Superoxide dismutase isoenzymes in the normal and diseased human cornea.

PURPOSE: The human cornea, a tissue much exposed to oxidative stress, is rich in extracellular superoxide dismutase (SOD). In this study, the contents and distributions of the SOD isoenzymes in the normal human cornea were compared with those in corneas affected by keratoconus and bullous keratopathy. METHODS: The central and peripheral parts of normal human corneas were analyzed separately. Central corneal buttons were obtained from patients with keratoconus and bullous keratopathy who were undergoing primary keratoplasty or retransplantation. SOD enzymatic activities were determined by a direct spectrophotometric method, and extracellular SOD and the cytosolic Cu- and Zn-containing SOD (CuZn-SOD) proteins were determined with ELISA and studied with immunohistochemistry. RESULTS: The total SOD content, and particularly the extracellular SOD content, was lower in the central than in the peripheral normal cornea. CuZn-SOD and extracellular SOD were demonstrated in all three corneal layers. CuZn-SOD was found in cells, whereas extracellular SOD appeared to be localized on cell surfaces, in basal membranes, and in the stroma. In keratoconus, corneal levels of extracellular SOD were half those in the control corneas, whereas CuZn-SOD and the mitochondrial Mn-containing SOD levels were normal. In bullous keratopathy, apart from edematous dilution, SOD isoenzyme levels were essentially normal. In a remarkable finding, the same pattern in SOD isoenzyme levels as in the original disease was also found at retransplantation. CONCLUSIONS: Extracellular SOD and CuZn-SOD show markedly different distribution patterns within the human cornea. Extracellular SOD activity in the central cornea is halved in keratoconus, compared with that in normal control corneas. The finding of a similar reduction at retransplantation in keratoconus suggests reduced corneal extracellular SOD synthesis in cells of the host as a cause of the low enzyme levels.

The interstitium of the human arterial wall contains very large amounts of extracellular superoxide dismutase.

The levels of the secreted, interstitially located extracellular superoxide dismutase (EC-SOD), the cytosolic copper-and-zinc-containing SOD (CuZn-SOD), and the mitochondrial manganese-containing SOD (Mn-SOD) were measured in the walls of human coronary arteries, proximal thoracic aortas, and saphenous veins. The blood vessel walls, particularly the arteries, were found to contain exceptionally large amounts of EC-SOD, whereas the levels of CuZn-SOD and Mn-SOD were relatively low compared with other tissues. Analysis of EC-SOD by immunohistochemistry indicates an even distribution in the vessel wall, including large amounts of the arterial intima. Arterial smooth muscle cells were found to secrete large amounts of EC-SOD and likely are the principal source of the enzyme in the vascular wall. The EC-SOD concentration in the human arterial wall extracellular space is high enough to efficiently suppress the putative pathological effects of the superoxide radical, such as oxidation of LDL and reaction with nitric oxide to form the deleterious peroxynitrite. The levels of EC-SOD in the aortic wall are found to vary widely among species and were on average 6440 U/g in humans, 4340 U/g in the cow, 2660 U/g in the pig, 160 U/g in the dog, 770 U/g in the mouse. There were only moderate differences in the amounts of CuZn-SOD and Mn-SOD. This wide variation in EC-SOD content suggests that the susceptibility to pathologies induced by superoxide radicals in the vascular wall interstitium should vary widely among species.