Purification of the pepsinogen A isozymogens by means of high resolution ion-exchange chromatography. Evidence for post-translational modifications

Determination of phosphoserine in human pepsinogens

It has previously been assumed that, in contrast to porcine pepsinogen, the human pepsinogens are not phosphorylated. The present investigations show that phosphorylation does contribute to the electrophoretic heterogeneity of the human pepsinogens. A new chromatographic method for analysis of phosphoamino acid was developed. Quantitative determinations of phosphoserine were carried out after hydrolysis in 6 mol l-1 HCl (4 h, 110 degrees C). The recovery value of an authentic sample of phosphoserine, treated in parallel with the unknown samples, was used for calculations.

Pepsinogens in health and disease

Pepsinogens, precursors of pepsins (potent and abundant digestive enzymes that are the primary products of the gastric chief cells), are members of the family of aspartic proteases. Because of the heterogeneity of pepsinogens, several classifications have appeared in the literature. I describe the recommended classification and nomenclature of the aspartic proteases and discuss their genetics, biochemistry (structure, activation of zymogens, mechanism of proteolytic activity and inhibitors), and physiology. The focus will be on the zymogens of pepsin, the so-called pepsinogens. The measurement of these enzymes in serum is a reliable noninvasive biochemical method for evaluating peptic secretion and obtaining information on the gastric mucosal status. A detailed review of the methods for the measurement of pepsinogens in serum, urine, and gastric mucosa is also provided. Data on pepsinogen levels in healthy subjects are discussed with respect to sex, age, smoking habit, and the presence of a circadian rhythm. The value of pepsinogen measurements in peptic ulcer to determine ulcer outcome and recurrence, in gastric cancer, and in Helicobacter pylori infection is reviewed. Finally, the effects of drugs on peptic secretion are discussed. In light of these data, the measurement of aspartic proteases, and in particular that of pepsinogen A and C, may be regarded as an effective biochemical approach to the evaluation and monitoring of patients with upper gastrointestinal diseases.

High-performance liquid chromatography: purification and chromatographic behaviour of molecular variants of pepsinogen A from human urine

By combining conventional DEAE chromatography with high-performance liquid chromatography on Sephacryl S-200 HR and Mono-Q columns, we have been able to isolate and fractionate human pepsinogen A (PGA) isozymogens from large amounts of urine. This method of fractionation is simple and allows one to obtain pepsinogen in a native non-denatured conformation. The isozymogens are homogeneous by electrophoretic and chromatographic criteria; this was confirmed by N-terminal amino acid sequencing. Purified PGA-3 and PGA-5 can be converted into an additional, more anionic, isoform on incubation at 37 degrees C. This isoform exists not only in vitro but also in vivo. The net negative charge of the PGA isozymogens is in the order PGA-5 less than deamidated PGA-5 less than PGA-3 less than deamidated PGA-3. Surprisingly, the elution order on the Mono-Q column was PGA-5/PGA-3/deamidated PGA-5/deamidated PGA-3. We have performed molecular modelling on PGA to investigate this phenomenon in terms of surface charge (not net charge) of the proteins. The model provides evidence that (1) only a fraction of the protein surface interacts with the support and (2) regions of localized charge at the protein surface may allow portions of the external surface to dominate chromatographic behaviour, resulting in a steering of the proteins with respect to the oppositely charged matrix. Pepsinogens may serve as model proteins for elucidating some of the variables that determine the chromatographic behaviour of proteins on ion-exchange columns.

Immunoblot technique to visualise serum pepsinogen A isozymogen patterns

Pepsinogen A (PGA) isozymogen patterns in urine and gastric mucosa can be visualised in non-denatured polyacrylamide gel electrophoresis by showing proteolytic activity after the conversion of pepsinogen into pepsin by acid. This method is not suitable for visualising PGA patterns in serum due to low PGA concentrations. To obtain a more sensitive visualisation method an immunoblotting technique was developed. PGA isozymogen patterns from urine and sonified gastric mucosa specimens obtained by immunoblotting were identical with those obtained by activity staining. The immunostaining method was at least 50 times more sensitive. PGA isozymogen patterns could be visualised in serum. Preliminary results suggest that the PGA patterns in serum and gastric mucosa are identical. As an association has been found between the genetically determined PGA isozymogen patterns in gastric mucosa and gastric malignancies in man, immunoblotting of PGA isozymogens in serum may provide a screening tool for subjects at risk of malignant gastric disease.

