G6PD and Parkinson disease: Intriguingly, the olfactory bulb—one of the earliest and most commonly affected regions in PD—has been shown to exhibit significantly higher activity of G6PD and another enzyme, 6-phosphogluconate dehydrogenase, compared to the cortex, hippocampus, striatum, or septum, with levels in the olfactory bulb being four times higher in a rat study (Ninfali et al., 1997).