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Chemical structure of one unit in a chondroitin sulfate chain. Chondroitin-4-sulfate: R1 = H; R2 = SO3H; R3 = H. Chondroitin-6-sulfate: R1 = SO3H; R2, R3 = H.
Chondroitin sulfate - alternate diagram

Chondroitin sulfate is a sulfated glycosaminoglycan (GAG) composed of a chain of alternating sugars (N-acetylgalactosamine and glucuronic acid). It is usually found attached to proteins as part of a proteoglycan. A chondroitin chain can have over 100 individual sugars, each of which can be sulfated in variable positions and quantities. Chondroitin sulfate is an important structural component of cartilage and provides much of its resistance to compression [1]. Along with glucosamine, chondroitin sulfate has become a widely used dietary supplement for treatment of osteoarthritis.

Contents

Terminology

Chondroitin sulfate was originally isolated well before the structure was characterised, leading to changes in terminology with time.[2] Early researchers identified different fractions of the substance with letters.

Letter identification Site of sulfation Systematic name
Chondroitin sulfate A carbon 4 of the N-acetylgalactosamine (GalNAc) sugar chondroitin-4-sulfate
Chondroitin sulfate C carbon 6 of the GalNAc sugar chondroitin-6-sulfate
Chondroitin sulfate D carbon 2 of the glucuronic acid and 6 of the GalNAc sugar chondroitin-2,6-sulfate
Chondroitin sulfate E carbons 4 and 6 of the GalNAc sugar chondroitin-4,6-sulfate

"Chondroitin sulfate B" is an old name for dermatan sulfate, and is no longer classified as a form of chondroitin sulfate.[3]

Chondroitin, without the "sulfate", has been used to describe a fraction with little or no sulfation.[4] However, this distinction is not used by all.

Although the name "chondroitin sulfate" suggests a salt with a sulfate counter-anion, this is not the case, as sulfate is covalently attached to the sugar. Rather, since the molecule has multiple negative charges at physiological pH, a cation is present in salts of chondroitin sulfate. Commercial preparations of chondroitin sulfate typically are the sodium salt. Barnhill et al. have suggested that all such preparations of chondroitin sulfate be referred to as "sodium chondroitin" regardless of their sulfation status.[5]

Structure

Chondroitin sulfate chains are unbranched polysaccharides of variable length containing two alternating monosaccharides: D-glucuronic acid (GlcA) and N-acetyl-D-galactosamine (GalNAc). Some GlcA residues are epimerized into L-iduronic acid (IdoA); the resulting disaccharide is then referred to as dermatan sulfate.

Protein attachment

Chondroitin sulfate chains are linked to hydroxyl groups on serine residues of certain proteins. Exactly how proteins are selected for attachment of glycosaminoglycans is not understood. Glycosylated serines are often followed by a glycine and have neighboring acidic residues, but this motif does not always predict glycosylation.

Attachment of the GAG chain begins with four monosaccharides in a fixed pattern: Xyl - Gal - Gal - GlcA. Each sugar is attached by a specific enzyme, allowing for multiple levels of control over GAG synthesis. Xylose begins to be attached to proteins in the endoplasmic reticulum, while the rest of the sugars are attached in the Golgi apparatus.[6]

Sulfation

Each monosaccharide may be left unsulfated, sulfated once, or sulfated twice. In the most common scenario, the hydroxyls of the 4 and 6 positions of the N-acetyl-galactosamine are sulfated, with some chains having the 2 position of glucuronic acid. Sulfation is mediated by specific sulfotransferases. Sulfation in these different positions confers specific biological activities to chondroitin GAG chains.

Function

Chondroitin's functions depend largely on the properties of the overall proteoglycan of which it is a part. These functions can be broadly divided into structural and regulatory roles. However, this division is not absolute, and some proteoglycans have both structural and regulatory roles (see versican).

Structural

Chondroitin sulfate is a major component of extracellular matrix, and is important in maintaining the structural integrity of the tissue. This function is typical of the large aggregating proteoglycans: aggrecan, versican, brevican, and neurocan, collectively termed the lecticans.

As part of aggrecan, chondroitin sulfate is a major component of cartilage. The tightly packed and highly charged sulfate groups of chondroitin sulfate generate electrostatic repulsion that provides much of the resistance of cartilage to compression. Loss of chondroitin sulfate from the cartilage is a major cause of osteoarthritis.

