Three samples containing varying amount of dystrophin obtained by mixing muscle extract from a DMD and age matched healthy donor at ratio of 25/75, 50/50 and 75/25 respectively were evaluated for dystrophin quantitation using the post-digestion spike-in approach with 30 nM of stable isotope standard peptides. Histograms represent intensity ratios of endogenous dystrophin peptides and filamin c peptide to their corresponding stable isotope labeled standard peptide. P1, P2 and P3 corresponds to dystrophin peptides [LLDLLEGLTGQK], [LLVEELPLR], [IFLTEQPLEGLEK] respectively and P4 correspond to filamin c peptide [VYNVTYTVK] that was used as an internal control. It is clearly seen that some ratios did not decrease with decreasing amounts of dystrophin, especially for the peptide P1, showing high variability for this approach.
