production yield of insulin, we have designed a mini-proinsulin (M2PI) having the central C-peptide region replaced with a sequence forming a reverse turn. The mini-proinsulin was fused at the N-terminus to a 21-residue fusion partner containing a His10 tag for affinity purification. The gene for the fusion protein was inserted downstream of the T7 promoter of the expression plasmid pET-3a, and the fusion proteins were produced as inclusion bodies in the Escherichia coli cytoplasm at levels up to 25% of the total cell protein. The protein was sulphonated, cleaved by CNBr and the M2PI mini-proinsulin was purified using ion-exchange chromatography. The refolding yield of M2PI was 20-40% better than that of proinsulin studied at the same molar concentrations, indicating that the short turn-forming sequence is more effective in the refolding process than the much longer C-peptide. Native human insulin was successfully generated by subsequent enzymic conversion of mini-proinsulin. The mini-proinsulin exhibited high receptor-binding activity, about 50% as potent as insulin, suggesting that this single-chained mini-proinsulin may provide a foundation in understanding the receptor-bound structure of insulin as well as the role of C-peptide in the folding and activity of proinsulin.

Structural aspects of the gastric H,K ATPase

The gastric H,K ATPase is an alpha beta heterodimeric member of the eukaryotic alkali-cation P-type ion-motive ATPase family. The alpha subunit is composed of 1033 amino acids and the beta subunit of 291 amino acids with 6 or 7 potential N-linked glycosylation sites. Much effort has been expended to define the membrane domain of P-type ATPases. A membrane domain of the large subunit consisting of 10 membrane-spanning sequences is suggested by a combination of methods such as (1) tryptic digestion, separation, and sequencing of membrane peptides, (2) labeling with extracytoplasmic reagents, and (3) in vitro translation of hydrophobic segments. The beta subunit has a single transmembrane segment with strong hydrophobic interactions with the alpha subunit. Blue native gel electrophoresis shows that the enzyme is an (alpha-beta)2 dimer. Cross-linking with Cu-phenanthroline provides evidence that association is between the alpha subunits, and the potential SH groups that are Cu sensitive are at cysteine 565 and cysteine 615, in the region of the large cytoplasmic loop between the fourth and fifth transmembrane segments. No cross-linking is observed in the membrane domain. ATP prevents cross-linking because of a conformational change at the surface of the protein induced by ATP or by direct binding of the nucleotide at the site of cross-linking. The WGA binding properties of the beta subunit allow investigation of the region of interaction with the alpha subunit. Thus, digestion of the enzyme by trypsin followed by SDS solubilization and selective elution from a WGA column resulted in coelution of the membrane fragment containing TM7 and TM8. This result demonstrates major hydrophobic interaction between the seventh and eighth transmembrane segments and the beta subunit. An antibody generated against rat parietal cells also recognized shared epitopes in the same region of both the alpha and beta subunits. Biochemical investigation of the arrangement of the transmembrane segments has been hindered by the lack of effective cross-linking reagents probably because of the compact arrangement of this domain, preventing even Cu access. A series of antiulcer drugs has been developed that have a unique chemistry related to their inhibition of the gastric H,K ATPase. They are 2-(substituted pyridyl methylsulfinyl) benzimidazoles, weak bases with a pKa of 4.0. After accumulation in the acidic space generated by the H,K ATPase either in vivo or in vitro, they undergo acid-catalyzed conversion to a tetracyclic sulfenamide which reacts with luminally accessible SH residues to form stable disulfide derivatives. In the particular case of pantoprazole, 2-(3,4-dimethoxy-2-pyridyl-methylsulfinyl)-5-difluoromethoxy benzimidazole, reaction is confined largely to cysteine 813, placed between the fifth and sixth transmembrane segments. The 5 azido analog of pantoprazole provided acid transport-dependent inhibition of the isolated transporting ATPase by this photoactivatable covalent SH reagent. The inhibited enzyme was then photolyzed, cleaved with trypsin, and the membrane fragments compared before and after photolysis. Disappearance of the segment corresponding to TM3,4 and a relative loss of the segment corresponding to TM7,8 suggests close proximity of these two membrane pairs to the loop joining the fifth and sixth transmembrane segments, in particular TM3,4. Use of this type of covalent, photoactivatable site-specific reagent to determine loop proximity can be extended to other acid transporters.

