Public Release: 

A unique hemoglobin may help the baby kangaroo's journey to the mother's pouch

A physiological process that is needed as a newborn wallaby changes from consuming its mother's oxygen to breathing outside air

American Physiological Society

San Diego, CA - A small species of kangaroo, known as the Tammar wallaby, is highly dependent on it's mother's pouch for its most crucial phase of development: immediately after birth. These native Australians are born after just 27 days of gestation, weighing in at a scant 350 milligrams. At birth, their eyes are not yet connected to their brain and their cerebral cortex possesses only one or two of the six layers it will need in adulthood. In many ways, the newborn Tammar is equivalent to a human embryo at seven weeks of gestation.

The blood of the newborn Tammar is embryonic in type and this persists for several days after birth. The red blood cells are all nucleated, which is characteristic of red cells in the embryos of other mammals, but is quite abnormal in any adult mammalian red cells. Additionally, there are four distinct hemoglobin types, each different from adult hemoglobin. Earlier research has revealed that these hemoglobins have features showing them to be embryonic in type. Before birth, the Tammar must get all its oxygen from the blood of the mother, across the yolk-sac placenta. After birth the animal must become more active and is largely air-breathing, although there is some oxygen uptake through their very thin skin.

The Study

A new study by an Australian research team has examined the embryonic-type hemoglobins from this species. The authors of a new study entitled, "Amino Acid Sequences of the Embryonic Globin Chains of a Marsupial, the Tammar Wallaby (Macropus eugenii)," are Robert Alastair Holland, from the University of New South Wales, Kensington (Sydney), New South Wales; Katherine H Gill, of MacQuarie University, North Ryde, New South Wales; Rory M Hope and David Wheeler, Adelaide University, Adelaide, South Australia; Steven J Cooper, from the South Australian Museum, Adelaide, South Australia; and Andrew A Gooley, Proteome Systems Limited, North Ryde, New South Wales, all in Australia. They will present their findings during the upcoming meeting, "The Power of Comparative Physiology: Evolution, Integration and Application," a scientific meeting of the American Physiological Society (APS). The gathering is being held August 24-28, 2002 at the Town & Country Hotel, San Diego, CA. Find more information log on to: http://www.the-aps.org/meetings/aps/san_diego/home.htm

The study of embryonic-type hemoglobins was undertaken to examine the findings that:

· Function laboratory studies revealed the oxygen affinity of the embryonic and newborn blood was surprisingly low. For an animal's blood to take up oxygen well at the intrauterine stage, its oxygen affinity should be higher than that of the mother's blood; and this is found in virtually all non-marsupial vertebrates. An objective was to assess the features of the embryonic hemoglobins that would give the blood this lower oxygen affinity.

· The same function studies showed that embryonic type hemoglobins did not function as tetrameric molecules (that is as molecules containing four sub-units), which is normal for vertebrate hemoglobins, but as bigger molecules, containing probably eight sub-units. The researchers sought to find if there were special features of the structure that would cause the molecules to associate into larger molecules.

Earlier work by the same team identified an extraordinary globin chain that is present in only small concentrations but is produced just around the time of birth. Its amino acid sequence has been shown to be more similar to bird and reptile globins than to any mammalian globin. Phylogenetic studies have shown that it is descended from a globin that split off from other globins before the bird-mammal evolutionary divergence over 300 million years ago. The gene for this globin has been found in other marsupials from different families, but not in any mammals other than marsupials. Researchers wanted to see if this gene is expressed in other marsupials or if the protein it codes for is found only in the Tammar. The persistence of this protein through evolution shows that the protein it codes for has some special and important function.

Methodology

All blood was removed from Tammar Wallaby newborns and the red cells were broken up to release the hemoglobin. The hemoglobins were separated by ion exchange chromatography, and for each, the chains were separated by another chromatography process. The chains were digested by the proteolytic enzyme, trypsin, and the digested fragments were analyzed by mass spectrometry to determine their molecular weight. The process also knocks off amino acids from either end of the peptide and the molecular weight of the new peptides is measured. With a general knowledge of the amino acid composition and sequence of hemoglobin, this enabled the determination of the sequence of each globin.

Results

Sequences were obtained for the chains. In one case, the so-called epsilon chain, the sequence showed it as similar to other mammalian embryonic beta-type globins. This epsilon was present in most of the hemoglobin present in the embryo and newborn. It showed no special features that would account for the special respiratory properties of mammalian blood.

There were two embryonic chains of the alpha type (known as zeta). They were similar to alpha-type embryonic chains in other species. One of these zeta chains had a polymorphism, with one amino acid difference at one site. In each chain there was an acetyl group on the amino end, which would block the ionic group there. This accounts for the decreased interaction between carriage of hydrogen ions and oxygen found in embryonic blood of this and other species. However, a careful examination of the sequence failed to find the explanation of the low oxygen affinity in embryonic blood. Nor did it reveal where two adjacent tetrameric molecules would link to form a larger molecule.

Finding two different embryonic alpha type chains was noteworthy. They differed in 14 amino acids out of a total of 141, and this shows that there were two genes each expressing a different zeta type globin. It is well known that in many mammals there is, as well as the zeta gene, a zeta pseudogene present. That is a DNA region that has many of the sequences to code for zeta but which has defects that prevent it producing the protein.

Conclusions

The findings demonstrate that there are more embryonic type hemoglobins in the Tammar than are found in most other species. The presence of the different hemoglobins to an extent not found in other animals suggests that in development there is the need for "flexibility" in hemoglobin function. This could be achieved by changing the proportion of the different molecules during development. This clearly happens when the omega chain is produced just a few days before birth; and also happens with the zeta chains.

The Tammar grows in the course of intrauterine development, making greater demands on oxygen supply, and then suddenly changes to a largely air breathing animal that has to make the climb from the birth canal up to the pouch. It is possible that the omega globin, the ancient molecule preserved in marsupials after 300 million years, plays an important role at this time. The omega chains in this mammal will not form a compound with the embryonic-type alpha chains, but only with the adult alpha chains, whose expression begins just at the time when the embryonic-type omega chains are first produced.

The study highlights the importance of working on the purified hemoglobins and not merely on the genes.

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The American Physiological Society (APS) is one of the world's most prestigious organizations for physiological scientists. These researchers specialize in understanding the processes and functions by which animals live, and thus ultimately underlie human health and disease. Founded in 1887 the Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals each year.

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