Regulatory

While it is a prescription or over-the-counter drug in 22 countries, chondroitin is regulated in the U.S. as a dietary supplement [1] by the Food and Drug Administration. As a result, in chondroitin sulfate supplements, there are no mandatory standards for formulation, and no guarantee that the product is correctly labelled. This is not the case of Europe, where there is a chondroitin sulfate formulation approved as a drug and considered as the reference product, with evidenced efficacy and safety demonstrated by clinical trials in osteoarthritic patients[7]. Adebowale et al. reported in 2000 that of 32 chondroitin supplements they analysed, only 5 were labeled correctly, and more than half contained less than 40% of the labeled amount.[8] Hence, the importance of testing the bioequivalence of all chondroitin sulfate formulations with the reference product approved in Europe. However, United States Pharmacopoeia (USP) testing standards now exist for the identification and quantification of chondroitin.

Chondroitin sulfate readily interacts with proteins in the extracellular matrix due to its negative charges. These interactions are important for regulating a diverse array of cellular activities. The lecticans are a major part of the brain extracellular matix, where the chondroitin sugar chains function to stabilize normal brain synapses as part of perineuronal nets. The levels of chondroitin sulfate proteoglycans are vastly increased after injury to the central nervous system where they act to prevent regeneration of damaged nerve endings. Although these functions are not as well characterized as those of heparan sulfate, new roles continue to be discovered for the chondroitin sulfate proteoglycans.

In cortical development, chondroitin sulfate is expressed by the Sub Plate and acts as a stop signal for neurons migrating from the Ventricular Zone. Neurons stopping here may then be programmed for further migration to specific layers in the cortical plate.

Medical use

Chondroitin is an ingredient found commonly in dietary supplements used as an alternative medicine to treat osteoarthritis and also approved and regulated as a symptomatic slow-acting drug for this disease (SYSADOA) in Europe and some other countries[9]. It is commonly sold together with glucosamine. Chondroitin and glucosamine are also used in veterinary medicine.[10]

Pharmacology

The dosage of oral chondroitin used in human clinical trials is 800–1,200 mg per day. Most chondroitin appears to be made from extracts of cartilaginous cow and pig tissues (cow trachea and pig ear and nose), but other sources such as shark, fish, and bird cartilage are also used. Since chondroitin is not a uniform substance, and is naturally present in a wide variety of forms, the precise composition of each supplement will vary.[5] In fact, although many food supplement companies produce their products in compliance with human food processing GMP, most of them do not produce their products in compliance with the GMP regulations for pharmaceuticals, resulting in products without pharmaceutical requirements[11].

Recent testing has revealed several flaws in the older testing methods. Without knowing the source of the chondroitin (e.g., shark, porcine, or bovine) and the approximate age of the animal, it is impossible to get a reliable reference standard, and, thus, results from previous testing had yielded percentages between 50 and 400%. In 2007, David Ji et al. reported in the Journal of Analytical Chemistry an extremely accurate method of quantification. The method included using an enzyme to break the chondroitin into its individual unsaturated disaccharides, and then measuring them using HPLC with an ion-pairing column and UV detection.[12]

Clinical studies have not identified any significant side-effects or overdoses of chondroitin sulfate, which suggest its long-term safety.[13] The Task Force of the European League Against Rheumatism (EULAR) committee recently granted chondroitin sulfate a level of toxicity of 6 in a 0-100 scale, confirming it is one of the safest drugs for osteoarthritis.[9] Moreover, its safety is supported by an absence of drug-drug interactions (chondroitin sulfate is not metabolized by cytochrome P450),[14] and the lack of safe alternatives for patients multi-medicated for osteoarthritis and other accompanying diseases, e.g. diabetes, hypertension, hyperlipidemia, etc.

Bioavailability and pharmacokinetics

Pharmacokinetic studies performed on humans and experimental animals after oral administration of chondroitin sulfate revealed that it can be absorbed orally. Chondroitin sulfate shows first-order kinetics up to single doses of 3,000 mg [15][16][17][18]. Multiple doses of 800 mg in patients with osteoarthritis do not alter the kinetics of chondroitin sulfate. The bioavailability of chondroitin sulfate ranges from 15% to 24% of the orally administered dose. More particularly, on the articular tissue, Ronca et al.[19] reported that chondroitin sulfate is not rapidly absorbed in the gastro-intestinal tract and a high content of labeled chondroitin sulfate is found in the synovial fluid and cartilage.

Mechanisms of action

The effect of chondroitin sulfate in patients with osteoarthritis is likely the result of a number of reactions including its anti-inflammatory activity, the stimulation of the synthesis of proteoglycans and hyaluronic acid, and the decrease in catabolic activity of chondrocytes inhibiting the synthesis of proteolytic enzymes, nitric oxide, and other substances that contribute to damage cartilage matrix and cause death of articular chondrocytes. A recent review summarizes data from relevant reports describing the biochemical basis of the effect of chondroitin sulfate on osteoarthritis articular tissues [20]. The rationale behind the use of chondroitin sulfate is based on the belief that osteoarthritis is associated with a local deficiency in some natural substances, including chondroitin sulfate.

