From Noblood

NOTE

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Hemoglobin (Hb) is seen as the obvious candidate for developing an effective blood substitute since it is the body's natural oxygen transporter. Hb-based substitutes currently under development use chemically modified hemoglobin.

In the normal circulatory system, the hemoglobin in red blood cells carries the majority of the oxygen. Since oxygen is not very soluble in water, the plasma alone is unable to carry very much. Hemoglobin-based oxygen carriers (HBOC) are solutions designed to allow hemoglobin molecules to circulate in the plasma, therefore increasing its oxygen carrying capacity. Several factors must be taken into account when designing an HBOC. These include the prevention of immunological responses, the source of the hemoglobin, and the methods for preventing the dissociation of hemoglobin. Observations on past models have lead to design changes and have spurred new ideas for uses of HBOC.

Immunological Response Prevention

Early attempts to make HBOC failed due to residual red blood cell stroma on the foreign hemoglobin. These residues caused an immunological response. This is now eliminated by ultrafiltration which creates stroma free hemoglobin.

Hemoglobin Sources

70,000 kg of hemoglobin would be needed to replace 20% of the red blood cell transfusions in the US, (Goodnough). Currently, different companies are using various hemoglobin sources.

Sources Include:

Expired donated human blood

• human protein
  • anticipated that supplies will be limited
• Recombinant human hemoglobin
  • human protein
• unclear if the technology to manufacture this on a large scale exists

Transgenic pig hemoglobin

• would be expensive and complex
• would require 100,000 transgenic animals

Bovine hemoglobin

• could be a byproduct of slaughter, inexpensive
  • need to develop pedigree herds
    

Dissociation Prevention

Early attempts to create HBOC failed due to severe renal toxicity. It was found that the dissociation of the hemoglobin tetramere leads to renal toxicity.

The following are current methods of dissociation prevention:

Conjugated polymerized hemoglobin

These molecules are conjugates of hemoglobin and polymers such as dextran or poylethylene. Conjugated hemoglobin has high viscosity and high oncotic pressure making them a possible candidate for use as plasma expanders.

Cross-Linked Hemoglobin

Reagents, such as gluteraldehyde, sebacyl chloride, or bis-(N-maleimidomethyl)ether are used to cross link reactive amino groups on the hemoglobin. Depending on the selectivity of the reagents, hemoglobin can be cross-linked intermolecularly, thus creating polyhemoglobin, or intramolecularly. Due to the stable cross-linking, companies such as Baxter have been able to use heat treatment to eliminate viruses.

Recombinant Hemoglobin

Hemoglobin with the 2 alpha units fused together are produced by genetically engineered Escherichia coli. The resulting amino acid substitution creates hemoglobin with a high oxygen affinity.