KINETIC STUDIES OF BROMELAIN CATALYSIS

Assessment of a novel mucolytic solution for dissolving mucus in pseudomyxoma peritonei: an ex vivo and in vitro study

Background

Pseudomyxoma peritonei (PMP) is difficult to treat. Intraperitoneal delivery of mucolytic solutions might potentially improve therapy, in addition to surgical cytoreduction and hyperthermic intraperitoneal chemotherapy.

Methods

Comparison of mucolytic effect of two formulations (control: bromelain 300 µg/mL+N-Acetylcystein 250 mM; test: bromelain 200 µg/mL+200 mM cysteamine) in vitro on a mucin producing cell lines (HT29) and ex vivo on mucus obtained from 18 PMP patients. Mucin plugs were classified according to their density into three categories: hard, semi hard and soft. Simulation of peritoneal washing ex vivo using a closed heated circulating pump.

Results

Solubilisation was faster with the test vs. the control formulation (90 vs. 180 min) for dissolving the soft mucin plugs (p < 0.05). The test solution was also more effective in dissolving the hard mucus plugs compared to control (82.5±2.74 % vs. 36.33±3.27 %). All mucin types disintegrated in simulated peritoneal washing. Cytotoxicity of the test solution on HT29 cell line was time-dependent.

Conclusions

The test formulation is more effective and faster than the control formulation in dissolving mucus plugs of various densities. Mucus plugs were all solubilised after 40 min in simulated peritoneal washing. This novel mucolytic formulation might pave the way for an effective and less invasive therapy of PMP in the future.

A formulation for in situ lysis of mucin secreted in pseudomyxoma peritonei

Although numerous clinical attempts have been made to disintegrate mucin secreted by pseudomyxoma peritonei (PMP), none are clinically recommended. Through examination of the pharmacologic characteristics of two novel agents, we titrated an optimized combination of bromelain and N-acetyl cysteine (NAC) that demonstrates in vitro and in vivo efficacy in the dissolution of mucinous ascites from PMP. In the in vitro experiments, 1 g of mucin was incubated in varying concentrations of bromelain (0-400 µg/ml) and NAC (0-5%) individually followed by a combination before arriving at a therapeutic combination dose of 300 µg/ml bromelain+4% NAC. This established an effective dose of bromelain 300 µg/ml+4% NAC at pH 7.0, when tested in a rat model implanted with 3 g of mucin intraperitoneally (IP). IP administration of the drug in a rat model of PMP was shown to result in mucin disintegration within 72 hr with no toxicity observed.

Self-sustained pH oscillations in immobilized proteolytic enzyme systems

This work represents our first step toward fulfilling the need to discover a model system for experimental investigations of temporal oscillations, pattern formations, and other non-linearity related dynamic behavior in immobilized-enzyme-membrane systems. In this paper, the regions in the parameter space where self-sustained pH oscillations can be induced were determined via extensive numerical simulation for five enzyme-membrane systems involving four proteolytic enzymes: alpha-chymotrypsin, trypsin, bromelain, and ficin. From this study, we concluded that, even with current enzyme-immobilization techniques, the possibility of experimentally observing self-sustained pH oscillations in a flat membrane immobilized with alpha-chymotrypsin and using N-acetyl-L-tryptophan ethyl ester as a substrate is high. Under suitable conditions and with extra efforts, self-sustained oscillations may also occur in membrane systems immobilized with ficin, trypsin and bromelain.