Sites of reaction of the gastric H,K-ATPase with extracytoplasmic thiol reagents

The vesicular gastric H,K-ATPase catalyzes an electroneutral H for K exchange allowing acidification of the intravesicular space. There is a total of 28 cysteines present in the alpha subunit of the gastric H,K-ATPase, of which 10 are found in the predicted transmembrane segments and their connecting loop, and 9 are present in the beta subunit, of which 6 are disulfide-linked. To determine which of these was accessible to extracytoplasmic attack, the enzyme was inhibited by four different substituted 2-pyridylmethylsulfinyl benzimidazoles, 5-methoxy-2-[(4-methoxy-3, 5-dimethyl-2-pyridyl)methylsulfinyl]-1H-benzimidazole (omeprazole), 2-[(4-trifluoroethoxy-3-methyl-2-pyridyl)methylsulfinyl]-1H-ben zimida zole (lansoprazole), 5-difluoromethoxy-2-[3, 4-methoxy-2-pyridyl)methylsulfinyl]-1H-benzimidazole (pantoprazole), and 2-[(4-(3-methoxypropoxy)-3-methyl)-2-pyridyl)methylsulfinyl]-1H-++ +benzi midazole (rabeprazole), under acid transporting conditions. All of these compounds are weak bases that accumulate in the acidic space generated by the pump and undergo an acid catalyzed rearrangement to a cationic sulfenamide, which forms disulfides with accessible cysteines. The relative rates of acid activation of these compounds corresponded to the relative rates of inhibition of ATPase activity and acid transport. Fragmentation of the enzyme by trypsin followed by SDS-polyacrylamide gel electrophoresis showed that omeprazole bound covalently to one of the two cysteines in the domains containing the fifth and sixth transmembrane segments and their extracytoplasmic loop and to cysteine 892 in the loop between the seventh and eighth transmembrane segments, but inhibition correlated with the reaction with cysteines in the fifth and sixth domain. Lansoprazole bound to the cysteines in these two domains as well as to cysteine 321 toward the extracytoplasmic end of the third transmembrane segments. Pantoprazole bound only to either cysteine 813 or 822 in the fifth and sixth transmembrane region. The inhibition of Rabeprazole correlated also with its binding to this part of the protein, but this compound continued to bind after full inhibition, eventually binding also to cysteines 321 and 892. No binding was found to any of the cysteines in the seventh to tenth transmembrane segments. Thermolysin digestion of the isolated omeprazole-labeled fifth and sixth transmembrane pair showed that cysteine 813 was the site of labeling. It is concluded that binding of these sided reagents to cysteine 813 in the loop between transmembrane (TM)5 and TM6 is sufficient for inhibition of ATPase activity and acid transport by the gastric acid pump. Of the 10 cysteines present in the membrane and extracytoplasmic domain, only three are exposed sufficiently to allow reactivity with these cationic thiol reagents. The binding to cysteine 813 defines the location of the extracytoplasmic loop between TM5 and TM6 and places the carboxylic acids 820 and 824 conserved between the gastric H,K- and the Na,K-ATPases in TM6, consistent with their assumed role in cation binding.

Dimerization of the gastric H+, K(+)-ATPase

When the gastric H+, K(+)-ATPase was solubilized by n-dodecyl beta-D-maltoside and electrophoresed in blue native-polyacrylamide gels (BN-PAGE), one major band at about 360 kDa was observed. Since this band was recognized by both monoclonal antibodies 1218 (anti-alpha) and wheat germ agglutinin (anti-beta), the H+, K(+)-ATPase in its native state exists in a dimeric (alpha beta)2 form. The site of interaction between the heterodimers was determined using Cu(2+)-phenanthroline cross-linking. The Cu(2+)-phenanthroline reagent reacted with the H+, K(2+)-ATPase activity. This cross-linking and enzyme inhibition were prevented by ATP. Cross-linking followed by N-ethylmaleimide blockade of maleimide-reactive SH groups, then reduction and fluorescein 5-maleimide labeling, then reduction and fluorescent tryptic peptide of about 6.5 kDa that had been cross-linked. Since its N-terminal amino acid is Val561, the peptide probably ends at Arg616 or Arg621 and Cys565 and/or Cys615 are probably within the region of closest contact between the two alpha-subunits.

