Cataract in a diabetic patients is because of accumulation of sorbital in lens. The enzyme responsible for this is

B i.e. NADPH + dependent aldolase reductase Lens Q, peripheral nerves, renal glomeruli & RBC's are the tissues where glucose can enter without the help of insulin. In diabetics, concentration of glucose rise with in lens which is conveed into sorbitol & fructose. Sorbitol does not easily diffuse through cell membrane & thus accumulate. It causes damage to lens protein and leads to cataract.