Similar Metabolites Formed from β-Carotene by Human Gastric-Mucosal Homogenates, Lipoxygenase, or Linoleic-Acid Hydroperoxide

To determine the basis for the formation of excentric cleavage products of β-carotene (β-C) after incubation with human gastric mucosal homogenates, we have studied the effect of lipoxygenase in β-C metabolism. β-C was incubated with human gastric mucosal homogenates, soybean lipoxygenase with linoleic acid, or the lipoxygenase primary product, 13( S)-hydroperoxy- cis,trans-9,11-octadecadienoic acid (13-LOOH). The β-C-metabolites, β-apo-14′, -12′, -10′, and -8′-carotenals, β-apo-13-carotenone, retinoic acid, and retinal were detected and quantified by HPLC after a 30-min incubation with 1.8 μM β-C. The products from the lipoxygenase plus linoleic acid incubation and from the lipoxygenase primary product, 13-LOOH, with β-C were exactly the same as the products from a human gastric mucosal homogenate incubation. Significantly larger amounts of the same β-C metabolites were formed when β-C was incubated with gastric mucosal homogenates and lipoxygenase together. Furthermore, nordihydroguaiaretic acid (NDGA), a specific lipoxygenase inhibitor, was found to significantly inhibit the formation of β-apo-carotenoids and retinoids produced by gastric mucosal homogenates incubated with β-C. The similarity of the β-C metabolites when β-C was incubated with human gastric mucosal homogenate, lipoxygenase plus linoleic acid, or 13-LOOH and the inhibition of β-C metabolite production by NDGA in gastric tissue incubation with β-C suggest that lipoxygenase is involved in β-C metabolism in gastric mucosa. The activity of 13-LOOH in our hands would indicate that an enzyme-linked process is occurring in gastric tissue producing fatty acid hydroperoxides, and that the hydroperoxide, or a radical species derived from it, is able to carry out the oxidation of β-C independently of the enzyme.