Autoimmune Thyroid Disease

An Unfortunate and Lengthy Adventure in Misdiagnosis

Salicylate oxidation

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Although salicylates have been used for centuries as treatment of inflammatory diseases, the mechanism of action of these drugs is still not clear. Aspirin (acetylsalicylic acid) and other nonsteroidal anti- inflammatory drugs (NSAID) inhibit prostaglandin biosynthesis, a property that appears to explain part of their anti-inflammatory activity. However, this mechanism does not appear to explain the anti- inflammatory properties of salicylic acid, which is a major metabolite of ASA in vivo. Results of prior studies in our laboratory have established that benzoic acid, the parent compound of the salicylate group of drugs, is decarboxylated and hydroxylated by the hydroxyl free radical (OH.) produced by stimulated granulocytes. These observations suggested that salicylates might be similarly metabolized by granulocytes. If so, the capacity of salicylates to rapidly react with OH. might relate directly to their known anti-inflammatory properties. Preliminary experiments established that salicylic acid and aspirin were decarboxylated by the hydroxyl free radical generated by the enzyme system xanthine-xanthine oxidase. We then studied the metabolism of salicylates by human granulocytes. Unstimulated granulocyte suspensions did not oxidize ASA or salicylic acid. However, suspensions stimulated by opsonized zymosan particles rapidly oxidized both substrates in pharmacological concentrations. The rate of oxidation of salicylic acid was 16-fold higher than benzoic acid, whereas the rate of oxidation of ASA was four-fold higher. The reaction was oxygen dependent and could be inhibited by known hydroxyl scavengers, particularly dimethylthiourea. The reaction could also be inhibited by superoxide dismutase and azide, indicating that O-2 and heme or an iron- dependent enzyme were required for the reaction. The reaction was not impaired by compounds known to react with the HOCL and the chloramines generated by stimulated PMN. Furthermore, salicylic acid in high concentrations did not impair the HMPS pathway, the production of O-2 or the production of H2O2 by granulocytes. These data provide evidence that salicylates are rapidly oxidized by the hydroxyl free radical produced by granulocytes and not O-2, H2O2, or HOCL. This capacity of salicylates to react rapidly and selectively react with OH. may directly relate to their anti-inflammatory properties. In addition, results of our experiments indicate that stimulated granulocytes acquire the capacity to metabolize these drugs. Therefore, several metabolites of salicylates may be produced at a site of inflammation, all of which may have altered biological activity compared with the parent compound. Oxidation of salicylates by stimulated granulocytes: evidence that these drugs act as free radical scavengers in biological systems

The difference in oxidation rate between salicylate and ASA might explain why there is a lower tolerance of salicylate than there is of pure ASA in a number of trials – perhaps it is the oxidised product that is more problematic?

The fact that salicylates are oxidised through xanthine oxidase is astonishing. Xanthine oxidase is a molybdenum dependent enzyme that produces bilirubin, bile, basically, that colours one’s poo brown. This might explain more fully the jaundice/yellow poo/hyper or hypo bilirubin problems in some salicylate intolerant people, especially after they go on the failsafe diet.

I’ve been wondering for a while about the anaemia link. Heme is a cofactor in the molybdoenzymes.


Written by alienrobotgirl

23 January, 2007 at 3:28 pm

Posted in The Science of FCI

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