Purification and characterization of pepsins A1 and A2 from the Antarctic rock cod Trematomus bernacchii

Biochemical characterization of acid proteases from the stomach of palometa (Pygocentrus nattereri, Kner 1858) with potential industrial application

Pepsin is one of the major enzymes with significant importance in the food industry, biomedicines, and pharmaceutical formulations. In this work, the main objective was to biochemically characterize a pepsin-like enzymatic extract obtained from Pygocentrus nattereri, a predatory freshwater fish, focusing on their potential industrial application. The obtained extract exhibited optimal activity at 45 °C and pH 1.0-2.0. These proteases remained stable after 2 h of incubation at temperatures ranging from 0° to 45 °C and within pH range of 1.0 to 7.0. Their activity was significantly affected in presence of pepstatin A and SDS, 10 μM and 3.46 mM respectively, while EDTA and PMSF showed partial inhibitory effects. Divalent cations (Ca2+ and Mg2+) did not inhibit the proteolytic activity of the extract; in fact, it improved at a 5 mM CaCl2 concentration. As the NaCl concentration increased, the enzyme activity decreased. However, after desalination, 90 % of the activity was recovered within the tested exposure time. Besides, this extract demonstrated exceptional versatility across diverse industrial applications, including collagen extraction augmentation, IgG hydrolysis facilitation, and silver and polyester recovery from X-ray films. Our results suggest that the obtained enzymatic extract has a wide range of potential applications.

Expression pattern and cellular localization of pepsinogen in early development and induced by different diets in the spotted knifejaw (Oplegnathus punctatus)

To solve the high mortality rate of early-stage larval feed conversion during aquaculture in Oplegnathus punctatus, the investigation of the structural and functional characteristics of the gastric tissue was conducted. Histological results showed that the gastric gland rudiment appeared at 17 dph. The basic structure of the stomach was fully developed between 26 and 35 dph. Two pepsinogen genes, named OpPGA1 and OpPGA2, were identified in the spotted knifejaw genome. qPCR results of developmental period showed that the two genes were low in expression during early development (5 and 15 dph). At 20 dph, the two genes started to show trace expression, and at 30 dph the mRNA expression levels of OpPGA1 and OpPGA2 reached the highest levels. Results of pepsin activity detection during the development period showed lower activity was detected 22 dph, followed by a peak at 30 dph. Under different feeding inductions, OpPGA1 showed the highest expression in the basic diet group and hard-shell group, while the expression level in the phytophagous group remained consistently low. The mRNA expression level of OpPGA2 in the phytophagous group was significantly higher than in other groups. Enzyme activity determination under different feeding inductions showed slightly higher enzyme activity in the basic diet group and crustacean group. The results of in situ hybridization showed that the mRNA of both OpPGA1 and OpPGA2 genes was both expressed in gastric gland cells. These information can contribute to the development of practical feeding methods in terms of digestive physiology for the development of larvae.

Hydrogen Deuterium Exchange and other Mass Spectrometry-based Approaches for Epitope Mapping

Antigen-antibody interactions are a fundamental subset of protein-protein interactions responsible for the "survival of the fittest". Determining the interacting interface of the antigen, called an epitope, and that on the antibody, called a paratope, is crucial to antibody development. Because each antigen presents multiple epitopes (unique footprints), sophisticated approaches are required to determine the target region for a given antibody. Although X-ray crystallography, Cryo-EM, and nuclear magnetic resonance can provide atomic details of an epitope, they are often laborious, poor in throughput, and insensitive. Mass spectrometry-based approaches offer rapid turnaround, intermediate structural resolution, and virtually no size limit for the antigen, making them a vital approach for epitope mapping. In this review, we describe in detail the principles of hydrogen deuterium exchange mass spectrometry in application to epitope mapping. We also show that a combination of MS-based approaches can assist or complement epitope mapping and push the limit of structural resolution to the residue level. We describe in detail the MS methods used in epitope mapping, provide our perspective about the approaches, and focus on elucidating the role that HDX-MS is playing now and in the future by organizing a discussion centered around several improvements in prototype instrument/applications used for epitope mapping. At the end, we provide a tabular summary of the current literature on HDX-MS-based epitope mapping.