Structure-taste correlations in sweet dihydrochalcone, sweet dihydroisocoumarin, and bitter flavone compounds

The dihydrochalcone derivatives of the bitter flavonoids naringin and neohesperedin are intensely sweet. Phyllodulcin is as sweet as the dihydrochalcones with similar taste properties although its structure apparently resembles that of bitter flavanone or flavone. Multifaceted approaches, including X-ray crystal structure analysis, energy calculation, and structure comparison, have been employed to clarify the structure-taste correlations in these classes of compounds. In the crystal, naringin dihydrochalcone assumes a 'J'-shaped conformation with a fully-extended dihydrochalcone moiety while neohesperidin dihydrochalcone assumes the same overall conformation but with a partially-extended moiety. A 2D conformational energy map of dihydrochalcone obtained using molecular mechanics revealed nine local minima. The pseudoequatorial and pseudoaxial forms of phyllodulcin have the same AM1 energies with a low energy barrier between them. The partially-extended form of dihydrochalcone and the pseudoequatorial form of phyllodulcin which are the maximally superposable conformers are proposed to be the active conformers. The major difference between the structures of flavone and phyllodulcin is not in the overall planarity but in the relative orientation of the pyrone and phenyl ring systems.

The pharmacology of the gastric acid pump: the H+,K+ ATPase

The gastric H+,K+ ATPase--the gastric acid pump--is the molecular target for the class of antisecretory drugs called the proton-pump inhibitors (PPIs). These compounds--omeprazole, lansoprazole, and pantoprazole--contain, as their core structure, 2-pyridyl methylsulfinyl benzimidazole. The H+,K+ ATPase is a heterodimer composed of a 1034-amino acid catalytic alpha peptide and a glycosylated 291-amino acid beta subunit. The alpha subunit probably contains 10 membrane-spanning sequences; the beta, a single transmembrane segment. The PPIs have a pKa of about 4.0; hence they accumulate only in the acidic secretory canaliculus of the stimulated parietal cell. Here they undergo conversion to a cationic sulfenamide, which then reacts with available cysteines on the extracytoplasmic face of the alpha subunit. Omeprazole reacts and forms disulfide bonds with cys813(822) and cys892; lansoprazole, with cys813(822), cys892, and cys321; and pantoprazole, with cys813 and -822. The antisecretory effect of the drugs reflects their short plasma half-life (approximately 60 min), the number of active pumps during that time, and the recovery of pumps following biosynthesis and reversal of inhibition. These drugs also show synergism with either amoxicillin or clari- thromycin in eradicating Helicobacter pylori, an organism shown to be important in duodenal and gastric ulcer disease. Their action is probably due to elevation of pH in the environment of the organism, rather than to any direct action.

Biochemical identification of transmembrane segments of the Ca(2+)-ATPase of sarcoplasmic reticulum

The transmembrane segments of sarcoplasmic reticulum Ca(2+)-ATPase were determined by trypsinization of cytoplasmic side-out intact sarcoplasmic reticulum vesicles. The membrane portion of tryptic digest comprising the transmembrane fragments, joined by the intravesicular segments, was separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after labeling with fluorescein 5-maleimide in the presence of sodium dodecyl sulfate. In this way, seven fluorescent bands of tryptic fragments below 11 kDa were observed which were derived from 4 pairs of membrane spanning segments and one hydrophobic sequence at the C-terminal end. Two peptides of 10.8 and 10.6 kDa had the identical N-terminal sequence beginning at Glu826, representing the transmembrane segments M7 and M8 and their connecting loop. A band at 8.1 kDa contained one peptide beginning at Tyr36 (M1/loop/M2). A 7.7-kDa peptide starting at Leu253 (M3/loop/M4) and a 7.3-kDa peptide beginning at Ala752 (M5/loop/M6) were also observed. A band at 6.7 kDa contained two peptides, one beginning at Ser48 (M1/loop/M2) and another beginning at Tyr763 (M5/loop/M6). In addition, a 4-kDa peptide beginning at Met925 was observed. The size of this peptide did not allow for a complete pair of transmembrane segments, but this peptide could have been derived from trypsinolysis between the last pair of membrane spanning segments. These data therefore provide biochemical evidence for at least 8 transmembrane segments and perhaps two more at the C-terminal end of the enzyme.

Identification of a region of the H,K-ATPase alpha subunit associated with the beta subunit

The gastric H,K-ATPase consists of an alpha, beta heterodimer. To determine which part of the alpha subunit is associated with the beta subunit, hog gastric vesicles were trypsinized, which left most of the beta subunit undigested. The vesicles were solubilized with either C12E8 (polyoxyethylene 8 lauryl ester) or Nonidet P-40 and then passed over a wheat germ agglutinin column, which retained the beta subunit. Among the fragments of membrane spanning segments obtained by digestion in the absence of K+, only one peptide fragment of the alpha subunit coeluted with the beta subunit, representing only the M7 and M8 membrane spanning segments of the alpha subunit along with their extracytoplasmic connecting loop. With K+ present during digestion, the fragments at 19-21 kDa and at 9.4 kDa were retained on the column, representing the M7 to C-terminal end and the M5/loop/M6 sector of the enzyme. These results provide biochemical evidence that a region of the alpha subunit containing the M7/loop/M8 sector is bound to the beta subunit under both digestion conditions; but when the C-terminal 180-amino acid sector of the alpha subunit remains intact, as found in the presence of K+, there is an additional retention of the M5/loop/M6 sector of the alpha subunit.

