In vitro antioxidant properties of potential biotransformation products of salicylate, sulphasalazine and amidopyrine.

2,3-and 2,5-dihydroxybenzoate (formed from salicylate by nonenzymatic or enzymatic hydroxylation), and 5-aminosalicylate (a prime metabolite of sulphasalazine) are highly efficient quenchers of the chemiluminescence (CL) produced by an oxy radical flux. Monohydric phenols (including salicylate) and meta-dihydric phenols are virtually inactive. These findings suggest that the para- or ortho-configuration of hydroxy/amino groups is important for this activity. These differences in activity between 2,3- and 2,5-dihydroxybenzoate, 5-aminosalicylate and monohydric phenols/2,4-, 2,6-dihydroxybenzoate were not seen in assays monitoring hydroxyl radicals. 4-aminophenazone (an oxidation product of both amidopyrine/aminopyrine and isopyrine), 4-hydroxyphenazone and some dietary catechols (and ascorbate), are also quenchers of oxy radical-associated CL.