Human pepsinogen C (progastricsin) polymorphism: evidence for a single locus located at 6p21.1-pter

A series of six clones containing the entire human pepsinogen C gene (PGC) was identified in a cosmid vector library by using cDNA and oligonucleotide probes. The 10.7-kb PGC gene includes nine exons and exhibits a high degree of sequence identity (60%) with the functionally related pepsinogen A genes. The predicted amino acid sequence was identical with the partial amino-terminal and carboxyl-terminal sequences of purified pepsinogen C. An informative restriction fragment length polymorphism was detected with several restriction enzymes and involved an insertion or deletion of 100 bp of intron sequence located between exons 7 and 8. Evidence that there is only a single PGC gene in humans is presented. The PGC gene and the prolactin gene were regionally localized to 6p21.1-pter by analysis of mouse X human somatic cell hybrids.

Identification of a Glu greater than Lys substitution in the activation segment of human pepsinogen A-3 and -5 isozymogens by peptide mapping using endoproteinase Lys-C

The isozymogens PGA-3 and PGA-5 of human pepsinogen A were digested with endoproteinase Lys-C. The peptides were separated by reverse-phase HPLC. PGA-5 showed a peak strongly absorbing at 254 nm absent in PGA-3. Analysis of amino acid composition using the Pico-Tag methodology combined with DABITC-sequencing reveals the sequence Tyr-Phe-Pro-Gln-Trp-Lys (peptide 37-43 of the activation segment). This confirms a study at the DNA level by our group [16] suggesting a Glu greater than Lys mutation at position 43 in the activation segment of PGA-5. Furthermore, it is proposed that the number of genetic variants of PGA is higher than is actually seen by electrophoresis.

Differential expression of pepsinogen isozymogens in a patient with Barrett esophagus

The pepsinogen A (PGA) isozymogens in the gastric mucosa and Barrett epithelium of a female patient with Barrett esophagus were studied on different occasions during a 3-year period by electrophoretic analysis of in vivo steady-state pepsinogen in biopsies by activity staining in combination with variant specific monoclonal antibodies and of de novo synthesized pepsinogen by autoradiography. In Barrett epithelium only one (Pg3) or two (Pg3 and Pg5) primary PGA gene products were detected, whereas in gastric mucosal biopsies three (Pg3, Pg4 and Pg5) primary gene products were demonstrated on all occasions. These differences strongly suggest differential expression/activation of individual gene numbers in the PGA gene cluster in Barrett esophagus and are in line with the preneoplastic nature of this condition. The mechanism behind this deregulation is currently under investigation by cell biology and molecular genetic techniques.

Discrepancies between gastric mucosal and urinary pepsinogen A patterns and in vitro synthesis and secretion of human pepsinogen

The relationship between electrophoretic pepsinogen A (PGA) patterns from urine and gastric mucosa was studied in healthy volunteers and in patients with various gastric disorders. Discrepancies between urinary and gastric PGA patterns were found in 63.3% of the individuals. In 9% of the subjects with these discrepancies, the phenotype class in urine was different from that in gastric mucosa. The differences were found in all diagnostic groups. The highest frequency of differences was found in patients with gastric ulcer. The differences were not related to the serum PGA level. More than 80% of the differences were caused by a lower relative intensity of pepsinogen A fraction 5 (Pg5) in urine than in gastric mucosa. The possible origin of differences in PGA isozymogen patterns was studied by organ culture of gastric biopsies. In vitro synthesis and secretion of pepsinogens were studied by electrophoresis and autoradiography. The synthesis rate of PGA in biopsies of 1-2 mm diameter was 40-100 ng/hr. Posttranslational modification of PGA isozymogens was demonstrated. Pg2 and part of Pg4 probably are secondary products of Pg3 and Pg5, respectively. In some individuals the secretion rate of Pg3 was low compared to the other isozymogens. The conversion of Pg3 into Pg2 and the differential secretion of the isozymogens may explain some of the discrepancies between gastric and urinary PGA patterns.

