In PD patients, the whole-blood GSH/GSSG ratio increased by 231% and red blood cell catalase activity by 215% (vs. 18% and 109% in controls, both P < 0.05), alongside significant rises in plasma cysteine (Cmax: 15.1 μg/mL in PD vs. 25.7 μg/mL in controls). Here, CAT is linked to Parkinson disease.