1 Avances en Química, (), 5- (007) Artículo Científico Kinetic of the enzymatic degradation of chitosan using bromelain: a viscosimetric study. Cristóbal Lárez Velásquez *, Dimas Medina, Carlos Torres, Enrique Millán () Grupo de Polímeros, Departamento de Química, Facultad de Ciencias, Universidad de Los Andes () Grupo de Electroquímica, Departamento de Química, Facultad de Ciencias, Universidad de Los Andes Mérida 50, Venezuela (*) Recibido: 0/07/007 Revisado: /08/007 Aceptado: /08/ Resumen: Cinética de la degradación enzimática de quitosano usando bromelina: un estudio viscosimétrico. Se realizó la degradación enzimática de quitosano (grado de acetilación (ga) y peso molecular promedio viscosimétrico (M V ) 0,85 y 0 kda, respectivamente) usando la enzima comercial bromelina. El decrecimiento del grado de polimerización viscosimétrico promedio (X V ) fue estudiado en una solución reguladora de ácido acético (0, M)/acetato de sodio (0, M) a 5 ºC. Bajo estas condiciones experimentales es posible correlacionar la viscosidad intrínseca instantánea [η] con M V a través de la ecuación de Mark-ouwink-Sakurada. Las ecuaciones de Sano, Solomon-Ciuta, Deb-Chatterjee y Ram-Yaseen, las cuales permiten calcular valores de [η] con una simple medida de viscosidad relativa (η rel ) y/o específica (η sp ), fueron empleadas para obtener valores instantáneos de [η]. También fueron obtenidos valores instantáneos de [η] a través de un proceso iterativo usando la ecuación de uggins. La velocidad inicial de depolimerización (v o ) fue obtenida de soluciones con diferentes concentraciones de quitosano y una concentración fija de bromelina. La constante de Michaelis-Menten (k M ) y la velocidad maxima (V m ) fueron obtenidas usando la representación de Lineweaver-Burk de la ecuación de Michaelis-Menten para cada juego de valores de [η], los cuales fueron calculados empleando todos los procedimientos mencionados. Los mejores parámetros cinéticos para la depolimerización del quitosano (k M =.9x0-7 mol.dl - ; V m = 9.09x0-8 mol.dl -.s - ) se obtuvieron cuando se empleó la ecuación de Sano para calcular los valores de [η]. Palabras clave: bromelina, estudios de degradación, depolimerización de quitosano, depolimerización enzimática. Abstract Enzymatic degradation of chitosan (degree of acetylation (da) and average viscosimetric molecular weight (M V ) of 0.85 and 0 kda, respectively) was carried out using commercial bromelain. Decrease of the average viscosimetric polymerization degree (X V ) was studied in a buffer solution of acetic acid (0. M)/sodium acetate (0. M) at 5 ºC. Under these experimental conditions it is possible to relate the instantaneous intrinsic viscosity [η] with M V through the Mark-ouwink-Sakurada equation. Equations of Sano, Solomon-Ciuta, Deb-Chatterjee and Ram-Yaseen, which allow to compute values of [η] with a single measurement of relative (η rel ) and/or specific viscosity (η sp ), were employed to obtain instantaneous values of [η]. Instantaneous values of [η] were also obtained through an iterative process using uggins equation. Initial velocity of depolymerization (v o ) was obtained from different concentration of chitosan solutions and a fixed bromelain concentration. Michaelis-Menten constant (k M ) and maximum velocity (V m ) were obtained using the Lineweaver-Burk representation of the Michaelis-Menten equation for each set of [η] which were calculated employing all procedures mentioned. Best kinetic parameters for chitosan depolymerization (k M =.9x0-7 mol.dl - ; V m =9.09x0-8 mol.dl -.s - ) were obtained when Sano`s equation was employed to calculate [η] values. Keywords: bromelain, degradation studies, chitosan depolymerization, enzymatic depolymerization. 5 Introduction Production of oligopolisaccharides from chitin and chitosan depolymerization is a growing research area due to the wide range of applications that have been derived from these materials. There is in vitro evidence, including some of medical significance, that chitosan oligomers enhance the antimicrobial activity against phytopathogenic fungi [-] and bacteria [-6]. Similarly, chitosan oligomers seem to improve the activity of the native chitosan in the preservation of foods, odontologic treatments, wound protection, etc. Chitosan depolymerization has been carried out by chemical (i.e. NO ), physical (i.e. sonication ) and biological (i.e. enzymes [9-]) methods. owever, when
