Tandem mass spectrometry (MS/MS) analysis of islets from stage II Fh1βKO mice islets detected succination of key cysteine residues in glyceraldehyde 3-phosphate dehydrogenase (GAPDH; residue C150) (Figures 5G and 5H), guanosine monophosphate reductase (GMPR; C186) (Figure S6A; Table S2), and Parkinson’s disease (autosomal recessive, early onset) 7 (PARK 7/DJ-1; C106). Here, GMPR is linked to Parkinson disease.