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CROCODILE BLOOD
CAMBRIDGE, ENGLAND- What do you get when you cross a
crocodile with a human? No, not an attorney, but an experiment
which provides clues about the evolution of hemoglobin and which
may lead to useful blood substitutes.
Crocodiles can remain underwater more than one hour. Indeed
they use this ability to kill their prey by drowning it.
Molecular biologists in Cambridge and Japan have found a quirk in
crocodile hemoglobin which appears to keep more oxygen
circulating in the blood. When a crocodile dives, the blood fills
with bicarbonate ions, the last product of respiration. As these
bicarbonate ions accumulate the oxygen affinity of the hemoglobin
is reduced. This in turn releases a large percentage of
hemoglobin-bound oxygen into the tissues.
This bicarbonate effect is considerably different from the
way diving mammals and seals stay under water for prolonged
periods. Diving mammals utilize muscle myoglobin to stay under
water. These mammals have more than 100 times the myoglobin
content as crocodiles, the researchers note.
Dr. Kiyoshi Nagai and colleagues identified a small part of
the crocodile hemoglobin molecule that binds bicarbonate ions.
They accomplished this by making numerous chimerical combinations
of human and crocodile hemoglobin and examining the oxygen
binding properties of the novel hemoglobins. Eventually, they
created a human hemoglobin with only a few crocodile hemoglobin
amino acids added which seemed to bind bicarbonate ions quite
effectively. The novel human/crocodile hemoglobin has been dubbed
'Hb scuba'.
The most important implication from this research is not the
creation of a new submarine race of humanity, the researchers
emphasize. Rather the findings reveal new data about hemoglobin
variety across species. The studies showed that no more than 12
amino acid replacements were required to create the bicarbonate
effect. This indicates how a relatively small modification in
molecular structure, subject to natural selection, can have
far-reaching effects which animals can put to good use. These
findings are also likely to open a new front in research effort
to develop blood substitutes.
This study was reported by Kiyoshi Nagai et al. in Nature,
v.373, 1/19/95, pp. 244-47.
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