Gastric acid secretion: activation and inhibition

Peripheral regulation of gastric acid secretion is initiated by the release of gastrin from the G cell. Gastrin then stimulates the cholecystokinin-B receptor on the enterochromaffin-like cell beginning a calcium signaling cascade. An exocytotic release of histamine follows with concomitant activation of a C1- current. The released histamine begins the H2-receptor mediated sequence of events in the parietal cell, which results in activation of the gastric H+/K+ - ATPase. This enzyme is the final common pathway of acid secretion. The H+/K+ - ATPase is composed of two subunits: the larger alpha-subunit couples ion transport to hydrolysis of ATP, the smaller beta-subunit is required for appropriate assembly of the holoenzyme. Both the membrane and extracytoplasmic domain contain the ion transport pathway, and therefore, this region is the target for the antisecretory drugs of the post-H2 era. The 100 kDa alpha-subunit has probably 10 membrane spanning segments with, therefore, five extracytoplasmic loops. The 35 kDA beta-subunit has a single membrane spanning segment, and most of this protein is extracytoplasmic with the six or seven N glycosylation consensus sequences occupied. Omeprazole is an acid-accumulated, acid-activated, prodrug that binds covalently to two cysteine residues at positions 813 (or 822) and 892, accessible from the acidic face of the pump. Lansoprazole binds to cys321, 813 (or 822) and 892; pantoprazole binds to cys813 and 822. The common binding site for these drugs (cys813 or 822) is responsible for the inhibition of acid transport. Covalent inhibition of the acid pump improves control of acid secretion, but since the effective half life of the inhibition in man is about 48 hr, full inhibition of acid secretion, perhaps necessary for eradication of Helicobacter pylori in combination with a single antibiotic, will require prolongation of the effect of this class of drug.

Continuing development of acid pump inhibitors: site of action of pantoprazole

Both receptor antagonists and acid pump inhibitors are clinically useful suppressants of acid secretion. The latter class of drugs, the substituted benzimidazoles, inhibit acid secretion more effectively and, therefore, provide superior symptom relief and healing in all acid-related diseases. The H2-receptor antagonists competitively block the action of histamine on the H2-receptors of parietal cells. This histamine is released from enterochromaffin-like cells (ECL cells) due to gastrin, acetylcholine or epinephrine stimulation. In addition, parietal cells have M3-receptors which can function independently of H2-receptors. Hence, there is no single common pathway for parietal cell stimulation. Stimulation of acid secretion by parietal cells requires activation of the acid pump, the gastric H+,K(+)-ATPase. The target site for the benzimidazoles is the activated gastric H+,K(+)-ATPase, and, in particular, the cysteines of the pump that are exposed to the acid space of the secretory canaliculus of the parietal cells. Pantoprazole in its protonated form selectively reacts with cysteines present in both the fifth and sixth membrane segments of the ATPase, explaining its mechanism of inhibiting proton transport by this enzyme.

Antibody epitope mapping of the gastric H+/K(+)-ATPase

Several antibodies against the gastric H+/K(+)-ATPase were analysed for the topological and sequence location of their epitopes. Topological mapping was done by comparing indirect immunofluorescent staining in intact and permeabilised rat parietal cells. Epitope definition was by Western analysis of intact and of trypsin or V8-proteinase-fragmented hog gastric ATPase combined with N-terminal sequencing of the fragments; by Western analysis of fragments of rabbit alpha subunit expressed in Escherichia coli; by analysis of rabbit alpha and beta subunits expressed in baculovirus-transfected SF 9 cells and by ELISA assay of synthetic octamers of one region of the hog alpha subunit. It was confirmed that the monoclonal antibody, mAb 95-111, recognised a cytoplasmic region between M4 and M5, close to the ATP-binding domain. The major epitope for monoclonal antibody mAb 12-18 was also in this region, but a second epitope was confirmed to be present in the M7/M8 region. The monoclonal antibody, mAb 146-14, was shown to recognise an extracytoplasmic epitope dependent on intact disulfide bonds, present in the rat and the rabbit, but absent in the hog beta subunit, due to non-conservative amino-acid substitutions. This antibody also recognised an epitope present in the alpha subunit of the H+/K(+)-ATPase at the M7 extracytoplasmic interface, perhaps indicating structural association of these two regions. The polyclonal antibody, pAb39, raised against the C-terminal portion of the enzyme, reacted only with the cytoplasmic surface of the H+/K(+)-ATPase, showing that the alpha subunit of the enzyme has an even number of membrane spanning segments.

