The need for an alternative to red cells in transfusions has led to the creation of hemoglobin-based oxygen carriers (HBOCs). However, evaluations of all products tested in clinical trials have noted cardiovascular complications, raising questions about their safety that led to the abandonment of all those products. It has been considered that the adverse side effects come from the scavenging of the vasodilator – nitric oxide (NO) by the deoxyheme sites of the hemoglobin derivatives. Another observation is that HBOCs with lower oxygen affinity than red cells release oxygen prematurely in arterioles, triggering an unwanted homeostatic response. Since the need for such a product remains critical, it is important to understand the reactivity patterns that contribute to the observed complications.
Various alterations of the protein have been attempted in order to reduce HBOC-induced vasoconstriction. Recent reports suggest that a safe and effective product should be pure, homogenous and have a high molecular weight along with appropriate oxygenation properties. While these properties are clearly important, vasodilatory features of hemoglobin through its nitrite reductase activity may also act as an in situ source of NO. It follows that HBOCs with an enhanced ability to produce NO from endogenous nitrite may serve to counteract vasoactivity associated with NO-scavenging by hemoglobin.
Here we characterize the effects of different protein modifications on the nitrite reductase activity of hemoglobin. We produced a variety of HBOCs that include cross-linked tetramers, polyethylene glycol (PEG) conjugates and bis-tetramers of hemoglobin. We report that the rate of NO production strongly depends on the conformational state of the protein, with R-state stabilized proteins (PEG-Hbs), exhibiting the fastest rates. In particular, we found that PEGylated bis-tetramers of hemoglobin (BT-PEG) exhibit increased nitrite reductase activity while retaining cooperativity and stability. Animal studies of BT-PEG demonstrated that this material is benign: it did not cause significant increases in systemic blood pressure in mice, the major side effect associated with existing HBOCs. BT-PEG exhibits an enhanced nitrite reductase activity together with sample purity and homogeneity, molecular size and shape, and appropriate oxygenation properties, characteristics of a clinically useful HBOC.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/31844 |
| Date | 10 January 2012 |
| Creators | Lui, Francine Evelyn |
| Contributors | Kluger, Ronald |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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