Characterization of an invertebrate transferrin from the crabCancer magister (Arthropoda)
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The protein has a molecular weight of 150,000±10,000 and consists of a single polypeptide chain. The iron content was found to be two molecules iron per molecule protein; the presence of a 52-fold molar excess of copper did not alter the iron binding properties.
Spectroscopical analysis carried out in the presence of bicarbonate yielded two absorption maxima at λ=276 and λ=465 nm.
The59Fe-tagged crab protein did not yield its iron to human apotransferrin when incubated in human plasma. When incubated with rat reticulocytes, the crab iron protein delivered its iron for hemoglobin and ferritin synthesis. This delivery can be completely abolished by adding an excess of rat transferrin.
When injected into rats in vivo, the crab protein delivered its iron to erythroid and non-erythroid tissue. The amount delivered to the erythroid marrow was shown to depend on iron requirements of that tissue.
When injected as59Fe-tagged protein into a crab, half-time disappearance of the radioiron is reached between 7 and 9 h. In the liver and in the carapace lining, the radioiron released is found in other iron binders.
Except for the significantly larger molecular weight, the crab iron binding protein fulfills all the criteria for transferrin. The findings negate the concept that transferrin is a newcomer in the evolutionary scene and to be found only in the phylum Chordata. Thus the need for a specific iron transport protein of the transferrin class is independent of the achievement of hemoglobin as a means to accomplish oxygen transport.
KeywordsFerritin Transferrin Single Polypeptide Chain Iron Binding Protein Iron Requirement
unsaturated iron binding capacity
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