Digestive proteinases from the marine fish processing wastes of the South-West Atlantic Ocean: Their partial characterization and comparison

Fish processing generates plenty of waste that is directly discarded in open-air dumps and water sources, or treated in the same way as urban solid waste, causing serious pollution problems. The waste represents a significant source of high-value bioproducts with potential applications in different industrial processes such as the production of feed, fertilizers, biodiesel and biogas, detergent additives and cosmetics. The objective of this study was to characterize and compare specific activities under different pH values and temperature conditions of acid and alkaline proteinases and viscera yield from the following fish species: Argentine hake Merluccius hubbsi, Brazilian flathead Percophis brasiliensis, Brazilian codling Urophycis brasiliensis and Stripped weakfish Cynoscion guatucupa. Individuals were fished off the coast of Mar del Plata (Argentina) by a commercial fleet and the viscera were immediately extracted and kept on ice until use. Stomach proteinases from four species had the highest activity at pH 2, with stability in the range of pH 2-4. The optimum pH was 11.5 from intestinal enzymes of C. guatucupa, M. hubbsi and P. brasiliensis and 9.5 from intestinal enzymes of U. brasiliensis. Alkaline proteinases from all species were highly stable in the range of 7-11.5. The optimum temperature of stomach proteinases from the four species studied were 30 and 50°C, with stability at 10 and 30°C during 150 min. The optimum temperature of intestinal enzymes from the tested species were 50°C with high stability at 10 and 30°C during 150 min. Alkaline proteinase from all species and acid proteinases from C. guatucupa were inactive at 70°C after 150 min, while there was a residual activity lower than 5% at 80°C on pre-incubated stomach enzymes of M.hubbsi, P. brasiliensis and U. brasiliensis after 5, 10 and 20 min, respectively. Digestive proteinases recovered in this study could be appropriate for technological usage, reducing manufacturing costs, obtaining revenue from fishery wastes, and contributing to the reduction of environmental pollution.

Advances in Hydrogen/Deuterium Exchange Mass Spectrometry and the Pursuit of Challenging Biological Systems

Solution-phase hydrogen/deuterium exchange (HDX) coupled to mass spectrometry (MS) is a widespread tool for structural analysis across academia and the biopharmaceutical industry. By monitoring the exchangeability of backbone amide protons, HDX-MS can reveal information about higher-order structure and dynamics throughout a protein, can track protein folding pathways, map interaction sites, and assess conformational states of protein samples. The combination of the versatility of the hydrogen/deuterium exchange reaction with the sensitivity of mass spectrometry has enabled the study of extremely challenging protein systems, some of which cannot be suitably studied using other techniques. Improvements over the past three decades have continually increased throughput, robustness, and expanded the limits of what is feasible for HDX-MS investigations. To provide an overview for researchers seeking to utilize and derive the most from HDX-MS for protein structural analysis, we summarize the fundamental principles, basic methodology, strengths and weaknesses, and the established applications of HDX-MS while highlighting new developments and applications.

Gas-Phase Hydrogen/Deuterium Scrambling in Negative-Ion Mode Tandem Mass Spectrometry

Hydrogen/deuterium exchange coupled with mass spectrometry (HDX MS) has become a powerful method to characterize protein conformational dynamics. Workflows typically utilize pepsin digestion prior to MS analysis to yield peptide level structural resolution. Tandem mass spectrometry (MS/MS) can potentially facilitate determination of site-specific deuteration to single-residue resolution. However, to be effective, MS/MS activation must minimize the occurrence of gas-phase intramolecular randomization of solution-generated deuterium labels. While significant work has focused on understanding this process in positive-ion mode, little is known about hydrogen/deuterium (H/D) scrambling processes in negative-ion mode. Here, we utilize selectively deuterated model peptides to investigate the extent of intramolecular H/D scrambling upon several negative-ion mode MS/MS techniques, including negative-ion collision-induced dissociation (nCID), electron detachment dissociation (EDD), negative-ion free radical-initiated peptide sequencing (nFRIPS), and negative-ion electron capture dissociation (niECD). H/D scrambling was extensive in deprotonated peptides upon nCID and nFRIPS. In fact, the energetics required to induce dissociation in nCID are sufficient to allow histidine C-2 and C_β hydrogen atoms to participate in the scrambling process. EDD and niECD demonstrated moderate H/D scrambling with niECD being superior in terms of minimizing hydrogen migration, achieving ~ 30% scrambling levels for small c-type fragment ions. We believe the observed scrambling is likely due to activation during ionization and ion transport rather than during the niECD event itself.

