beta-carotene directly influences the amount of soybean lipoxygenase (LOX) in the reaction medium available for the catalytic conversion of linoleic acid into corresponding hydroperoxides. Inhibition of LOX by beta-carotene was predominantly of the noncompetitive type. The mean K (m(app)) value was estimated to be 13.25 mu M +/- 0.15 mu M for linoleic acid while the mean K (i) value was estimated to be 2.56 mu M +/- 0.14 mu M for beta-carotene. V (m(app)) values decreased from 1.90 Delta A (234 nm)/min to 0.38 Delta A (234 nm)/min as the concentration of beta-carotene increased. The IC50 value for beta-carotene was 2.49 mu M +/- 0.21 mu M. It is shown here that beta-carotene is an efficient LOX inhibitor, effective at low-level concentration enzyme-dependent peroxidation of linoleic acid. Presence of beta-carotene keeps LOX in its reduced form of Fe(II) and the oxidation of linoleic acid is prevented.