Objectives. The aim of the present investigation was to study the activity of glucose-6-phosphate dehydrogenase (G6PD) and correlate its activity to protein oxidation markers in type 2 diabetic patients under poor glycemic control. Methods. G6PD activity, protein carbonyl group concentration, and total thiol group content were measured in blood samples of 40 patients with type 2 diabetes mellitus under poor glycemic control and 20 healthy control subjects. Results. G6PD activity and total thiol group content decreased significantly while glycated hemoglobin (HbA1C) and protein carbonyl group concentration increased significantly in diabetic patients than in the controls ( ). In addition, Obtained results revealed that, in diabetics, G6PD activity negatively correlated to protein carbonyl and HbA1C ( and ？0.65, resp.), while positively correlated to total thiol ( ) and protein carbonyl negatively correlated to total thiol ( ), while positively correlated to HbA1C ( ). Also in controls, G6PD activity negatively correlated to protein carbonyl and HbA1C ( and ？0.56, resp.), while positively correlated to total thiol ( ) and protein carbonyl negatively correlated to total thiol ( ), while positively correlated to HbA1C ( ). Conclusions. We concluded that G6PD activity decreased in diabetics than in controls and was negatively correlated to oxidative stress markers and HbA1C. G6PD activity can be taken as a biomarker of oxidative stress and poor glycemic control in type 2 diabetic patients. 1. Introduction Diabetes mellitus is a metabolic disease characterized by hyperglycemia due to defects in insulin metabolism. If the hyperglycemia of diabetes is not managed properly, it causes long-term damage, dysfunction, and failure of different organs, notably the eyes, kidneys, nerves, heart, and blood vessels [1]. An increase in oxidative stress has been observed in diabetic patients, which may be due to an increase in processes that produce oxidants or due to a decrease in the antioxidant defense mechanisms [2]. Glucose-6-phosphate dehydrogenase (EC1.1.1.49; D-Glucose-6-phosphate: NADP+ oxidoreductase) is the rate-limiting enzyme of the pentose phosphate pathway. It is required for the antioxidant defense because it produces NADPH, the main cellular reductant and the fuel for glutathione recycling within the cells [3] G6PD plays a central role in cell metabolism and was found to play pathophysiologic roles in many diseases like diabetes, aldosterone-induced endothelial dysfunction, and cancer. The central role of G6PD is being the major source of NADPH, a hydrogen
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