Methods for Measuring Relative Oxidation Levels of a Protein

A method for assessing the oxidation states of a protein in a sample, the method comprising the steps of contacting the sample with a first label adapted to selectively bind to at least one reduced cysteine group of the protein therein to form a first labelled sample; forming a sub-sample of the first labelled sample; treating the sub-sample to selectively reduce at least one reversibly oxidised cysteine group of the protein therein to form a treated sub-sample; contacting the treated sub-sample with a second label adapted to selectively bind to a reduced cysteine group of the protein to form a second labelled sample; and assessing the first and second labelled samples for a plurality of oxidation states of the protein.