Kinetic analysis of the induction period in lipoxygenase catalysis
The dioxygenation of 50 pM linoleate at 0.1 pM (13s)-hydroperoxylinoleate, 240 pM 02, pH 10, and 25 O C , catalyzed by varying amounts of soybean lipoxygenase-1, was studied with rapid kinetic techniques. The aim was to assess the effect of transient redistributions of the Fe(I1) and Fe(II1) enzyme forms on the shape of the reaction progress curves. Reactions initiated with iron(I1) lipoxygenase show an initial increase in rate, the “kinetic lag phase” or “induction period”. The duration of this induction period varies from approximately 1 s at [lipoxygenase] > 20 nM to 5 s at [lipoxygenase] = 3 nM. At [lipoxygenase] < 2 nM, the duration of the induction period in these curves is inversely proportional to [lipoxygenase]. The integrated steady-state rate equation for the single fatty acid binding site model of lipoxygenase catalysis [Schilstra et al. (1992) Biochemistry 31,7692-76991 also shows an induction period whose duration is inversely proportional to [lipoxygenase]. These observations, in combination with nonsteady- state numerical simulations, lead to the conclusion that, at [lipoxygenase] < 2 nM, pre-steady-state redistributions of enzyme intermediates occur fast with respect to the rate at which the concentrations of substrates and products change. At higher lipoxygenase concentrations, the pre-steady-state redistributions contribute significantly to the induction period. From a nonlinear least-squares fit to the steady-state rate equation of data obtained at lipoxygenase concentrations of 0.5 and 1 nM, it was calculated that 1% of the linoleate radicals that are formed after hydrogen abstraction dissociate from the active site before enzymic oxygen insertion has occurred.