CAT and Parkinson disease: CAT is one of the crucial antioxidant enzymes that fights against oxidative stress by destroying cellular hydrogen peroxide to produce water and oxygen and maintaining an optimum level of the molecule in the cell, essential for cellular signalling processes. CAT deficiency can play an important role in the pathogenesis of several human diseases (e.g., Parkinson's disease, diabetes mellitus, schizophrenia and bipolar disorder).