Chemical reagents and methods for detection and quantification of proteins in complex mixtures

The invention provides a reagent comprising an affinity tag, a detectable moiety, a linker, an isotope tag and a reactive group. The invention also provides methods of using a reagent of the invention. The methods can be used to label a polypeptide in a sample by contacting a sample with a reagent of the invention under conditions allowing the reactive group to bind to one or more polypeptides in the sample. The invention additionally provides methods of isolating, identifying and quantifying a polypeptide in a sample. The invention further provides methods of diagnosing a disease using a reagent of the invention.