2 6 C. Lárez Velásquez, D. Medina, C. Torres, E. Millán /Avances en Química, (), 7- (007) the proteolytic enzyme bromelain has been used as depolymerization agent, literature is scarce and not conclusive. Thus, while some authors have found that this enzyme is able to produce depolymerization in a good extension [,], others have reported a low activity on chitosan degradation . Susceptibility of chitosan to various nonspecific enzymes (i.e. wheat germ lipase , lysozyme , papain , cellulase , hemicellulase , b-glucosidase , etc.) has been reported. owever, some authors have attributed this behavior to enzyme purity, which could be contaminated with chitosanases [,9]. On the other hand, based on results obtained from a genetically engineered lipase  others authors assume that these enzymes can catalyze this degradation reaction by themselves. Presumably, hydrolysis of chitosan by this enzymatic preparation follows an acid-base mechanism  Chitosan depolymerization by a nonspecific enzyme such as bromelain may possess much practical importance because this is easily obtained from stem of pineapple, which implies low production costs and use of biocompatible materials. Specific enzymes with high activity toward chitosan depolymerization (i.e. chitosanase) are usually very expensive and most of them unavailable in bulk for commercial exploitation . Usually, most of chitosanases are obtained from microbial sources although it has been reported the isolation and purification from some plants (sweet orange callous tissue  and cucumber ). In order to obtain quantitative data on the chitosan depolymerization by bromelain we have carried out kinetic studies by mean of a viscosimetric method. Viscosimetric data were evaluated using several of the viscosimetric equations more reported in the literature to compute [η] by mean of a single measure (Table ). Additionally, an iterative process based on uggin s equation was employed in order to compare with results coming from of the single point methods. Depolymerization was carried out in acetic acid (0. M)/sodium acetate (0. M) buffer solution at 5 ºC. This medium allows to directly relates changes in the viscosimetric measurements to molecular weight variation of the biopolymer. Experimental Chemicals: chitosan (Fluka BioChemika, MW) was purified dissolving the biopolymer in acetic acid and precipitating it by neutralization (p 7) with NaO, then the polymer is filtrated and exhaustively rinsed with deionized water (Millipore) and dried at 50 ºC;.0x0-5 g/dl bromelain (Sigma Chemical Co.) solution in 0.08 EDTA disodium salt dihydrate (J.T. Baker) was prepared previous to kinetics studies. Chitosan characterization: M v was determined from viscosimetric measurements in acetic acid (0. M)/sodium acetate (0. M) buffer solution. An Ubbelohde type viscometer (Canon) was used. Temperature was maintained at 5.0±0. ºC using a thermostated water bath (Gallenkamp). -nmr spectroscopy in D O + trifluroacetic acid (Bruker, DRX 00 MZ) analysis shows an acetylation degee of 0.6. Viscosimetric kinetics studies: a variable amount of standard buffered chitosan solution and fresh buffer solution, both filtrated through a funnel of irsch with fritted disc (Sovirel No. ), were loaded into the viscosimeter (under controlled temperature) in the adequate proportion (see table ). After thermal stabilization five viscosimetric measurements of the unload enzyme solution were ran to ensurechitosan solutions stability under time experimental time. Then a determined volume of enzyme solution (0 µl) was introduced in the system, stirred and, immediately, consecutive viscosimetric measurements were carried out along the time. Each set of solution concentrations was ran the same day and this procedure was repeated three times with very similar results. Data used to construct table was selected from the better set of results because it is difficult to carry out the viscosimetric measures at the same times in each run. Viscosimetric data have been treated according to following procedure: a) Relative viscosity (η r ) and/or specific viscosity (η sp ) are related to intrinsic viscosity ([η]) using any of the empirical equations that allow to calculate [η] from a single measurement. Table shows some of these equations, although Chee  has concluded that some of them (i.e. equations, and ) produce unacceptable results even when diluted polymer solutions are used. b) Average viscosimetric molecular weight (M v ) can be obtained directly from [η] using Mark-ouwink- Sakurada (MS) equation ([η] = KM v α ). Chitosan K and α values have been reported by Rinaudo et al (K = 7.x0 - and α = 0.76 for a chitosan with da = 0.5) in acetic acid/sodium acetate buffer solution at 5 ºC . c) Initial velocity (v o ) for depolymerization process can be obtained by plotting M V (or X V ) vs time d) Kinetics parameters (Michaelis-Menten constant (K M ) and maximum velocity (V m )) are obtained by processing v o values according to Michaelis-Menten equation, plotting /v o vs /c po (Lineweaver-Burk representation).
3 C. Lárez Velásquez, D. Medina, C. Torres, E. Millán /Avances en Química, (), 7- (007) 7 Table. Empirical equations used to intrinsic viscosity determination through the single-point method. Equation Number Reference [η] = 8[(η rel ) /8 - ]/c p ()  [η] = [(η sp - lnη rel )/(c p ) ] / ()  [η] = [(lnη rel - η sp + ½(η sp ) )] / /c p ()  [η] = (η sp + lnη rel )/c p ()  (a) (b) Fig..- (a) η sp with time response for a chitosan/bromelain solution ( g/dl and.0x0-5 g/dl, respectively) in acetic acid (0.0 M)/sodium acetate (0.0 M) buffer solution at 5 ºC for long time reaction. (b) η sp with time response for diverse chitosan concentrations at fixed bromelain concentrations (9.8x0-6 g/dl) in acetic acid (0.0 M)/sodium acetate (0.0 M) buffer solution at 5 ºC for initial stages of depolymerization process. Results and discussion Experimental M v value obtained from the starting chitosan sample (09 KDa), perfectly agree with a previous study on similar commercial chitosan sample . Data were plotted according uggins equation (η sp /c p = [η]+ k [η] c p ) and a value for [η] =.0 dl/g was obtained. On the other hand, a uggins constant (k ) value of 0.56 was also obtained, indicating non appreciable aggregation in the system. Figure (a) shows long time η sp changes vs. reaction time for a chitosan solution ( g/dl) and bromelain (.0x0-5 g/dl) in acetic acid (0.0 M)/sodium acetate (0.0 M) buffer solution at 5 ºC. Similar studies with different enzyme/substrate ratios were conducted previously. As it can be appreciated, a typical macromolecular enzymatic depolymerization behavior, with an appreciable η sp decreasing along the time, is observed. This result demonstrates that bromelain, in this medium, has a good activity in spite of the low concentration of enzyme employed in this assay. Figure (b) shows η sp changes vs. reaction time for the initial stages of the chitosan enzymatic degradation under the experimental conditions showed in table. Average viscosimetric polymerization degree (X v ) values (M v = M o X v with M o = average molar mass of the repetitive units= 67. g/mol) and [η] values were obtained from figure b data, for each time, employing MS equation and equations - (table ), respectively. Similar results
