A Unique Hemoglobin, Possibly 300 Million Years Old,
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
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.
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.
-end-
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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EDITOR’S NOTE: For further
information or to schedule an interview, contact Donna Krupa at 703.967.2751
(cell), or by email at djkrupa1@aol.com.