N-Glycosylation Improves the Pepsin Resistance of Histidine Acid Phosphatase Phytases by Enhancing Their Stability at Acidic pHs and Reducing Pepsin's Accessibility to Its Cleavage Sites

N-Glycosylation can modulate enzyme structure and function. In this study, we identified two pepsin-resistant histidine acid phosphatase (HAP) phytases from Yersinia kristensenii (YkAPPA) and Yersinia rohdei (YrAPPA), each having an N-glycosylation motif, and one pepsin-sensitive HAP phytase from Yersinia enterocolitica (YeAPPA) that lacked an N-glycosylation site. Site-directed mutagenesis was employed to construct mutants by altering the N-glycosylation status of each enzyme, and the mutant and wild-type enzymes were expressed in Pichia pastoris for biochemical characterization. Compared with those of the N-glycosylation site deletion mutants and N-deglycosylated enzymes, all N-glycosylated counterparts exhibited enhanced pepsin resistance. Introduction of the N-glycosylation site into YeAPPA as YkAPPA and YrAPPA conferred pepsin resistance, shifted the pH optimum (0.5 and 1.5 pH units downward, respectively) and improved stability at acidic pH (83.2 and 98.8% residual activities at pH 2.0 for 1 h). Replacing the pepsin cleavage sites L197 and L396 in the immediate vicinity of the N-glycosylation motifs of YkAPPA and YrAPPA with V promoted their resistance to pepsin digestion when produced in Escherichia coli but had no effect on the pepsin resistance of N-glycosylated enzymes produced in P. pastoris. Thus, N-glycosylation may improve pepsin resistance by enhancing the stability at acidic pH and reducing pepsin's accessibility to peptic cleavage sites. This study provides a strategy, namely, the manipulation of N-glycosylation, for improvement of phytase properties for use in animal feed.

The effect of temperature adaptation on the ubiquitin-proteasome pathway in notothenioid fishes

There is an accumulating body of evidence suggesting that the sub-zero Antarctic marine environment places physiological constraints on protein homeostasis. Levels of ubiquitin (Ub)-conjugated proteins, 20S proteasome activity and mRNA expression of many proteins involved in both the ubiquitin (Ub) tagging of damaged proteins as well as the different complexes of the 26S proteasome were measured to examine whether there is thermal compensation of the Ub-proteasome pathway in Antarctic fishes to better understand the efficiency of the protein degradation machinery in polar species. Both Antarctic (Trematomus bernacchii, Pagothenia borchgrevinki) and non-Antarctic (Notothenia angustata, Bovichtus variegatus) notothenioids were included in this study to investigate the mechanisms of cold adaptation of this pathway in polar species. Overall, there were significant differences in the levels of Ub-conjugated proteins between the Antarctic notothenioids and B. variegatus, with N. angustata possessing levels very similar to the Antarctic fishes. Proteasome activity in the gills of Antarctic fishes demonstrated a high degree of temperature compensation such that activity levels were similar to activities measured in their temperate relatives at ecologically relevant temperatures. A similar level of thermal compensation of proteasome activity was not present in the liver of two Antarctic fishes. Higher gill proteasome activity is likely due in part to higher cellular levels of proteins involved in the Ub-proteasome pathway, as evidenced by high mRNA expression of relevant genes. Reduced activity of the Ub-proteasome pathway does not appear to be the mechanism responsible for elevated levels of denatured proteins in Antarctic fishes, at least in the gills.

Sulfonamide inhibition studies of the γ-carbonic anhydrase from the Antarctic bacterium Pseudoalteromonas haloplanktis

The Antarctic bacterium Pseudoalteromonas haloplanktis encodes for a γ-class carbonic anhydrase (CA, EC 4.2.1.1), which was cloned, purified and characterized. The enzyme (PhaCAγ) has a good catalytic activity for the physiologic reaction of CO2 hydration to bicarbonate and protons, with a k(cat) of 1.4×10(5) s(-1) and a k(cat)/K(m) of 1.9×10(6) M(-1)×s(-1). A series of sulfonamides and a sulfamate were investigated as inhibitors of the new enzyme. Methazolamide and indisulam showed the best inhibitory properties (K(I)s of 86.7-94.7 nM). This contribution shed new light on γ-CAs inhibition profiles with a relevant class of pharmacologic agents.