The site of action of pantoprazole in the gastric H+/K(+)-ATPase

Pantoprazole is a pyridinyl-2-methylenesulfinyl-2-benzimidazole derivative. This compound inhibits the vesicular gastric H+/K(+)-ATPase (cytoplasmic side out) under acid transporting conditions by accumulating in the acid space generated by the pump. Pantoprazole is then converted in an acid-catalysed reaction to a cationic sulfenamide and reacts with cysteines available in or from the acidic extracytoplasmic space. This compound binds to the hog gastric H+/K(+)-ATPase with a stoichiometry of 3 nmol per mg protein, resulting in 94% inhibition of ATPase activity. Tryptic cleavage of the intact vesicles which had been reacted with [14C]pantoprazole at a 1 to 4 trypsin to protein ratio removed most of the cytoplasmic domain leaving the pairs of membrane spanning segments and their connecting extracytoplasmic loops intact. The peptides remaining in the membrane were dissolved in SDS and available cysteine residues labelled with fluorescein-5-maleimide. The peptides were separated on Tricine gradient gels, transferred to PVDF membranes and identified by fluorescence and radioactivity. From N-terminal sequence, fluorescence and molecular mass, it is concluded that pantoprazole is able to label both Cys-813 and Cys-822. These cysteines are predicted to be located in the extracytoplasmic loop connecting membrane segments 5 and 6 and in membrane segment 6. The major cytoplasmic tryptic cleavage site at this location moved from position 776 in unmodified enzyme to positions 784 and 792 following pantoprazole labelling, showing that the configuration of this region changed with pantoprazole labelling. A similar result was obtained by reduction of the enzyme with dithiothreitol. Covalent binding of the cationic sulfenamide to this region of the enzyme is able to block the conformation necessary for phosphorylation of the enzyme by ATP, accounting for its inhibitory effect on acid secretion.

Membrane topology and omeprazole labeling of the gastric H+,K(+)-adenosinetriphosphatase

The gastric H+,K(+)-ATPase is an alpha beta heterodimer with close homology to the Na+,K(+)-ATPase. Digestion of intact cytoplasmic-side-out vesicles at a trypsin to protein ratio of 1/4 removed most of the cytoplasmic protein, leaving membrane-spanning pairs in high yield. These were visualized on gels and poly(vinylidene difluoride) (PVDF) membranes by sodium dodecyl sulfate solubilization of the membrane-embedded segments and labeling of the cysteine residues with fluorescein maleimide prior to electrophoresis. The membrane-spanning residues of the alpha subunit were found between positions 104 and 162 (M1/M2), 291 and 358(M3/M4), 776 and 835 (M5/M6), and 853 and 946 (M7/M8). Although this method did not detect membrane retention of the hydrophobic sequences subsequent to position 946, it provided biochemical evidence for at least eight membrane segments in the catalytic subunit. Intact vesicles containing this enzyme transport acid in the presence of KCl, valinomycin, and MgATP. Omeprazole accumulates in these acidified vesicles and converts to a cationic sulfenamide. This forms disulfides with accessible cysteines. The reaction with this extracytoplasmic thiol reagent inhibits ATPase activity. Full inhibition was obtained with a stoichiometry of 2.2 mol of omeprazole bound/mg of protein. Only the alpha subunit was labeled. The cysteines reacting with omeprazole were defined by proteolytic cleavage of 3H- or 14C-omeprazole-labeled enzyme followed by peptide sequencing of fragments separated on tricine gradient gels and transferred to PVDF membranes. Tryptic digestion at a 1/40 trypsin to protein ratio in the presence of ligands that stabilize the E2P form of the enzyme produced two large fragments, one of 68 kDa stretching from Glu47 to probably Arg666 that contained minor labeling and the other of 333 kDa beginning at Ala671 and extending to probably Arg946 that contained greater than 85% of the label. Digestion of labeled vesicles at 1/75 or 1/4 trypsin to protein ratios gave radioactive patterns consistent with labeling at Cys813 and/or Cys822 and at Cys892 and/or Cys927 and/or Cys938. V8 protease digestion of the solubilized alpha subunit produced a fragment extending from Ser838 to possible Asp900 that was omeprazole-labeled, showing that Cys892 was labeled and Cys927 and Cys938 were not. Hence, omeprazole labels the H+,K(+)-ATPase at cysteines within the M5/M6 and M7/M8 regions of the alpha subunit, accounting for its inhibitory action in vivo and in vitro.

