TH and Parkinson disease: Additional processes that might contribute to susceptibility of DA neurons in PD include: (1) toxicity of tetrahydrobiopterin (an obligatory cofactor for TH) via increased DA production and the cofactor's autooxidation [52] and (2) increasing catecholamine concentrations via overexpression of α-synuclein and resulting disruption of vesicular pH that perturbs the ability of vesicles to store neurotransmitters [53].