Kinetic mechanism of guinea pig neutrophil 5-lipoxygenase.

The kinetic mechanism of guinea pig neutrophil 5-lipoxygenase was investigated using a continuous spectrophotometric assay that monitors product diene formation at 236 nm due to substrate oxygenation. Progress curves for reactions with both arachidonic acid and eicosapentaenoic acid are characterized by 1-3-min lag phases in the attainment of steady-state velocities and product inhibition, as indicated by the total cessation of the reaction prior to complete depletion of substrate. The dependence of the steady-state velocity on arachidonic acid concentration appears to follow Michaelis-Menten kinetics, with Vmax = 4.2 +/- 0.4 nmol of 5-hydroxy-6,8,11,14-eicosatetraenoic acid/min/mg of protein and Ks = 25 +/- 4 microM. The addition of Ca2+ results in an overall activation: lag phases are shortened to 10-20 s, Vmax increases to 24 +/- 2 nmol/min/mg of protein, and Ks decreases to 7.7 +/- 1.7 microM; and a change in a mechanism to one involving substrate inhibition (Kss = 13 +/- 1 microM). The observed activation by Ca2+ has a half-maximal response at around 30 microM. In the presence of Ca2+, ATP causes an increase in Vmax to 30 +/- 4 nmol/min/mg of protein without changing Ks or Kss and a reduction of the lag to less than 5 s. The half-maximal response for ATP is 31 +/- 7 microM. Oxygenation of eicosapentaenoic acid in the presence of Ca2+ and ATP occurs with similar kinetics, except for significantly less substrate inhibition: Vmax = 31 +/- 6 nmol/min/mg of protein, Ks = 7 +/- 1 microM, and Kss = 33 +/- 2 microM. This is the first report suggesting a kinetic mechanism for 5-lipoxygenase, which accounts for substrate inhibition, regulation by Ca2+, and ATP and substrate specificity.