Inhibition of papain by isothiocyanates

During the tapping of papaya latex for papain (EC 3.4.22.2), benzyl isothiocyanate is enzymatically produced from benzylglucosinolate, a major component of the latex fluid. Benzyl isothiocyanate inhibits papain hydrolysis of alpha-N-benzoyl-L-arginine ethyl ester (Bz-Arg-OEt). Since tha availability of the papain sulfhydryl group to 5,5'-dithiobis-2-nitrobenzoid acid (Nbs2) is inversely related to the extensiveness of the isothiocyanate inhibition, and treatment by benzyl isothiocyanate reduces the affinity of activated papain to a mercurial-Sepharose column, it is proposed that the papain sulfhydryl reacts with the electrophilic functional group of this inhibitor. Fifteen isothiocyanates were selected and both the chemical reactivity antors are involved in the isothiocyanate-papain inhibition: (1) chemical reactivity of isothiocyanates, (2) presence or absence of an aromatic substitution, and (3) the spatial relationship of the aromatic moiety to the -N = C = S group of isothiocyanates. These data further suggest the presence of an 'aromatic site' near the Cys-25 sulfhydryl group of activated papain. Based on the existing model of papain molecule, the imidazole group of His-159 coincides well with our proposed aromatic site. The prospects of using isothiocyanates as chemical probes for the study of chemical environment of active sites in other enzymes are briefly discussed.

Photosensitized inactivation of stem bromelain. Oxidation of histidine, methionine, and tryptophan residues

Pineapple stem bromelain was photooxidized in the presence of Methylene Blue used as a sensitizer. The essential sulfhydryl group of the enzyme protein rapidly became inaccessible to react with 5,5'-dithiobis(nitrobenzoic acid), but the reactivity was readily regained to the original level upon treatment with dithiothreitol. Even after such reduction, the photooxidized enzyme showed a markedly decreased hydrolytic activity on casein. Spectral examination revealed that the oxidized enzyme had tyrosine residues intact. Amino acid analysis showed significant decreases in histidine, ethionine, and tryptophan residues. Photoinactivation occurred in a similar manner also in the presence of tetrathionate which reversibly blocked the essential sulfhydryl group. It is concluded that the irreversible photoinactivation of stem bromelain must be related to the oxidation of histidine, methionine, and tryptophan residues. When the photooxidation was carried out a different pH values ranging from 4.0 to 8.3, the inactivation and the decrease in histidine content were found to be markedly pH dependent. Thus, the photooxidation experiment provided a method for directly measuring the apparent pKa of the ionization of the single histidine residue in stem bromelain. Apparent pKa values of 6.4 and 7.1 were obtained for the histidine imidazole in the absence and in presence of tetrathionate, respectively. In view of these normal pKa values for an imidazole, a mechanism of ionization of the active-site group in a plant thiol proteinase is proposed, in which the validity of mechanism involving a close electronic interaction between histidine and cysteine residues is seriously questioned.

The structure and mechanism of stem bromelain. Evaluation of the homogeneity of purified stem bromelain, determination of the molecular weight and kinetic analysis of the bromelain-catalysed hydrolysis of N-benzyloxycarbonyl-L-phenylalanyl-L-serine methyl ester

1. Purified stem bromelain (EC 3.4.22.4) was eluted from Sephadex G-100 as a single peak. The specific activity across the elution peak was approximately constant towards p-nitrophenyl hippurate but increased with elution volume with N(2)-benzoyl-l-arginine ethyl ester as substrate. 2. The apparent molecular weight, determined by elution analysis on Sephadex G-100, is 22500+/-1500, an anomalously low value. 3. Purified stem bromelain was eluted from CM-cellulose CM-32 as a single peak and behaved as a single species during column electrophoresis on Sephadex G-100. 4. Purified stem bromelain migrates as a single band during polyacrylamide-gel electrophoresis under a wide variety of conditions. 5. The molecular weight determined by polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate is 28500+/-1000. 6. Sedimentation-velocity and equilibrium-ultracentrifugation experiments, under a variety of conditions, indicate that bromelain is an apparently homogeneous single peptide chain of mol.wt. 28400+/-1400. 7. The N-terminal amino acid composition is 0.64+/-0.04mol of valine and 0.36+/-0.04mol of alanine per mol of enzyme of mol.wt. 28500. (The amino acid recovery of the cyanate N-terminal amino acid analysis was standardized by inclusion of carbamoyl-norleucine at the cyclization stage.) 8. The pH-dependence of the Michaelis parameters of the bromelain-catalysed hydrolysis of N-benzyloxycarbonyl-l-phenylalanyl-l-serine methyl ester was determined. 9. The magnitude and pH-dependence of the Michaelis parameters have been interpreted in terms of the mechanism of the enzyme. 10. The enzyme is able to bind N-benzyloxycarbonyl-l-phenylalanyl-l-serine methyl ester relatively strongly but seems unable to make use of the binding energy to promote catalysis.

