Chitosan

Chitosan is a linear polysaccharide composed of randomly distributed ß-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It has a number of commercial and possible biomedical uses.

Manufacture and properties


Chitosan is produced commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans (crabs, shrimp, etc.). The degree of deacetylation (%DA) can be determined by NMR spectroscopy, and the %DA in commercial chitosans is in the range 60-100 %.

The amino group in chitosan has a pKa value of ~6.5, thus, chitosan is positively charged and soluble in acidic to neutral solution with a charge density dependent on pH and the %DA-value. In other words, chitosan is bioadhesive and readily binds to negatively charged surfaces such as mucosal membranes. Chitosan enhances the transport of polar drugs across epithelial surfaces, and is biocompatible and biodegradable. Purified qualities of chitosans are available for biomedical applications.

Chitosan and its derivatives such as trimethylchitosan (where the amino group has been trimethylated) have been used in non-viral gene delivery. Trimethylchitosan, or quaternised chitosan, has been shown to transfect breast cancer cells; with increased degree of trimethylation increasing the cytotoxicity and at approximately 50% trimethylation the derivative is the most efficient at gene delivery. Oligomeric derivatives (3-6 kDa) are relatively non-toxic and have good gene delivery properties.

Usage
Chitosan is used primarily as a plant growth enhancer, and as a substance that boosts the ability of plants to defend against fungal infections. It is approved for use outdoors and indoors on many plants grown commercially and by consumers. The active ingredient is found in the shells of crustaceans, such as lobsters, crabs, and shrimp, and in certain other organisms. Given its low potential for toxicity and its abundance in the natural environment, chitosan is not expected to harm people, pets, wildlife, or the environment when used according to label directions.

Chitosan can also be used in water processing engineering as a part of a filtration process. Chitosan causes the fine sediment particles to bind together and is subsequently removed with the sediment during sand filtration. Chitosan also removes phosphorus, heavy minerals, and oils from the water. Chitosan is an important additive in the filtration process. Sand filtration apparently can remove up to 50% of the turbidity alone while the Chitosan with sand filtration removes up to 99% turbidity.

Chitosan is also useful in other filtration situations, where one may need to remove suspended particles from a liquid. Chitosan, in combination with bentonite, gelatin, silica gel, isinglass, or other fining agents is used to clarify wine, mead, and beer. Added late in the brewing process, chitosan improves flocculation, and removes yeast cells, fruit particles, and other detritus that cause hazy wine. Chitosan combined with colloidal silica is becoming a popular fining agent for white wines, because chitosan does not require acidic tannins (found primarily in red wines) to flocculate with.

Biomedical use
Chitosan's properties allow it to rapidly clot blood, and has recently gained approval in the USA for use in bandages and other hemostatic agents. Chitosan purified from shrimp shells is used in a granular hemostatic product, Celox, made by Medtrade Biopolymers Inc. of Crewe, England and in the chitosan dressings made by HemCon Medical Technologies Inc. of Portland, OR, USA. The Hemcon product reduces blood loss in comparison to gauze dressings and increases patient survival. Hemcon products have been sold to the United States Army, who have already used the bandages on the battlefields of Iraq. Chitosan is hypoallergenic, and has natural anti-bacterial properties, further supporting its use in field bandages.

Claimed benefits
Chitosan is frequently sold in tablet form at health stores as a 'fat attractor': It is supposed to have the capability of attracting fat from the digestive system and expelling it from the body so that users can, it is claimed, lose weight without eating less. However, some scientific research suggests that these claims are likely without substance. At best, unmodified chitosan would remove roughly 30 calories per day from a person's diet. Modfied chitosan is claimed to absorb anywhere up to three to six times its weight in fat and oils. Detractors claim that using chitosan may have the deleterious effect of rendering ineffective certain minerals found in foodstuffs and required by the body in order to remain healthy.

Discussion on Chitosan from Researchers
With the unavailability of specific research studies to support the claims made on chitosan as a revolutionary weight loss supplements, one must be careful on what is fact and what is speculation. The following are conclusions and specific discussion made from researchers, although take note that their specific studies were not given with precise accounts of their experimentation. It is now generally accepted that soluble dietary fibers increase gastrointestinal lumen viscosity (Edwards, 1990) and delay gastric emptying (Chang, 1983). Chitosans have specifically been shown to alter bile acid composition, increase neutral sterol excretion and reduce ileal fat digestibility (Fukada, 1991; Maezaki, 1993; Razdan & Pettersson, 1994). The mechanisms by which chitosans achieve these effects are not fully established, although increased intestinal viscosity and increased bile acid-binding capacity are two proposals currently favored (Furda, 1990). Since polyglucosamines are the second-most-ubiquitous dietary fiber after cellulose, it is reasonable to assume that much more research regarding the nutritional significance of these important dietary fibers is to be expected (Knorr, 1991). Chitosan has such characteristics that are associated with a dietary fiber which are assumed to be related to the reductions in cholesterol as well as increases in the excretion of neutral steroids observed in animal experiments (Furda, 1990; Ikeda, 1993; Razdan & Pettersson, 1994). Chitosan, which is largely deacetylated, contains cationic groups located on the polyglucosamine chain (Sugano, 1993). Thus, chitosan may have a bile acid-binding capacity, causing entrapment or disintegration of mixed micelles in the duodenum and ileum (Furda, 1990). This interruption in bile acid circulation would lead to reduced lipid absorption and increased sterol excretion. Chitosan is relatively insoluble in water but is soluble in dilute acids, giving rise to highly-viscous dietary fibers (Furda, 1990). It has been suggested that viscous dietary fibers such as chitosan inhibit uptake of dietary lipids by increasing the thickness of the intestinal lumen boundary layer, a proposal again supported by numerous animal experiments (Sugano, 1993; Ikeda, 1993).