Products of lipoxygenase metabolism play a role in the pathogenesis of psoriasis. Four protoberberine alkaloids, berberine, oxyberberine, jatrorrhizine, columbamine, and two aporphine alkaloids, magnoflorine, and corytuberine, isolated from Mahonia aquifolium, were tested for lipoxygenase inhibition. Oxyberberine, corytuberine, and columbamine were the most potent lipoxygenase inhibitors tested, whereas berberine and magnoflorine exhibited only low potencies. A strong linear correlation (r = 0.866) between lipoxygenase inhibition and lipid antioxidant properties of these compounds was found. These data suggest that the mechanism of lipoxygenase inhibition by these alkaloids may be linked to the inhibition of lipid hydroperoxide substrate accumulation. Inhibition of lipoxygenase by these compounds may contribute to the therapeutic effect of M. aquifolium extracts in the treatment of psoriasis.