Kinetics of inhibition of firefly luciferase by oxyluciferin and dehydroluciferyl-adenylate

The inhibition mechanisms of the firefly luciferase (Luc) by the two major products of the reactions catalysed by Luc, oxyluciferin and dehydroluciferyl-adenylate (L-AMP), were investigated. Light production in the presence and absence of these inhibitors (0.5 to 2 mu M oxyluciferin; 0.0025 to 1.25 mu M L-AMP) has been measured in 50 mM Hepes buffer (pH = 7.5), 10 nM Luc, 250 mu M ATP and D-Luciferin (from 3.75 up to 120 mu M). Nonlinear regression analysis with the appropriate kinetic models (Henri-Michaelis-Menten and Williarn-Morrison equations) reveals that oxyluciferin is a competitive inhibitor of luciferase (K-i = 0.50 +/- 30.03 mu M) while L-AMP act as a tight-binding competitive inhibitor (K-i = 3.8 +/- 0.7 nM). The K-m values obtained both for oxyluciferin and L-AMP were 14.7 +/- 0.7 and 14.9 +/- 0.2 mu M, respectively. L-AMP is a stronger inhibitor of Luc than oxyluciferin and the major responsible for the characteristic flash profile of in vitro Luc biolumincscence.