Hemoglobin: The Basic Building Block of Blood Substitutes

Hemoglobin is the body’s principal oxygen delivery agent and is found in the blood’s red cells. It consists of four globin protein chains, each containing a heme molecule. As blood flows past the lungs’ walls, the heme molecules bind oxygen. Then as the blood flows through the body, the oxygen is released to tissues as needed.

Scientists have adopted a strategy of using hemoglobin as the basic building block of blood substitutes (Hemoglobin Based Oxygen Carriers – HBOC’s). But first, the hemoglobin must be removed from the cell. In this state it is referred to as “acellular hemoglobin”. While acellular hemoglobin has the desired oxygen carrying abilities, it poses safety problems that must be adequately addressed.

First Generation of HBOC’s – Polymerized Acellular Hemoglobin & 15% Solution

Acellular hemoglobin cannot be used on its own because of its small size. It is so tiny that it can filter through the wall of the blood vessels to create toxic conditions. It is also excreted rapidly through the kidneys. First Generation developers tried to overcome this size problem by cross linking the globin chains to prevent the four chains from falling away from each other.

Next, they created a large molecule consisting of several cross-linked hemes. This chemical process is referred to as “polymerizing”. These polymerized molecules are then dissolved to yield solutions having hemoglobin content in the range of 10% - 14%. The relatively short half-life of the polymerized molecule after transfusion into the patient and the high hemoglobin content however posed dangers.

Dextran–Hemoglobin: One Stable Bonding Port and a 6% Solution

The only element dextran-hemoglobin has in common with the First Generation blood substitutes is acellular hemoglobin. All else is different. Instead of polymerizing cross-linked globin into large unstable molecules with imprecisely specified connecting points, the inventors of dextran-hemoglobin elected to conjugate (link) acellular hemoglobin to dextran, a component used for over 5 decades in blood plasma expanders. There are two clearly specified points of linkage between the dextran and the hemoglobin that are placed away from the heme-pocket on the protein to prevent instability and heme loss. The result is a highly stable molecule. Moreover, the inventors designed a molecule that can effectively deliver oxygen at 6% hemoglobin content instead of 10%-14% with a long half-life in the circulatory system (57 hours). The molecule does not enter the kidneys or lymphatic space and is chemically more stable than hemoglobin.

First Generation of HBOC’s – Symptoms of Acellular Hemoglobin Toxicity

Once in circulation, the polymerized chains are removed quickly. In 22 hours or less, half of the original amount has broken down into waste material. This means that the product does not stay in functional mode for long. Also it means that the body has to deal with excessive quantities of fragmented material in a short period of time. This instability and the resulting release of bilirubin and iron cause a number of toxic conditions.

As the polymerized hemoglobin molecule is removed from the circulation and broken down, fragments of acellular hemoglobin and its component heme groups are metabolized, mainly in the liver. Due to the high concentration of hemoglobin (10% - 14%) and the short half-life (7 ½ - 22 hours) the normal process of metabolic degradation is overloaded.

Acellular hemoglobin oxidized and creates free radicals that damage tissues. And HBOC with a high concentration (10% - 14%) of hemoglobin and a low oxygen affinity results in high oxygen levels in blood which in turn mean a much higher rate of oxidization and the production of free radicals that could reach dangerous levels. Such oxidative damage is well known, for example in reperfusion injury to the heart of clogged arteries.

On metabolism, hemes split apart, releasing iron. Excessive iron loading is known to cause injury to the body.

The pigment component of the heme group is converted into bilirubin. This rise in bilirubin content of the blood produces jaundice in the patient and can cause brain damage, as is well know in the case of hr-negative infants.

Dextran-Hemoglobin: Addressing the Toxicity Issues

The dextran-hemoglobin molecule is highly stable. The two clearly specified connecting ports (very stable co-valent bonds) mean that there is little likelihood of acellular hemoglobin breaking off from the dextran component.

Dextran-Hemoglobin has a half-life of 57 hours (vs about 22 hrs for First Generation products) which means that it stays functional in circulation for 2.6 times as long. This slower rate of metabolism coupled with the use of 6% instead of 10% - 14% hemoglobin content means that the body is able to process and eliminate the by-products in an orderly fashion without danger of overload.

Formation of injurious free radicals is reduced by high affinity for oxygen.

Molecular stability and lower hemoglobin content eliminates the danger of acellular hemoglobin clogging the delicate tubes of the Henley’s loops in the kidneys.

Molecular stability and lower hemoglobin content protects against the possibility of dangerous levels of iron deposits in the liver.