Purification and characterization of recombinant polymeric hemoglobin P1 of Glycera dibranchiata

Purification of Lumbricus terrestris erythrocruorin (LtEc) with anion exchange chromatography

The naturally extracellular hemoglobin (erythrocruorin) of the Canadian nightcrawler, Lumbricus terrestris (LtEc), is a unique oxygen transport protein that may be an effective substitute for donated human blood. Indeed, this ultra-high molecular weight (~3.6 MDa) hemoglobin has already been shown to avoid the side effects associated with previous hemoglobin-based oxygen carriers and its high thermal stability (T_m = 56°C) and resistance to heme oxidation (k_ox = 0.04 hr^-1 × 10³ at 20°C) allow it to be stored for long periods of time without refrigeration. However, before it can be tested in human clinical trials, an effective and scalable purification process for LtEc must be developed. We have previously purified LtEc for animal studies with tangential flow filtration (TFF), which allows rapid and scalable purification of LtEc based on its relatively large size, but that type of size-based purification may not be able to specifically remove some impurities and high MW (>500 kDa) contaminants like endotoxin (MW = ~1-4 MDa). Anion exchange (AEX) and immobilized metal affinity chromatography (IMAC) are two purification methods that have been previously used to purify mammalian hemoglobins, but they have not yet been used to purify large invertebrate hemoglobins like LtEc. Therefore, the goal of this study was to determine if AEX and IMAC resins could successfully purify LtEc from crude earthworm homogenate, while also preserving its macromolecular structure and function. Both processes were able to produce purified LtEc with low levels of endotoxin, but IMAC purification induced significantly higher levels of heme oxidation and subunit dissociation than AEX. In addition, the IMAC process required an additional desalting step to enable LtEc binding. In contrast, AEX produced highly pure LtEc that was not dissociated. LtEc purified by AEX also exhibits similar oxygen binding characteristics (P₅₀ = 27.33 ± 1.82 mm Hg, n = 1.58 ± 0.17) to TFF-purified LtEc (P₅₀ = 28.84 ± 0.40 mm Hg, n = 1.93 ± 0.02). Therefore, AEX appears to be the optimal method for LtEc purification.

Purification of diverse hemoglobins by metal salt precipitation

Although donated blood is the preferred material for transfusion, its limited availability and stringent storage requirements have motivated the development of blood substitutes. The giant extracellular hemoglobin (aka erythrocruorin) of the earthworm Lumbricus terrestris (LtEc) has shown promise as a blood substitute, but an efficient purification method for LtEc must be developed to meet the potential large demand for blood substitutes. In this work, an optimized purification process that uses divalent and trivalent metal salts to selectively precipitate human, earthworm, and bloodworm hemoglobin (HbA, LtEc, and GdHb, respectively) from crude solutions was developed. Although several metal ions were able to selectively precipitate LtEc, Zn(2+) and Ni(2+) provided the lowest heme oxidation and highest overall yield of LtEc. In contrast, Zn(2+) was the only metal ion that completely precipitated HbA and GdHb. Polyacrylamide gel electrophoresis (PAGE) analysis shows that metal precipitation removes several impurities to provide highly pure hemoglobin samples. Heme oxidation levels were relatively low for Zn(2+)-purified HbA and LtEc (2.4±1.3% and 5.3±2.1%, respectively), but slightly higher for Ni(2+)-purified LtEc (8.4±1.2%). The oxygen affinity and cooperativity of the precipitated samples are also identical to samples purified with tangential flow filtration (TFF) alone, indicating the metal precipitation does not significantly affect the function of the hemoglobins. Overall, these results show that hemoglobins from several different species can be highly purified using a combination of metal (Zn(2+)) precipitation and tangential flow filtration.

Recombinant functional multidomain hemoglobin from the gastropod Biomphalaria glabrata

The extracellular hemoglobin multimer of the planorbid snail Biomphalaria glabrata, intermediate host of the human parasite Schistosoma mansoni, is presumed to be a 1.44 MDa complex of six 240 kDa polypeptide subunits, arranged as three disulfide-bridged dimers. The complete amino acid sequence of two subunit types (BgHb1 and BgHb2), and the partial sequence of a third type (BgHb3) are known. Each subunit encompasses 13 paralogus heme domains, and N-terminally a smaller plug domain responsible for subunit dimerization. We report here the recombinant expression of different functional fragments of BgHb2 in Escherichia coli, and of the complete functional subunits BgHb1 and BgHb2 in insect cells; BgHb1 was also expressed as disulfide-bridged dimer (480 kDa). Oxygen-binding measurements of the recombinant products show a P(50) of about 7 mmHg and the absence of a significant cooperativity or Bohr effect. The covalently linked dimer of BgHb1, but not the monomer, is capable to form aggregates closely resembling native BgHb molecules in the electron microscope.