The inhibition mechanism of soybean lipoxygenase (LOX) by beta-carotene was studied. Addition of beta-carotene into the reaction mixture decreased the rate of conjugated diene formation. Increasing the concentration of beta-carotene in the reaction mixture resulted in a decrease in the rate of conjugated diene formation. Although the rate of conjugated diene formation was lower in the presence of beta-carotene, the same amounts of linoleic acid hydroperoxides were formed by the enzyme at the end of the reaction, both with and without beta-carotene in the reaction medium. The rates of conjugated diene formation for 40, 20, 10 and 4 U mL(-1) LOX enzyme were almost equal to zero when the concentrations of beta-carotene were 20, 17.5, 15 and 10 mu mol L-1 in model reaction systems, respectively. beta-Carotene directly influences the amount of enzyme in the reaction medium available for the catalytic conversion of linoleic acid into corresponding hydroperoxides. The results obtained here suggest that beta-carotene reacts with linoleyl radical (L-.) at the beginning of the chain reaction, preventing the accumulation of conjugated diene forms (LOO., LOO- and LOOH). Since L-. transforms back to its original form of LH, the enzyme cannot complete the chain reaction and thus remains at inactive Fe(II) form. (c) 2005 Society of Chemical Industry.