Reaction of Nitric Oxide with Synthetic Hemoprotein, Human Serum Albumin Incorporating Tetraphenylporphinatoiron(II) Derivatives

The reaction of nitric oxide (NO) with a synthetic hemoprotein, the recombinant human serum albumin (rHSA) incorporating eight tetraphenylporphinatoiron(II) derivatives bearing a covalently linked axial base (FeP) [rHSA-FeP], has been investigated. The UV--vis absorption spectrum of the phosphate buffer solution (pH 7.3) of rHSA-FeP showed maxima at 425 and 546 nm upon the addition of NO. The carbonyl rHSA-FeP, in which FePs are six-coordinate CO-adducts, also moved to the same species after bubbling with NO gas. ESR spectroscopy revealed that the incorporated FePs in the albumin formed six-coordinate nitrosyl complexes; the proximal imidazole moiety does not dissociate from the central iron when NO binds to the trans side. The NO-binding affinity of rHSA-FeP (P(1/2)(NO), 1.7 x 10(-6) Torr, pH 7.3, 298 K) was significantly lower than that of FeP itself (P(1/2)(NO), 1.8 x 10(-8) Torr in toluene). Kinetically, this arises from the decreased association rate constant (k(on)(NO), 8.9 x 10(8) M(-1) s(-1) --> 1.5 x 10(7) M(-1) s(-1)). Since NO-association is diffusion controlled, incorporation of the synthetic heme into the albumin matrix appears to restrict the NO access to the central iron(II).