Topology and sites in the H,K-ATPase

Understanding the membrane topology of the EP-type pumps has been approached largely by analysis of hydrophobicity plots, which are confusing in the COOH-terminal third of the proteins. Each pair of predicted membrane-spanning segments with the extracytoplasmic loop contains at least one cysteine, allowing fluorescent labeling of these regions of the enzymes by cysteine reagents once the cytoplasmic domain has been removed. The membrane segment arrangement of the H,K and sr Ca ATPases was investigated by tryptic cleavage of intact cytoplasmic face-out vesicles. This was followed by fluorescein or coumarin maleimide labeling of the SDS solubilized residual membrane fragments, tricine gradient gel separation, and sequencing. The presence of four membrane-spanning pairs was demonstrated for the alpha subunit of the H,K-ATPase, with no membrane retention of H9 and H10, although H9 has four cysteines based on cDNA sequencing. A similar observation was made for the Ca pump, except that fluorescein-labeled H9 was detected in the membrane with a molecular weight of 4 kD, showing that cleavage had occurred at lys958 predicted to be extracytoplasmic in a 10 membrane segment model. It seems likely that for both these enzymes the membrane domain contains only 8 alpha helical spanning segments. Cleavage at ala236 in the beta subunit was found only in leaky, not in ion-tight vesicles, arguing for a single membrane segment in this subunit. In the H,K-ATPase additional evidence for the presence and arrangement of the first, third, and fourth pair of segments was obtained by labeling the intact enzyme with extracytoplasmic inhibitory reagents. The K competitive reagent, an imidazopyridine, MeDAZIP+, labeled the first pair of membrane segments. The acid-activated SH reagent class, the pyridinyl methyl sulfinyl benzimidazoles, labeled cysteines 813 and 822 in the M5/M6 region as well as cysteine 892 in the extracytoplasmic loop between M7 and M8. No labeling of the beta subunit was found, indicating the presence of three disulfide bonds in the extracytoplasmic domain of this subunit. Both sets of extracytoplasmic reagents are predicted to bind close to the fatty acid/phospholipid head group interface. Inhibition by these reagents shows that conformational changes are transmitted between cytoplasmic and extracytoplasmic domains.

Structural aspects of the gastric H,K-ATPase

The gastric H,K-ATPase is an alpha, beta heterodimer. The large catalytic subunit is composed, in the case of the hog enzyme, of 1033 amino acids, whereas the beta subunit is composed of about 291 amino acids and is heavily glycosylated. The membrane topology of the alpha subunit is difficult to predict using hydropathy analysis. Tryptic hydrolysis of intact, inside out vesicles followed by cysteine labelling with fluorescein-5-maleimide provided experimental evidence for an 8 membrane spanning model for the alpha subunit, between residues 104 and 162 (M1/M2), 291 and 358 (M3/M4), 776 and 835 (M5/M6), and 853 and 946 (M7/M8). No evidence was found for a pair of segments (M9/M10) towards the C terminal end of the molecule, contrary to predictions for the Na,K- and Ca-ATPases. Iodination of intact vesicles followed by carboxypeptidase Y cleavage of the C terminal tyrosines showed that the C terminal end of the alpha subunit was cytoplasmic. The epitope for antibody 146 was extracytoplasmic and located between residues 871 to 874 between M7/M8. The binding site of the K competitive imidazo-pyridine, SCH28080, was to the extracytoplasmic loop between M1 and M2, whereas the binding of the covalent SH reagent generated from acid activation of omeprazole in acid transporting vesicles was to 2 cysteines at positions 813 (or 822) and 892 predicted to be in the extracytoplasmic loops connecting M5/M6 and M7/M8, respectively. The beta subunit was only hydrolysed in broken vesicles. A fragment beginning at position 236 was liberated under these conditions only in the presence of reducing agents, showing that cysteine 210 and 263 were disulfide linked.(ABSTRACT TRUNCATED AT 250 WORDS)