Letter: 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 tool to evaluate V_max/K_max of peptides for protein kinases.