DEGRADATION OF FULLY WATER-SOLUBLE, PARTIALLY N-ACETYLATED CHITOSANS WITH LYSOZYME

Chitosans, prepared by homogeneous N-deacetylation of chitin, with degrees of N-acetylation ranging from 4 to 60% (F(A) = 0.04 to 0.60) exhibiting full water solubility and known random distribution of acetyl groups, were degraded with lysozyme. Initial degradation rates (r) were determined from plots of the viscosity decrease (DELAT1/[eta]) against time of degradation. The time course of degradation of chitosans with lysozyme were non-linear, while the time course of degradation of chitosans with an oxidative-reductive depolymerization reaction (using H2O2) showed the expected linear relationship for a first-order, random depolymerization reaction, independent of the chemical composition of the chitosan. The effect of lysozyme concentration and substrate concentration on the initial degradation rates were determined, showing that this lysozyme-chitosan system obeys Michaelis-Menten kinetics. The initial degradation rates of chitosan with lysozyme increased strongly with increasing fraction of acetylated units (F(A)). From a Michaelis-Menten analysis of the degradation data that assumes different catalytic activities of lysozyme for the different hexameric substrates in the polysaccharide chain, it is concluded that the hexameric substrates that contain three-four or more acetylated units contribute mostly to the initial degradation rate when lysozyme degrades partially N-acetylated chitosans. A chitosan with a very low fraction of acetylated units (F(A) = 0.010) was studied as an enzyme inhibitor. Initial degradation rates of chitosan (with different F(A) values) decreased as the inhibitor concentration increased, while the relative rates stayed constant, indicating that the ratio between initial reaction rates for productive sites (hexamers containing three-four or more N-acetylated units) are unaffected by non-productive sites, as deduced from the theory of competing substrates.