LysoPC inhibition of group IA sPLA2 activity. (A.) Relationship between substrate concentration and velocity in reactions of group IA sPLA2 in the presence of high levels of substrate but without inhibitor. The data suggested there was endogenous inhibition at the highest substrate concentrations as the plot deviated from the typical hyperbolic curves shown in B. (B.) Plots of Vo versus substrate concentration for group IA sPLA2 activity in the presence of different concentrations of lysoPC 18:0. In this lower range of substrate concentrations, typical hyperbolic curves were obtained. Note that progressive inhibition of activity was found up to 40 μM lysoPC. All reactions were with 15 nM sPLA2. The kinetic parameters for these reactions calculated by nonlinear regression are shown in Table 2: PDA-pyrenyldecanoic acid.