Design, conformational, functional and physiological characterization of recombinant polymeric heme-proteins

Genetic engineering offers the opportunity to design and construct various mutant hemoglobins possessing conformational and functional characteristics that render them suitable for use as therapeutic agents in a variety of clinical situations. Problems related to the use of Hb solutions for transfusion are the rapid loss of Hb through the kidney's glomeruli and vasoconstriction. The latter could be caused by nitric oxide (NO) depletion in the wall of the vasculature as a result of hemoglobin extravasation. It may also be caused by an autoregulatory mechanism elicited by excess oxygen delivered from plasmahemoglobin. Another potential problem is that at the physiological colloid-osmotic pressure of human plasma, only a limited amount of Hb may be safely infused; thus, the oxygen-carrying capacity of blood may not be fully restored. Polymers of tetrameric Hb have the potential advantage of being transfused in much larger amounts, extravasating less across the endothelium and producing less hypertension. Recombinant hemoglobins as possible hemoglobin-based oxygen carriers (HBOCs) have been produced in several laboratories. Whatever the precise mechanism of the hemoglobin hemodynamic effect, its effect seems to be minimized by elimination of low molecular weight molecules. This is supported by the findings that large molecules, such as polymerized hemoglobin, PEG-modified hemoglobin and dextran-conjugated hemoglobin appear to be much less vasoactive than, for example, -Hb, DCLHb or rHb1.1. One promising approach to produce a safe hemoglobin based blood substitute would therefore be to construct molecules that form polymers.