Ascorbate protects against tert-butyl hydroperoxide inhibition of erythrocyte membrane Ca2+ + Mg2(+)-ATPase.

The incubation of erythrocyte suspensions or isolated membranes containing a residual amount of hemoglobin (0.04% of original cellular hemoglobin) with tert-butyl hydroperoxide (tBHP, 0.5 mM) caused significant inhibition of basal and calmodulin-stimulated Ca2+ + Mg2(+)-ATPase activities and the formation of thiobarbituric acid reactive products measured as malondialdehyde. In contrast, the treatment of white ghosts (membranes not containing hemoglobin) with tBHP (0.5 mM) did not lead to appreciable enzyme inhibition within the first 20 min and did not result in malondialdehyde (MDA) formation. However, the addition of either 10 microM hemin or 100 microM ferrous chloride + 1 mM ADP to white ghosts produced hydroperoxide effects similar to those in pink ghosts (membranes with 0.04% hemoglobin). The concentrations of hemin and ferrous chloride which caused half-maximal inhibition of Ca2+ + Mg2(+)-ATPase activity at 10 min were 0.5 and 30 microM, respectively. The effects of several antioxidants (mannitol, thiourea, hydroxyurea, butylated hydroxytoluene, and ascorbate) were investigated for their protective effects against oxidative changes resulting from tBHP treatment. Over a 30-min incubation period only ascorbate significantly reduced the enzyme inhibition, MDA formation, and protein polymerization. Thiourea and hydroxyurea decreased MDA formation and protein polymerization but failed to protect against the enzyme inhibition. Butylated hydroxytoluene was similar to thiourea and hydroxyurea but with better protection at 10 min. Mannitol, under these conditions, was an ineffective antioxidant for all parameters tested.