EFFECTS OF DEFEROXIMINE ON CHONDROCYTE ALKALINE-PHOSPHATASE ACTIVITY - PROXIDANT ROLE OF DEFEROXIMINE IN THALASSEMIA

The homozygous form of beta-thalassemia, the most common single gene d isorder, is treated by red cell transfusion therapy. Following transfu sion, the chelator, deferoximine, is administered to patients to remov e excess iron. However, when this drug is given to young children, met aphyseal dysplasia and abnormalities of linear growth are frequently o bserved. To explore the notion that deferoximine interferes with endoc hondral growth by chelating zinc, we examined the effect of the drug o n chondrocytes maintained in long-term culture. We found that deferoxi mine caused a dose-dependent inhibition of a wide range of functions i ncluding cell proliferation, protein synthesis (and possibly under-hyd roxylation of type X collagen), and mineral deposition. Directly relev ant to the mineralization process was the observation that the drug dr amatically lowered the activity of alkaline phosphatase, a zinc-requir ing enzyme. To test the hypothesis that enzyme inhibition was due to c helation of zinc by deferoximine, the cell culture medium was suppleme nted with excess zinc. However, this treatment did not overcome the de feroximine-dependent change in enzyme activity. We next examined the p ossibility that deferoximine, in the presence of ascorbate, could form a free radical system that would serve to inactivate the enzyme. Usin g alkaline phosphatase extracted from chick cartilage, we noted that t he activity of the phosphatase was markedly reduced in the presence of deferoximine and ascorbate. These effects were consistant with the no tion that deferoximine and ascorbate can act as a prooxidant couple. T his conclusion was confirmed when we measured the oxidative activities of the system using nitroblue tetrazolium and cytochrome c. Indeed, w e noted that deferoximine markedly activates the autocatalytic oxidati on of ascorbate. We next investigated the possibility that the change in alkaline phosphatase activity was due to the formation of reactive oxygen radicals. Though oxygen radical scavengers and dis-proportionat ing agents did not change the activity of the enzyme, alpha-tocopherol provided complete protection. In conclusion, the deferoximine-ascorba te couple inactivates chondrocyte alkaline phosphatase probably by gen eration of free radicals. As free radicals can damage cartilage as wel l as other tissues, clinical regimens that are directed at elevating a scorbate levels in thalassemia need to be carefully reviewed.