Kinetics of the hydrolysis of N-benzoyl-L-serine methyl ester catalysed by bromelain and by papain. Analysis of modifier mechanisms by lattice nomography, computational methods of parameter evaluation for substrate-activated catalyses and consequences of postulated non-productive binding in bromelain- and papain-catalysed hydrolyses

1. N-Benzoyl-l-serine methyl ester was synthesized and evaluated as a substrate for bromelain (EC 3.4.22.4) and for papain (EC 3.4.22.2). 2. For the bromelain-catalysed hydrolysis at pH7.0, plots of [S(0)]/v(i) (initial substrate concn./initial velocity) versus [S(0)] are markedly curved, concave downwards. 3. Analysis by lattice nomography of a modifier kinetic mechanism in which the modifier is substrate reveals that concave-down [S(0)]/v(i) versus [S(0)] plots can arise when the ratio of the rate constants that characterize the breakdown of the binary (ES) and ternary (SES) complexes is either less than or greater than 1. In the latter case, there are severe restrictions on the values that may be taken by the ratio of the dissociation constants of the productive and non-productive binary complexes. 4. Concave-down [S(0)]/v(i) versus [S(0)] plots cannot arise from compulsory substrate activation. 5. Computational methods, based on function minimization, for determination of the apparent parameters that characterize a non-compulsory substrate-activated catalysis are described. 6. In an attempt to interpret the catalysis by bromelain of the hydrolysis of N-benzoyl-l-serine methyl ester in terms of substrate activation, the general substrate-activation model was simplified to one in which only one binary ES complex (that which gives rise directly to products) can form. 7. In terms of this model, the bromelain-catalysed hydrolysis of N-benzoyl-l-serine methyl ester at pH7.0, I=0.1 and 25 degrees C is characterized by K(m) (1) (the dissociation constant of ES)=1.22+/-0.73mm, k (the rate constant for the breakdown of ES to E+products, P)=1.57x10(-2)+/-0.32x10(-2)s(-1), K(a) (2) (the dissociation constant that characterizes the breakdown of SES to ES and S)=0.38+/-0.06m, and k' (the rate constant for the breakdown of SES to E+P+S)=0.45+/-0.04s(-1). 8. These parameters are compared with those in the literature that characterize the bromelain-catalysed hydrolysis of alpha-N-benzoyl-l-arginine ethyl ester and of alpha-N-benzoyl-l-arginine amide; K(m) (1) and k for the serine ester hydrolysis are somewhat similar to K(m) and k(cat.) for the arginine amide hydrolysis and K(as) and k' for the serine ester hydrolysis are somewhat similar to K(m) and k(cat.) for the arginine ester hydrolysis. 9. A previous interpretation of the inter-relationships of the values of k(cat.) and K(m) for the bromelain-catalysed hydrolysis of the arginine ester and amide substrates is discussed critically and an alternative interpretation involving substantial non-productive binding of the arginine amide substrate to bromelain is suggested. 10. The parameters for the bromelain-catalysed hydrolysis of the serine ester substrate are tentatively interpreted in terms of non-productive binding in the binary complex and a decrease of this type of binding by ternary complex-formation. 11. The Michaelis parameters for the papain-catalysed hydrolysis of the serine ester substrate (K(m)=52+/-4mm, k(cat.)=2.80+/-0.1s(-1) at pH7.0, I=0.1, 25.0 degrees C) are similar to those for the papain-catalysed hydrolysis of methyl hippurate. 12. Urea and guanidine hydrochloride at concentrations of 1m have only small effects on the kinetic parameters for the hydrolysis of the serine ester substrate catalysed by bromelain and by papain.