Sulfonamide inhibition studies of the γ-carbonic anhydrase from the Antarctic cyanobacterium Nostoc commune

A carbonic anhydrase (CA, EC 4.2.1.1) belonging to the γ-class has been cloned, purified and characterized from the Antarctic cyanobacterium Nostoc commune. The enzyme showed a good catalytic activity for the physiologic reaction (hydration of carbon dioxide to bicarbonate and a proton) with the following kinetic parameters, kcat of 9.5×10(5)s(-1) and kcat/KM of 8.3×10(7)M(-1)s(-1), being the γ-CA with the highest catalytic activity described so far. A range of aromatic/heterocyclic sulfonamides and one sulfamate were investigated as inhibitors of the new enzyme, denominated here NcoCA. The best NcoCA inhibitors were some sulfonylated sulfanilamide derivatives possessing elongated molecules, aminobenzolamide, acetazolamide, benzolamide, dorzolamide, brinzolamide and topiramate, which showed inhibition constants in the range of 40.3-92.3nM. As 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and γ-CAs are closely associated in carboxysomes of cyanobacteria for enhancing the affinity of RubisCO for CO2 and the efficiency of photosynthesis, investigation of this new enzyme and its affinity for modulators of its activity may bring new insights in these crucial processes.

Pepsinogens and pepsins from largemouth bass, Micropterus salmoides: purification and characterization with special reference to high proteolytic activities of bass enzymes

Six pepsinogens were purified from the gastric mucosa of largemouth bass (Micropterus salmoides) by DEAE-Sephacel chromatography, Sephadex G-100 gel filtration, and Mono Q FPLC. The potential specific activities of two major pepsinogens, PG1-1 and PG2-2, against hemoglobin were 51 and 118 units/mg protein, respectively. The activity of pepsin 2-2 was the highest among the pepsins reported to date; this might be linked to the strongly carnivorous diet of the largemouth bass. The molecular masses of PG1-1 and PG2-2 were 39.0 and 41.0 kDa, respectively. The N-terminal amino acid sequences of PG1-1 and PG2-2 were LVQVPLEVGQTAREYLE- and LVRLPLIVGKTARQALLE-, respectively, showing similarities with those of fish type-A pepsinogens. The optimal pHs for hemoglobin-digestive activity of pepsins 1-1 and 2-2 were around 1.5 and 2.0, respectively, though both pepsins retained considerable activity at pHs over 3.5. They showed maximal activity around 50 and 40 °C, respectively. They were inhibited by pepstatin similarly to porcine pepsin A. The cleavage specificities clarified with oxidized insulin B chain were shown to be restricted to a few bonds consisting of hydrophobic/aromatic residues, such as the Leu(15)-Tyr(16), Phe(24)-Phe(25) and Phe(25)-Tyr(26) bonds. When hemoglobin was used as a substrate, the kcat/Km value of bass pepsin 2-2 was 4.6- to 36.8-fold larger than those of other fish pepsins. In the case of substance P, an ideal pepsin substrate mimic, the kcat/Km values were about 200-fold larger than those of porcine pepsin A, supporting the high activity of the bass pepsin.

Analytical Aspects of Hydrogen Exchange Mass Spectrometry

This article reviews the analytical aspects of measuring hydrogen exchange by mass spectrometry (HX MS). We describe the nature of analytical selectivity in hydrogen exchange, then review the analytical tools required to accomplish fragmentation, separation, and the mass spectrometry measurements under restrictive exchange quench conditions. In contrast to analytical quantitation that relies on measurements of peak intensity or area, quantitation in HX MS depends on measuring a mass change with respect to an undeuterated or deuterated control, resulting in a value between zero and the maximum amount of deuterium that can be incorporated. Reliable quantitation is a function of experimental fidelity and to achieve high measurement reproducibility, a large number of experimental variables must be controlled during sample preparation and analysis. The method also reports on important qualitative aspects of the sample, including conformational heterogeneity and population dynamics.

Minimizing carry-over in an online pepsin digestion system used for the H/D exchange mass spectrometric analysis of an IgG1 monoclonal antibody

Chromatographic carry-over can severely distort measurements of amide H/D exchange in proteins analyzed by LC/MS. In this work, we explored the origin of carry-over in the online digestion of an IgG1 monoclonal antibody using an immobilized pepsin column under quenched H/D exchange conditions (pH 2.5, 0 °C). From a consensus list of 169 different peptides consistently detected during digestion of this large, ~150 kDa protein, approximately 30% of the peptic peptides exhibited carry-over. The majority of carry-over originates from the online digestion. Carry-over can be substantially decreased by washing the online digestion flow-path and pepsin column with two wash cocktails: [acetonitrile (5%)/isopropanol (5%)/acetic acid (20%) in water] and [2 M guanidine hydrochloride in 100 mM phosphate buffer pH 2.5]. Extended use of this two-step washing procedure does not adversely affect the specificity or activity of the immobilized pepsin column. The results suggest that although the mechanism of carry-over appears to be chemical in nature, and not hydrodynamic, carry-over cannot be attributed to a single factor such as mass, abundance, pI, or hydrophobicity of the peptides.