Recently, new mechanisms of action have been described for chondroitin sulfate. In an in vitro study, chondroitin sulfate reduced the IL-1β-induced nuclear factor-kB (Nf-kB) translocation in chondrocytes [21]. In addition, chondroitin sulfate has recently shown a positive effect on osteoarthritic structural changes occurred in the subchondral bone[22].

Clinical trials

In several prospective controlled studies, chondroitin sulfate had been thought to decrease pain, improve functional disability, reduce NSAID or acetaminophen consumption, and provide good tolerability with an additional carry-over effect[23][24][25][26][27][28][29].

Due to the popularity of the glucosamine-chondroitin supplement and the apparent lack of reliable information about its usefulness in treating osteoarthritis,[30] the National Institutes of Health funded a study to test the effects of chondroitin and glucosamine on osteoarthritis of the knee. This multicenter, placebo-controlled, double-blind, six month long trial found that glucosamine plus chondroitin had no statistically significant effect on symptoms of osteoarthritis in the overall group of osteoarthritis patients .[31] The control group of patients who took celecoxib (a commonly used osteoarthritis drug) did have a statistically significant improvement in their symptoms. These results indicate that glucosamine and chondroitin do not effectively relieve osteoarthiritic pain in the overall group of osteoarthritis patients.[2]

Recently, a review by Bruyere et al. about glucosamine and chondroitin sulfate for the treatment of knee and hip osteoarthritis concludes that both products act as valuable symptomatic therapies for osteoarthritis disease with some potential structure-modifying effects[32].

At present, OARSI (OsteoArthritis Research Society International) is recommending chondroitin sulfate as the second-most-effective treatment for moderate cases of osteoarthritis[33] Likewise, the European League Against Rheumatism (EULAR) supports the usefulness of chondroitin sulfate in the management of knee osteoarthritis and grants the highest level of evidence, 1A, and strength of the recommendation, A, to this product[9].

Contamination in heparin

On Wednesday, March 19, 2008 the U.S. Food and Drug Administration (FDA) identified "oversulfated chondroitin sulfate" as a contaminant in heparin originating from China.[34]

In this regard, "it is very important to remark on the relevant chemical differences between the chondroitin sulfate formulation approved in Europe as a drug and considered the reference product, and the “oversulfated chondroitin sulfate” identified as a contaminant in heparin originating from China."

The “oversulfated chondroitin sulfate” is not a product extracted from biological sources; it is synthesized through a sulfation reaction from the biological molecule. This is a semi-synthesis process that uses naturally derived chondroitin sulfate as a reagent in combination with various potentially dangerous chemicals, though their significance in the toxicity of "oversulfated chondroitin sulfate" is not known.

The resulting product contains 3 or 4 sulfate groups per disaccharide, and, therefore, its structure differs considerably from the original one (see #Sulfation section above). Furthermore, analysis of the contaminant unexpectedly revealed an unusual type of sulfation not found in any natural sources of chondroitin sulfate. In addition, a tetrasulfated disaccharide repeat unit has not been isolated to date from animal tissues[35]

Thus, chondroitin sulfate is merely the substrate of the reaction: The final, oversulfated molecule constitutes a new entity, whose pharmacological and clinical properties are most likely very different from the biological molecule, as it is demonstrated in a recent article published in New England Journal of Medicine. In this study[36], Sasisekharan and colleagues showed that the oversulfated chondroitin sulfate (OSCS) found in contaminated lots of unfractionated heparin, as well as a synthetically generated OSCS reference standard, directly activated the kinin–kallikrein pathway in human plasma, which can lead to the generation of bradykinin, a potent vasoactive mediator. In addition, OSCS induced generation of C3a and C5a, potent anaphylatoxins derived from complement proteins. Chondroitin sulfate A was also tested and it showed no induction of amidollytic activity. Screening of plasma samples from various species indicated that swine and humans are sensitive to the effects of OSCS in a similar manner. OSCS-containing heparin and synthetically derived OSCS-induced hypotension associated with kallikrein activation when administered by intravenous infusion in swine. In contrast, none of the three pigs treated with chondroitin sulfate A showed any significant changes in blood pressure or heart rate.

See also

References

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  2. ^ P. A. Levene and F. B. La Forge (1913). "On Chondroitin Sulphuric Acid". J. Biol. Chem. 15: 69–79.  Free PDF online
  3. ^ MeSH Chondroitin+sulfates
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