The key findings of the present study include: (i) Reduction of β/α-cell ratio in human islets from patients with T1DM and T2DM; (ii) consistent decrease in β/α-cell ratio during islet isolation and following islet transplantation; (iii) greater vulnerability of β-cell than α-cell to oxidative stress; and (iv) lower expression of the key antioxidant enzymes catalase and GPX in human β-cells than α-cells, a phenomenon which could, at least in part, account for the higher oxidative damage and lower viability of β-cell following exposure to oxidative and nitrosative stress observed. Here, CAT is linked to type 2 diabetes mellitus.