Accessing the reproducibility and specificity of pepsin and other aspartic proteases

The aspartic protease pepsin is less specific than other endoproteinases. Because aspartic proteases like pepsin are active at low pH, they are utilized in hydrogen deuterium exchange mass spectrometry (HDX MS) experiments for digestion under hydrogen exchange quench conditions. We investigated the reproducibility, both qualitatively and quantitatively, of online and offline pepsin digestion to understand the compliment of reproducible pepsin fragments that can be expected during a typical pepsin digestion. The collection of reproducible peptides was identified from >30 replicate digestions of the same protein and it was found that the number of reproducible peptides produced during pepsin digestion becomes constant above 5-6 replicate digestions. We also investigated a new aspartic protease from the stomach of the rice field eel (Monopterus albus Zuiew) and compared digestion efficiency and specificity to porcine pepsin and aspergillopepsin. Unique cleavage specificity was found for rice field eel pepsin at arginine, asparagine, and glycine. Different peptides produced by the various proteases can enhance protein sequence coverage and improve the spatial resolution of HDX MS data. This article is part of a Special Issue entitled: Mass spectrometry in structural biology.

Hydrogen exchange mass spectrometry: are we out of the quicksand?

Although the use of hydrogen exchange (HX) mass spectrometry (MS) to study proteins and protein conformation is now over 20 years old, the perception lingers that it still has "issues." Is this method, in fact, still in the quicksand with many remaining obstacles to overcome? We do not think so. This critical insight addresses the "issues" and explores several broad questions including, have the limitations of HX MS been surmounted and has HX MS achieved "indispensable" status in the pantheon of protein structural analysis tools.

Purification and characterization of pepsinogens and pepsins from the stomach of rice field eel (Monopterus albus Zuiew)

Three pepsinogens (PG1, PG2, and PG3) were highly purified from the stomach of freshwater fish rice field eel (Monopterus albus Zuiew) by ammonium sulfate fractionation and chromatographies on DEAE-Sephacel, Sephacryl S-200 HR. The molecular masses of the three purified PGs were all estimated as 36 kDa using SDS-PAGE. Two-dimensional gel electrophoresis (2D-PAGE) showed that pI values of the three PGs were 5.1, 4.8, and 4.6, respectively. All the PGs converted into corresponding pepsins quickly at pH 2.0, and their activities could be specifically inhibited by aspartic proteinase inhibitor pepstatin A. Optimum pH and temperature of the enzymes for hydrolyzing hemoglobin were 3.0-3.5 and 40-45 °C. The K (m) values of them were 1.2 × 10⁻⁴ M, 8.7 × 10⁻⁵ M, and 6.9 × 10⁻⁵ M, respectively. The turnover numbers (k(cat)) of them were 23.2, 24.0, and 42.6 s⁻¹. Purified pepsins were effective in the degradation of fish muscular proteins, suggesting their digestive functions physiologically.

