Soybean lipoxygenase inhibition: studies with the sulphasalazine metabolites N-acetylaminosalicylic acid, 5-aminosalicylic acid and sulphapyridine.

Soybean lipoxygenase inhibition has been proposed as an in vitro biochemical model for the antiinflammatory action of certain drugs used in the treatment of ulcerative colitis. In an extension of a recent study which showed that therapeutically active compounds, such as sulphasalazine and its colonic metabolite 5-aminosalicylic acid were soybean lipoxygenase inhibitors, it has now been shown that N-acetylaminosalicylic acid, the principal metabolite of 5-aminosalicylic acid, also inhibits soybean lipoxygenase in a dose dependent and noncompetitive manner (Ki 3.0 X 10(-8) M, IC50 250 microM). Sulphapyridine, the other major metabolite of sulphasalazine, which has been demonstrated to be inactive in the treatment of ulcerative colitis, did not inhibit the lipoxygenase activity. The findings further support the hypothesis that only the therapeutically active compounds are soybean lipoxygenase inhibitors.