Reactions of free haematins and haemoproteins with nitric oxide and certain other substances

Abstract

It is shown that free hematins will react with nitric oxide under certain conditions giving compounds with characteristic absorption spectra. The reactions of nitric oxide with both protohematin and urohematin were examined. In acid solution each of these hematins changes color from reddish brown to a bright red in the presence of nitric oxide, and 2 strong absorption bands appear in the green region of the spectrum while the Soret band of the hematin is shifted to a longer wavelength. Urohematin in alkaline solution reacts similarly with nitric oxide and can be reduced to give a NO-urohem compound with a single broad absorption band in the visible region. The results obtained with alkaline protohematin are less clear-cut. The absorption spectra of the NO-compounds of proto-and uro-hematin under various conditions were determined. No spectroscopic evidence was obtained for any reaction between fluoride, azide or sulfide with free hematins, although these substances all form well-defined compounds with methemoglobin and other hemoproteins. The reactions of different substances with free iron-porphyrins were compared with those given with hemoproteins. Of all the substances examined only nitric oxide and the cyanide ion are capable of co-ordinating with the bivalent and tervalent iron of heme and hematin respectively, both when they are free and when they form the prosthetic groups of hemoproteins.