4 8 C. Lárez Velásquez, D. Medina, C. Torres, E. Millán /Avances en Química, (), 7- (007) were obtained for all samples, showing good correlation (table ) when these data were used to obtain values of the Table.- Results obtained during degradation studies of different chitosan solutions using bromelain. Correlation coefficient (R ) are referred to plots of X v vs. t Sample V standar chitosan (ml) V buffer (ml) c po. (mol ch.dl - ) x x x x x x0-7.0x0-7.96x0-7.87x0-7.7x x x x0-7.86x0-7.86x x x x0-7.6x0-7.5x x0-7.00x0-7.50x x x0-7.99x0-7.76x0-7.x0-7.x x0-7.58x x0-7.6x0-7.5x0-7 Eq. X V initial v om (mol ru.mol - ch.s - ) C bromelain init = 9.8x0-6 g/dl; V bromelain init = 0 µl; C standar chitosan = 0.9 g/dl v o (mol ru.dl -.s - ).80x0-8.80x0-8.76x0-8.7x0-8.70x0-8.59x0-8.56x0-8.6x0-8.5x0-8.5x0-8.66x0-8.x x0-8.86x0-8.86x0-8.76x0-8.55x0-8.9x0-8.7x x x0-8.x0-8.09x0-8.8x0-8 5.x x0-8.60x0-8.6x0-8.60x x x0-8.97x0-8.67x0-8.88x0-8 R average depolymerization initial velocity (v om ). It is important to clarify that v om, such as it has been determined by plotting X v vs. reaction time (figure ), represents an average depolymerization initial velocity value (i.e. average of the moles of repetitive units (mol ru ) broken by mol of chitosan by time, expressed in mol ru.mol ch -.s - ) but it does not measure the variation of the concentration of repetitive units being depolymerized. Then, total depolymerization initial velocity (v o ) is obtained, in each sample, by the product v om c p, and expressed as mol ru.dl -.s -. Table shows several interesting results, as for example:
5 C. Lárez Velásquez, D. Medina, C. Torres, E. Millán /Avances en Química, (), 7- (007) 9 Fig..- X v changes at the initial stages of depolymerization of a chitosan (0.086 g/dl) solution using bromelain (9.8x0-6 g/dl) in acetic acid (0.0 M)/sodium acetate (0.0 M) buffer at 5 ºC. Values for X v were obtained according to equations ( ), (o), ( ) and ( ). Grey symbol correspond to X v initial values obtained using the respective equation; bold symbols represent data used to compute v o. a) Values of v om have a better linear correlation coefficient (R ) for most of samples when equation was used to compute [η] values. b) X v initial values obtained for each sample tend to increase with chitosan concentration when equations - were used to calculate [η] values. c) X v initial values for each sample (values obtained before adding enzyme to each chitosan solution) are less disperse when equation was used to calculate [η] (see table ), including values obtained through an iterative data treatment using the uggins equation. [η] values obtained employing this iterative process were computed reordering uggins equation as: [η] = η sp /c p k [η] c p then, [η] values are placed into the right side of the Table.- Standard deviation obtained for the average X v initial when equations - and uggins equation are used to calculate [η]. X v initial values were employed in these calculations for all chitosan solutions. Equation Average X v Standard % initial deviation deviation uggins equation until an equal value for [η] is produced. This intrinsic viscosity value obtained from the iterative treatment, correspond to [η] for the polymer in the experimental conditions studied (c p and η sp ). k value (0.56) obtained from the initial viscosimetric study is used and it is supposed that remain unvaried during the depolymerization initial stages studied. On the other hand, we have supposed that initial changes in viscosity are only due to chitosan molecular weight decrease and there are not contributions from conformational changes in the polymeric chain due to a molecular weight transition. According to Tsaih , for a chitosan of similar acetylation degree, this transition is expected to appear when X V is around 