M. Hatori et al., EFFECTS OF DEFEROXIMINE ON CHONDROCYTE ALKALINE-PHOSPHATASE ACTIVITY - PROXIDANT ROLE OF DEFEROXIMINE IN THALASSEMIA, Calcified tissue international, 57(3), 1995, pp. 229-236
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.