The ferric form of soybean lipoxygenase catalyzes an elimination reaction on 12-iodo-cis-9-octadecenoic acid (12-IODE) to produce iodide ions and 9,11-octadecadienoic acid (9, 11-ODA). If excess 13(S)-hydroperoxy-cis-9,trans-11-octadecadienoic acid (13-HPOD) is present, the reaction proceeds until about one-half of the racemic 12-IODE is consumed; in the absence of excess 13-HPOD, the reaction stops after about three turnovers. Ferric lipoxygenase also catalyzes the conversion of 12-bromo-cis-9-octadecenoic acid (12-BrODE) to 9,11-ODA at a rate that is less than 25% of that observed with 12-IODE. These elimination reactions cannot be detected with ferrous lipoxygenase or with lipoxygenase that has been inactivated by 5,8,11,14-eicosatetraynoic acid. In the case of 12-IODE, elimination is accompanied by a loss of enzymatic activity; at pH 9.0, about 10 iodide ions are produced per molecule of enzyme inactivated. No inactivation can be detected with 12-BrODE. Ascorbate and hydroxylamine, which can act as free-radical traps, block the inactivation by 12-IODE but do not inhibit the elimination reaction. When the enzyme is inactivated by [1-14C]-12-IODE at pH 9.0, the amount of radioactivity that is covalently bound to the protein is less than 30% of that expected for 1:1 incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)