Enhanced oxygen unloading by an interdimerically crosslinked hemoglobin in an isolated perfused rabbit heart.

Abstract

Coronary perfusion has shown that an intramolecularly crosslinked hemoglobin (Hb) with a very low affinity for 02 (Hb crosslinked covalently between the f3 chains with 2-nor-2-formylpyridoxal 5'-phosphate, HbXL) has several advantages over ordinary Hb. As predicted from in vitro oxygenation curves, much more 02 was unloaded to the heart at three different heart rates, at two perfusion rates, and when the perfusate was equilibrated with 25% as well as 95% 02. In all cases, the improved 02 unloading occurred at higher tissue 02 pressures than with normal Hb. The greater 02 consumption with HbXL was accompanied by better mechanical performance because, after 90 min of perfusion, the HbXL-perfused hearts maintained two-thirds of their original contractility (dp/dt), while that of the Hb-perfused hearts had declined to one-fifth. A special advantage of HbXL is its ability to unload significant amounts of 02 even at low temperature (100C), in contrast to whole blood. This should make it useful for supporting aerobic metabolism during low-temperature cardioplegia in cardiac surgery and for organ preservation.The use of hemoglobin (Hb) as a blood substitute has been widely investigated (1, 2) because it has certain advantages over whole blood, such as low viscosity, colloidal osmotic pressure, longer shelf life, and absence of antigens and viral contaminants. However, the use of Hb solutions as a blood substitute is vitiated by two serious drawbacks. The 02 affinity of Hb in the absence of 2, 3-diphosphoglycerate is too high for useful 02 unloading, and the dissociation of HbO2 into dimers leads to rapid plasma disappearance and elimination by the kidneys (3). Therefore, attempts have been made to overcome these two limitations by chemical modification of the Hb. These have included substitution with pyridoxal phosphate to lower the 02 affinity (4-8) and various crosslinking procedures to increase intravascular retention (9-11). Another chemical modification that lowers the 02 affinity substantially has been described recently (12). Unfortunately, pyridoxylation does not prevent dissociation of hemoglobin into half molecules, whereas the introduction of crosslinks has generally increased rather than decreased the 02 affinity. For this reason, Hbs have been used that are both pyridoxylated and polymerized by intermolecular or intramolecular crosslinks (9-11). However, even this approach is undesirable because it leads to heterogeneous polydispersed mixtures of widely different molecular weight with obvious osmotic consequences. Equally important is the flat oxygenation curve that inevitably results from a mixture of Hbs with different 02 affinities.We previously have described a method for crosslinking Hb interdimerically between the p chains with a dialdehyde derivative of pyridoxal phosphate, 2-nor-2-formylpyridoxal 5'-phosphate, forming HbXL (13). This modification fulfills both objectives at the same time because it leads to a Hb that has a profoundly lowered 02 affinity, does not dissociate, and consists of ...

HbXL99 alpha: a hemoglobin derivative that is cross-linked between the alpha subunits is useful as a blood substitute.

“…Secondly, the increase in the oxygen affinity of hemoglobin because of the absence of 2,3-bisphosphoglycerate (P2-glycerate) limits the unloading of oxygen to the tissues. Under physiological conditions, the P50 (partial pressure of 02 at half-saturation of hemoglobin) of isolated hemoglobin A is approximately 12 mm Hg (1 mm Hg = 133.3 kPa), compared to 26 mm Hg for whole blood (4). As a result, the amount of oxygen released by hemoglobin at the normal tissue partial pressures of oxygen (approximately 40 mm Hg) is decreased by nearly half.…”

“…Pyridoxal phosphate and other polyanionic affinity reagents that react with hemoglobin at the P2-glycerate site have been used to lower the oxygen affinity of such derivatives (5)(6)(7)9). The bifunctional analog 2-nor-2-formylpyridoxal 5'-phosphate, studied extensively by Ruth and Reinhold Benesch and their coworkers (10)(11)(12), provides a means of both cross-linking the tetramer between the 13 subunits and reducing the oxygen affinity. However, the reagent itself is difficult to synthesize, making it infeasible to prepare this derivative in the large amounts that would be needed for clinical use.…”

Comparison of the Pharmacokinetic Properties of Hemoglobin-Based Oxygen Carriers

“…2-nor-2-formylpyridoxal 5′-phosphate (NFPLP) can form a cross link between the two β chains of Hb, resulting in an intramolecular cross-linked form of Hb [18]. Using this characteristic, Bakker et al prepared an Hb that was modified by cross-linking with NFPLP (HbNFPLP) in the 1980s [19,20].…”

Current status of oxygen carriers (‘blood substitutes’): 2006

“…described an array of small molecules that could bind in the 2,3 diphosphoglycerate‐binding site to ensure that oxygen affinity (i.e. the P50) was similar to that of blood [36,37]. A comparison of the key properties of representative first‐generation (PolyHeme ® ) and second‐generation (Hemospan ® ) products is shown in Table 2, and examples of the oxygen‐binding curves of several representative products are shown in Fig.…”