The most general indicator, and the most commonly used marker, of protein oxidation is protein carbonyl content. Redox cycling cations such as Fe2+ or Cu2+ can bind to cation binding locations on proteins and with the aid of further attack by H2O2 or O2 can transform side-chain amine groups on several amino acids (i.e., lysine, arginine, proline, or histidine) into carbonyls. Cayman's Protein Carbonyl Fluorometric Assay Kit provides a reliable and sensitive method for determining protein carbonyl concentration in plasma, serum, cell lysate, and tissue homogenate samples. The assay relies on the 1:1 binding of a fluorophore to the protein carbonyl. Once bound, excess fluorophore is washed away. Any remaining fluorescence is directly proportional to protein carbonyl concentration.