Modulation of the oxygen affinity of cobalt-porphyrin by globin

We have combined two extreme effects which influence the oxygen affinity to obtain a cobalt-based oxygen carrier with an affinity similar to that of h uman adult hemoglobin (HbA). The goal was to obtain an oxygen transporter w ith a lower oxidation rate, Exchange of the heme group (Fe-protoporphyrin I S) in Hb with a cobalt-porphyrin leads to a reduction in oxygen affinity by over a factor of 10, an oxygen affinity too low for use as a blood substit ute, At the other extreme, certain globin sequences are known to provide a very high oxygen affinity; for example, Hb Ascaris displays an oxygen affin ity 1000 times higher than HbA, We demonstrate here that these opposing eff ects can be additive, yielding an oxygen affinity similar to that of HbA, b ut with oxygen binding to a cobalt atom, We have tested the effect of subst itution of cobalt-porphyrin for heme in normal HbA, sperm whale (SW) Mb (Mb ), and high affinity globins for leghemoglobin, two trematode Hbs: Paramphi stomum epiclitum (Pc) and Gastrothylax crumenifer (Gc), As for HbA or SW Mb , the transition from heme to cobalt-porphyrin in the trematode Hbs leads t o a large decrease In the oxygen affinity, with oxygen partial pressures fo r half saturation (P-50) of 5 and 25 mm Hg at 37 degrees C for cobalt-Pc an d cobalt-Cc, respectively. A critical parameter for Hb-based blood substitu tes is the autoxidation rate;,while both metals oxidize to an inactive stat e, we observed a decrease in the oxidation rate of over an order of magnitu de for cobalt versus iron, for similar oxygen affinities, The time constant s for autoxidation at 37 degrees C were 250 and 100 h for Pc and Cc, respec tively. (C) 2000 Federation of European Biochemical Societies.