A study of the properties of two porphyringlobin species formed in the reaction of protoporphyrin IX with human globin.

Globin was prepared from the main (A(0)) component of human haemoglobin and reacted with protoporphyrin IX; the product, when subjected to chromatography on CM-Sephadex, separated into fast- and slow-moving species. These were isolated for examination. The dissociation constant for the tetramer-dimer equilibrium of fast-moving porphyringlobin was determined at 2.8x10(-6)m; this is to be compared with values of 2.2x10(-6)m and 8x10(-8)m determined for oxyhaemoglobin and the slow-moving porphyringlobin respectively. It was also shown that the thiol groups of fast-moving porphyringlobin react with 4,4'dithiodipyridine at an identical rate with those of oxyhaemoglobin; in comparison, the rates of reaction of deoxyhaemoglobin and porphyringlobin are much slower but are again identical with one another. The quenching of porphyringlobin fluorescence by I(-) ions was also studied. The quenching could not be represented by a simple Stern-Volmer relationship (whereas that of porphyrin-apomyoglobin is), but was represented by a model in which the fluorescence of fast-moving porphyringlobin was more accessible to the quencher than that of the slow-moving component. Similarly, fast-moving porphyringlobin was photodecomposed more rapidly by oxygen than the slow-moving species.