Superoxide ion as a primary reductant in ascorbate-mediated ferritin iron release.
The mechanism of ascorbate-promoted ferritin iron reduction under aerobic conditions was studied. The initial rate of ferritin iron release was determined by spectrophotometric measurement of the Fe(ferrozine)3(2+) complex which absorbs at 562 nm. Variation of the initial ferrozine concentration had no influence on the rate of iron release suggesting that ferrozine does not participate in the rate-determining step. Experimental measurements of the initial rate of iron release as a function of ascorbate concentration resulted in saturation kinetics with Vmax = 2.0 X 10(-7) M.min-1 and KM = 1.3 X 10(-3) M. The effect of pH was quite pronounced with a maximal rate of iron release at pH 7.0. Stoichiometric measurements on the reaction mixture, with added catalase, resulted in a ratio of 2 Fe(II) released per ascorbate. Ascorbate-mediated iron release was inhibited 85% by superoxide dismutase, but 0% inhibition was noted with aposuperoxide dismutase. It is proposed that superoxide ion, generated during the iron-promoted oxidation of ascorbate, acts as a reductant of ferritin iron. A mechanism of ferritin iron release consistent with these experimental observations is discussed.
Boyer
R F
RF
Department of Chemistry, Hope College, Holland, MI 49423.
McCleary
C J
CJ
D001205:Ascorbic Acid / Q000032:analysis*; D005293:Ferritins / Q000032:analysis*; D007501:Iron / Q000378:metabolism*; D010084:Oxidation-Reduction; D013055:Spectrophotometry, Infrared; D013481:Superoxides / Q000378:metabolism*
D013481:Superoxides; D005293:Ferritins; D007501:Iron; D001205:Ascorbic Acid