Onion and garlic essential oils were previously shown to inhibit mouse skin tumor promotion, as were the enzymes, lipoxygenase, and cyclooxygenase. In the present study, the inhibition of soybean lipoxygenase (EC 22.214.171.124) by onion and garlic components and related compounds was investigated. The IC50 values as well as the kinetic inhibition constants were determined for the most active compounds. Di‐(1‐propenyl) sulfide, an analog of the substrate moiety required for oxygenase action, was the only irreversible inhibitor observed with Ki =59 μM and k3 =0.53/min. Inhibition in the presence of substrate was uncompetitive at 88 and 132 μM linoleic acid with Ki =129 μM. At 173 μM linoleic acid, however, inhibition was competitive with Ki =66 μM. Dially trisulfide, allyl methyl trisulfide, and diallyl disulfide were competitive inhibitors, while 1‐propenylpropyl sulfide and (E, Z)‐4,5,9‐trithiadodeca‐1,6,11‐triene 9‐oxide (ajoene) were mixed inhibitors. Nordihydroguaiaretic acid (NDGA), the most potent lipoxygenase inhibitor, was a competitive inhibitor with Ki =0.29 μM. The results indicate a relative potency of inhibition for structural features in the following order: di(1‐propenyl) sulfide > an alkenyl trisulfide > an alkenyl disulfide. Di(n‐propyl) disulfide, a major onion oil component, inhibited neither lipoxygenase nor promotion. Di(1‐propenyl) sulfide and ajoene inhibited both. This suggests that the inhibition of lipoxygenase may be involved in anti‐promotion.
- Garlic and Onion Sulfides
- Soybean Lipoxygenase Inhibition
- Tumor Promotion Inhibition