Kinetic studies on the reduction of iron(III)deuteroporphyrin--human serum albumin complex with dithionite ion.

The effects of human serum albumin (HSA) on the rate of dithionite reduction of iron(III)deuteroporphyrin (iron(III)Dp) have been investigated in order to further characterize the porphyrin binding site and the changes manifested in this site under various conditions. These studies were performed under pseudo-first-order conditions, and in the presence of carbon monoxide as a "trapping agent" for the reduced iron(II)porphyrin. The rate of reduction of the free iron(III)Dp in phosphate buffer at pH 7.4 follows second-order kinetics with a rate constant (4.2 X 10(9) M-1 s-1) suggestive of a diffusion-controlled process. A six-orders of magnitude decrease in the rate of reduction was observed with iron(III)Dp was complexed with HSA. This result is consistent with HSA-bound porphyrin being less accessible to the aqueous environment. Additional studies demonstrated that both pH and anions induce various alterations in the complex that are reflected in the rate of reduction of iron(III)porphyrin.