By a number of different technics, confirmatory evidence has been obtained that Cr51 activity of Hb A1 tagged with Na2Cr51O4 in a ratio of approximately 1 mole chromium to 1000 mole of hemoglobin is largely associated with the β polypeptide chains. No significant difference in the site or strength of chromium binding could be demonstrated in abnormal hemoglobins resulting from point amino acid substitutions (Hbs S and C).

The results obtained with Hb F were very suggestive of γ chain binding, but an apparently weak Hb F-Cr51 bond or increased elution rate markedly hampered such studies.

The propensity of Cr51 for preferential tagging of β chains was seen not only when hemoglobin was tagged intact, but also when isolated β chains were tagged. This may indicate that there is a specific locus or chemical configuration within the β chains where Cr51 tagging occurs. Knowledge of the exact total amino acid sequence of α, β, and γ chains should soon be available and may permit a precise identification of such a Cr51 binding site.

Whether the results of this type of highly artificial manipulation of hemoglobin has significance in Cr51 red cell survival studies in clinical situations where large amounts of Hb F occur is uncertain. Conflicting results of red cell survival studies of the fetal red cells suggests that it may.27,28 Further investigation of this problem is currently in progress.

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