Biomedical Engineering Reference
In-Depth Information
wide variety of enzymes is involved in chitin and chitosan degradation,
including endo- and exo-chitinases, chitosanases, chitodextrinases, chi-
totriosidases, chitobiosidases, N-acetylhexosaminidases and enzymes of
the terminal metabolism of N-acetyl glucosamine and glucosamine [5].
h e occurrence of enzymes catalysing degradation of chitin chitosan has
been found not only in microorganisms, but also in plants, arthropods
and mammals. Chitinase activity has been discovered in human serum
and the enzyme, as well as its production by cloning, is patented [6, 7].
Evidences collected suggests that signii cant portions of chitin-based
dressings are depolymerised and that oligomers are further hydrolysed
to
N
-acetylglucosamine, a common aminosugar in the body which either
enters the innate metabolic pathway to be incorporated into glycoproteins
or is excreted as carbon dioxide [8].
Lysozyme is the primary enzyme responsible for
in vivo
degradation
of chitosan through hydrolysis of acetylated residues. Lysozyme may be
mainly responsible for chitosan degradation in serum, but may not be the
only enzyme involved
in vivo
degradation [9, 10]. h e mechanism of deg-
radation by these enzymes is not clearly elucidated.
h e biodegradation of chitosan is a phenomenon dependent on the
several factors, especially DDA, MW and degree of crystallinity. h elower
MW chitosans exert faster degradation in comparison to high MW chito-
sans [11]. h e
in vitro
and
in vivo
degradation rate of chitosan is inversely
related to the DDA [12, 13].
,
In vitro
studies showed that the chitosan with
a DDA of 97 % was not degraded at all, while that with a 50 % degraded
with maximum rate [14]. Another study found that the chitosans with
DDA from 30-70 % were degraded well (above 50 %) within 4 weeks,
whereas, the degradation of samples with very low or high deacetylation
was minimal over this period [15]. In addition, it seems that at least three
consecutive N-acetylated residues are necessary to be recognised by the
enzyme [16]. h e crystallinity of chitosan depends on DDA; so the rate
of degradation, in turn, depends on crystallinity [14, 17]. Susceptibility
of β-chitin to lysozyme was found to be much more than that of α-chitin
due to weak intermolecular forces. In addition, the water content, the
shape and the state of surface of the material, also inl uences biodegrada-
tion [18, 19]. Chemically modii ed chitosans are also endowed with sus-
ceptibility to lysozyme
in vivo
. It depends on the substitution on nitrogen
of glucosamine unit and/or on various substituents at the 3-O, 6-O- posi-
tion of the glucosamine and N-acetylglucosamine residues [20, 21].
Chitosan is susceptible to enzymatic degradation by other non-specii c
enzymes from a variety of sources such as cellulases [22], hemicellulases
[23], proteases [24], lipases [25] and collagenases [26].