0 (M V KDa), which is far below than experimental X V values used for our kinetic studies. igher X v initial values dispersion observed when equations - are employed to calculate [η], which come from the increase of X v values when concentration is increased, may possibly be attributed to the logarithmic contribution in these equations. These logarithmic contributions are more remarkable for η sp > or η rel >, as it is observed for more concentrated solutions. On the other hand, due to equation has not any logarithmic contribution produces less dispersed results, which do not show an increasing trend when concentration is increased. Figure shows the Lineweaver-Burk or double reciprocal representation (/v o vs /c po ) using v o values obtained with all the procedures used to compute instantaneous [η] values. v o values were obtained from similar plots as in figure, using either equations - or the iterative process of uggins equation. Acceptables linear correlation coefficients are observed for most curves, particularly for curve obtained when Sano s equation was used, although K M and V max values obtained from each curves are very different. These values (Table ) were calculate from the slope (m = K M /V max ) and intercept (i = /V max ) obtained from each plot in figure. Table. K M and V max values obtained for the initial enzymatic degradation of chitosan with bromelain using the different procedures to calculate [η]. Equation used to calculate [η] K M (mol.dl - ) V max (mol.dl -.s - ).9x x x0-7.7x x x x0-6 -.x uggins 5.50x0-7.7x R
6 0 C. Lárez Velásquez, D. Medina, C. Torres, E. Millán /Avances en Química, (), 7- (007) Fig..- Lineweaver-Burk representation (double reciprocal) for depolymerization of chitosan solutions using bromelain (9.8x0-6 g/dl) in acetic acid (0.0 M)/sodium acetate (0.0 M) buffer to 5 ºC. Table 5.- K M and V max values reported in some enzymatic systems using chitosan as substrate. Enzyme Sustrate p T (ºC) K m * V max * ydrolase purified from commercial stem bromelain  Papain purified from commercial sample  Pronase from Streptomyces griseus  Chitosan (M w 7 kda, DD 0,7) prepared from shrimp chitin %.50 µmol GlcN/min/mg g.dl - 5.8x0-8 mol.s -.mg mg.ml nmoles.min -.mg g.dl -.8x0-9 mol.s -.mg - Commercial chitosan mg.ml -.8 nmoles.min -.mg g.dl -.x0-9 mol.s -.mg - ydrolase purified from commercial cellulase  Commercial chitosan (M 560 kda, DD 0,90) mg/ml 0.00 g.dl µmol.min - mg x0-9 mol.s -.mg - Chitosanase purified from Bacillus sp.  Chitosan (DD 0.798) mg.ml g.dl - 7.7x0-6 mol.s - mg - Commercial bromelain without purification (this work) Commercial chitosan (M 0 kda, DD 0,85) g.dl x0-8 mol.s -.mg - * Bold values are reported by the authors, after conversion to unique type of units for a better comparison.
7 C. Lárez Velásquez, D. Medina, C. Torres, E. Millán /Avances en Química, (), 7- (007) By considering that Sano s equation produces (a) an average X V initial value more similar to that obtained through the graphic method using uggins equation, (b) a less dispersed X v initial values and (c) a better correlation of the Lineweaver-Burk representation, k M and V max values obtained with this equation have been selected as the more reliable results. A final consideration to support this selection is based on the fact that Sano`s equation is the only relationship that include c po = K M values in the concentration range evaluated to calculate [η]. Finally, we have found that k M and V max values obtained employing this equation are in an order of magnitude similar to those reported for chitosan enzymatic depolymerization systems, as it can be appreciated in the Table 5. Conclusions Kinetic studies of initial depolymerization of the biopolymer chitosan using the non specific enzyme bromelain has been carried out through viscosimetric analysis. Instantaneous intrinsic viscosities during degradation