Venom on ice: first insights into Antarctic octopus venoms

The venom of Antarctic octopus remains completely unstudied. Here, a preliminary investigation was conducted into the properties of posterior salivary gland (PSG) extracts from four Antarctica eledonine (Incirrata; Octopodidae) species (Adelieledone polymorpha, Megaleledone setebos, Pareledone aequipapillae, and Pareledone turqueti) collected from the coast off George V's Land, Antarctica. Specimens were assayed for alkaline phosphatase (ALP), acetylcholinesterase (AChE), proteolytic, phospholipase A(2) (PLA(2)), and haemolytic activities. For comparison, stomach tissue from Cirroctopus sp. (Cirrata; Cirroctopodidae) was also assayed for ALP, AChE, proteolytic and haemolytic activities. Dietary and morphological data were collected from the literature to explore the ecological importance of venom, taking an adaptive evolutionary approach. Of the incirrate species, three showed activities in all assays, while P. turqueti did not exhibit any haemolytic activity. There was evidence for cold-adaptation of ALP in all incirrates, while proteolytic activity in all except P. turqueti. Cirroctopus sp. stomach tissue extract showed ALP, AChE and some proteolytic activity. It was concluded that the AChE activity seen in the PSG extracts was possibly due to a release of household proteins, and not one of the secreted salivary toxins. Although venom undoubtedly plays an important part in prey capture and processing by Antarctica eledonines, no obvious adaptations to differences in diet or morphology were apparent from the enzymatic and haemolytic assays. However, several morphological features including enlarged PSG, small buccal mass, and small beak suggest such adaptations are present. Future studies should be conducted on several levels: Venomic, providing more detailed information on the venom compositions as well as the venom components themselves; ecological, for example application of serological or genetic methods in identifying stomach contents; and behavioural, including observations on capture of different types of prey.

Investigating solution-phase protein structure and dynamics by hydrogen exchange mass spectrometry

By taking advantage of labeling methods such as hydrogen exchange (HX), many details about protein conformation, dynamics, and interactions can be revealed by mass spectrometry. In this unit, hydrogen exchange theory is discussed as it applies to HX-MS protocols, the practice of HX-MS including data analysis and interpretation is explained in detail, and recent advancements in technology which greatly increase the depth of information gained from the technique are highlighted.

Study on pepsinogens and pepsins from snakehead (Channa argus)

Three pepsinogens (PG1, PG2, and PG3) were highly purified from the stomach of freshwater fish snakehead (Channa argus) by ammonium sulfate fractionation, anion exchange, and gel filtration. Two-dimensional gel electrophoresis and native-PAGE analysis revealed that their molecular masses were 37, 38, and 36 kDa and their isoelectric points 4.8, 4.4, 4.0, respectively. All of the pepsinogens converted into their active form pepsins under pH 2.0 by one-step pathway or stepwise pathway. The three pepsins showed maximal activity at pH 3.0, 3.5, and 3.0 with optimum temperature at 45, 40, and 40 degrees C, respectively, using hemoglobin as substrate. All of the pepsins were completely inhibited by pepstatin A, a typical aspartic proteinase inhibitor. The N-terminal amino acid sequences of the three pepsinogens were determined to the 34th, 25th, and 28th amino acid residues, respectively. Western blot analysis of the three PGs exhibited different immunological reactions.

Aspartic proteinases in Antarctic fish

The present review surveys several recent studies of the aspartic proteinases from Antarctic Notothenioidei, a dominating fish group that has developed a number of adjustments at the molecular level to maintain metabolic function at low temperatures. Given the unique peculiarities of the Antarctic environment, studying the features of Antarctic aspartic proteinases could provide new insights into the role of these proteins in fish physiology. We describe here: (1) the biochemical properties of a cathepsin D purified from the liver of the hemoglobinless icefish Chionodraco hamatus; (2) the biochemical characterization of Trematomus bernacchii pepsins variants A1 and A2 obtained by heterologous expression in bacteria; and (3) the identification of two closely related, novel aspartic proteinases from the liver of the two Antarctic fish species mentioned above. Overall, the results show that Notothenioidei aspartic proteinases display a number of characteristics that are remarkably different from those of mammalian aspartic proteinases, including high turnover number or high catalytic efficiency. We have named the newly identified aspartic proteinases "Nothepsins" and classified them relative to aspartic proteinases from other species.

Pepsinogens and pepsins from mandarin fish (Siniperca chuatsi)

Four pepsinogens (PG-I, PG-II, PG-III(a), and PG-III(b)) were highly purified from the stomach of the freshwater fish mandarin fish (Siniperca chuatsi) by ammonium sulfate fractionation, anion exchange, and gel filtration. The molecular masses of the four purified PGs were 36, 35, 38, and 35 kDa, respectively. All the pepsinogens converted into their active form pepsins within a few minutes under pH 2.0. The optimum pH and temperature of the four enzymes were 3.0-3.5 and 45-50 degrees C, using hemoglobin as a substrate. The N-terminal amino acid sequences of PG-I and PG-II were determined to the 12th and 17th amino acid residues, respectively. Western blot analysis using antisea bream polyclonal antibodies cross reacted with PG-I, PG-II, and PG-III(b) while no cross reaction with PG-III(a) was detected, suggesting the diversity of pepsinogens in fish.