Molecular cloning of a pair of human pepsinogen A genes which differ by a Glu----Lys mutation in the activation peptide

Hepatitis C virus (HCV) belonging to the family Flaviviridae has infected 3% of the population worldwide and 6% of the population in Pakistan. The only recommended standard treatment is pegylated INF-α plus ribavirin. Due to less compatibility of the standard treatment, thirteen medicinal plants were collected from different areas of Pakistan on the basis of undocumented antiviral reports against different viral infections. Medicinal plants were air dried, extracted and screened out against HCV by infecting HCV inoculums of 3a genotype in liver cells. RT-PCR results demonstrate that acetonic and methanolic extract of Acacia nilotica (AN) showed more than 50% reduction at non toxic concentration. From the above results, it can be concluded that by selecting different molecular targets, specific structure-activity relationship can be achieved by doing mechanistic analysis. So, additional studies are required for the isolation and recognition of antiviral compound in AN to establish its importance as antiviral drug against HCV. For further research, we will scrutinize the synergistic effect of active antiviral compound in combination with standard PEG INF-α and ribavirin which may be helpful in exploring further gateways for antiviral therapy against HCV.

Molecular cloning of a pair of human pepsinogen A genes which differ by a Glu----Lys mutation in the activation peptide

Three human cosmid clones containing pepsinogen A (PGA) encoding sequences were isolated from a genomic bank derived from a single individual. One cosmid contains two PGA genes in tandem in a head-to-tail orientation, while the other two cosmids each contain a single PGA gene. The three cosmids were characterized by restriction mapping and sequence analysis (exons 1 and 2 and flanking regions). As judged from these data, three of the four PGA genes isolated appear to be nearly identical, but one of the tandem genes is clearly different from the other genes. The first exon of all four genes codes for the same amino acid sequence. However, in the second exon of one of the tandem genes we found a nucleotide substitution giving rise to a Glu----Lys substitution of the 43rd amino acid residue of the activation peptide, leading to a charge difference of the corresponding isozymogens. The presence of two distinct PGA genes in the isolated gene pair conclusively proves the multigene structure of the PGA system. These genes might be responsible for at least part of the electrophoretic polymorphism at the protein level.

Immunochemical, electrophoretic, and genetic heterogeneity of pepsinogen I. Characterization with monoclonal antibodies

Two immunologic subclasses of human pepsinogen I alpha-PG I and beta-PG I, have been identified based on their reactivity toward a murine monoclonal antibody that recognizes an epitope on the alpha-PG I isozymogens. The antibody was used to purify the major alpha- and beta-isozymogens from gastric mucosa and to determine their contributions to the previously described genetic polymorphism of PG I. The alpha-epitope was localized to the pepsin region of the molecules. The two major alpha-PG I isozymogens (Pg 3 alpha and Pg 5 alpha) and the major beta-PG I isozymogen (Pg 4 beta) were demonstrated to contain net charge differences located in the respective pepsin and activation peptide regions. We propose that the alpha- and beta-subclasses contain net charge amino acid substitutions encoded by the corresponding pepsinogen genes: PGA3, PGA4, and PGA5.

Pepsinogen A polymorphism in gastric mucosa and urine, with special reference to patients with gastric cancer

Electrophoretic pepsinogen A patterns were determined in gastric fundic mucosa biopsies from 601 patients with various gastric disorders and 25 healthy volunteers. Pepsinogen A patterns with an intense fraction 5 appeared to be associated with gastric cancer and premalignant changes of the stomach (p less than 10(-9)). In 60 individuals pepsinogen A patterns were determined in normal mucosa from different parts of the stomach. No differences were found between these patterns. In 29 out of 59 gastric cancer patients pepsinogen A could be demonstrated in the macroscopically malignant tissue. In two cases a different pattern compared with uninvolved fundic mucosa was observed. During a follow up study, major changes in the pepsinogen A pattern were observed in 7 out of 56 patients. In 8.6% of the examined patients urinary pepsinogen A patterns differed considerably as compared with the pattern observed in the gastric fundus. The results suggest that the highly significant association between intense Pg5 (the product of the D gene) and gastric cancer or its precursors may be caused by genetic as well as non-genetic factors.