Patent ID: 7635573

Claim:
A mass spectroscopic method for comparing protein levels in two or more samples comprising steps of: a. labeling the N- or C-terminus of proteins in a first protein sample with a first detectable isotopic label to generate a first labeled protein sample comprising a first set of labeled proteins; b. labeling the proteins of a second protein sample at said terminus with a second detectable isotopic label to generate a second labeled protein sample comprising a second set of labeled proteins, wherein the second detectable isotopic label is distinguishable from the first detectable isotopic label; c. cleaving the proteins of the first and second sets of labeled proteins with a cleaving agent, thereby producing a mixture comprising isotopically labeled proteins and non-labeled peptides; d. separating the isotopically labeled proteins from the non-labeled peptides; e. measuring the level of the isotopically labeled proteins in each sample by a mass spectroscopic technique; and f. comparing levels of the isotopically labeled proteins in the first and second samples.