Correlation between phosphorylation ratios by matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis and enzyme kinetics

To identify the correlation between the phosphorylation ratios by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-ToF MS) analysis and enzyme kinetics (K-m, V-max and V-max/K-m) it is important to understand whether MALDI-ToF MS can be applied for monitoring the properties of peptides that are substrates of protein kinases. The correlation between phosphorylation ratios and enzyme kinetics was examined using peptides for protein kinase C (PKC) and for 60 kDa phosphoprotein, encoded by the cellular sarcoma gene (c-Src). Phosphorylation ratios, analyzed by MALDI-ToF MS, showed a higher correlation coefficient (r=> +0.7) for V-max/K-m compared with that (r= < +/- 0.6) for K-m or V-max. For ion modes, a higher correlation coefficient between phosphorylation ratios and V-max/K-m was identified in the positive mode (r= > +0.7) compared with that in the negative mode (r= < +0.5). These results suggest that MALDI-ToF MS is a useful toot to evaluate V-max/K-m of peptides for protein kinases.