Figure 3.

MALDI-ToF analysis of isolated creatine kinase (CK) (0.5 mg/mL) incubated with 50 μM quercetin, 0.4 nM HRP and 50 μM H₂O₂ with or without 50 μM ascorbate for 5 min at 37 °C. After trypsin digestion the mass spectrum of digested CK was measured. The control spectrum of CK displayed a peak at m/z 1130 and no peak at m/z 1430. The incubation with quercetin showed a peak at m/z 1430 which corresponds to the mass of the adduct of quercetin quinone (300 dalton) with the peptide having mass m/z 1130, whereas the peak at m/z 1130 decreased. The amino acid sequence of the peptide is GYTLPPHCSR, containing cysteine 146. The spectrum of CK incubated with quercetin in combination with ascorbate also showed a peak at m/z 1430. The peak at m/z 1130 was also present, but the relative intensity (RI) was less than the untreated CK.