were calculated by either an iterative process using uggins equation or through four empirical equations employing the single point method under experimental conditions where chitosan MS viscosimetric constants are well known. Sano s equation produced the more reliable results (K M = 0. g.dl - and V max =.8x0-7 mol.s -.mg - ), which are comparable to results obtained from chitosan degradation with specific and nonspecific enzyme. Acknowledgements This work was financially supported by Consejo de Desarrollo Científico, umanístico y Tecnológico de la Universidad de Los Andes, Mérida, Venezuela. Proyect code: C B. References.- C.R. Allan, L.A. adwiger. The fungicidal effect of chitosan on fungi of varying cell wall composition. Exp. Mycol.,, (979)..- D.F. Kendra, L.A. adwiger. Characterisation of the smallest chitosan oligomer that is maximally antigungal to Fusarium solani and elicits pisatin formation in Pisum sativum. Exp. Mycol., 8, 76-8 (98)..- S. Roller, N. Covill. The antifungal properties of chitosan in laboratory media and apple juice. Int. J. Food Microbiol., 7, (999)..- X.F. Liu, Y.L. Guan, D.Z. Yang, Z. Li, K. De Yao. Antibacterial action of chitosan and carboxymethylated chitosan. J. Appl. Polym. Sci., 79, -5 (00) Seo, K. Mitsuhashi,. Tanibe. Antibacterial and antifungal fiber blended by chitosan. In Advances in Chitin and Chitosan. C.J. Brine, P.A. Sandford, J.P. Zikakis et al. Eds. Elsevier Science Publ. Ltd, London, 0 (99). 6.- R. Muzzarelli, R. Tarsi, O. Filippini, E. Giovanetti, G. Biagini, P.E. Varaldo. Antimicrobial properties of N- carboxybutyl chitosan. Antimicrobial Agents and Chemotherapia. (0), 09 0 (990). 7.- G.G. Allan, M. Peyron. Molecular weight manipulation of chitosan. I: Kinetics of depolymerization by nitrous acid. Carbohydr. Res., 77(), 57-7 (995). 8.- M.L. Tsaih, R.. Chen. Effect of degree of deacetylation of chitosan on the kinetics of ultrasonic degradation of chitosan. J. Appl. Polym. Sci., 90, 56-5 (00). 9.- R.A.A. Muzzarelli, V. Stanic, V. Ramos. Enzymatic depolymerization of chitins and chitosans. In Carbohydrate Biotechnology Protocols. C. Bucke, Ed. umana Press, London (999). 0.- C.Y. Cheng, Y.K. Li. An Aspergillus chitosanase with potential for large scale preparation of chitosan oligosaccharides. Biotechnology and Applied Biochemistry,, 97-0 (000)..- A.B.V. Kumar, M.C. Varadaraj, L.R. Gowda, R.N. Tharanathan. Characterization of chito-oligosaccharides prepared by chitosanolysis with the aid of papain and Pronase, and their bactericidal action against Bacillus cereus and Escherichia coli. Biochem. J., 9, (005)..- L.F. Lee, W.K. Lee, M.Y. Maskat, R.M. Illias, S.A. Aziz, K. Kamarulzaman,. Osman. Partial Depolymerization of chitosan with the aid of Bromelain. Pakistan Journal of Biological Sciences, 8(), 7-77 (005)..- M. Yalpani and D. Pantaleone. An examination of the unusual susceptibilities of aminoglycans to enzymatic hydrolysis. Carbohydr. Res., 56, (99)..- T.A. Nicholas. Antimicrobial use of native and enzymatically degraded chitosans for seafood protections. Thesis. Univesity of Maine (00). 5.- R.A.A. Muzzarelli, W.S. Xia, M. Tomasetti., P. Ilari. Depolymerization of chitosan and substitute chitosans with the aid of a wheat germ lipase preparation. Enzyme Microb. Technol., 7, 5 55 (995). 6.- B.T. Stokke, K.M. Vårum,.K. olme, R.J.N. jerde, O. Smidsrød. Sequence specificities for lysozyme depolymerization of partially N-acetylated chitosans. Canadian Journal of Chemistry,7, (995). 7.- G.J. Wu, G.J. Tsai. Cellulase degradation of shrimp chitosan for the preparation of a water-soluble hydrolysate with immunoactivity. Fisheries Science, 70 (6), 0 (00). 8.- C. Qin, Y. Du, L. Zong, F. Zeng, Y. Liu, B. Zhou. Effect of hemicellulase in the molecular weight and structure of chitosan. Polymer Degradation and Stability, 80, 5 (00).
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