G6PD and Parkinson disease: Additionally, the decreased energy contribution from the PPP predicted in PD models under lower energy demand is consistent the decreased levels of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, key enzymes of the PPP, observed in the putamen and cerebellum of PD patients (Dunn et al., 2014; Camandola and Mattson, 2017).