Novel haemoglobin mutation (α127Lys→Glu) increases oxygen affinity and has a minor effect on haptoglobin binding

To determine if a new haemoglobin (Hb) variant was the underlying cause of erythrocytosis in a subject with a high apparent HbA(1)c.

Haemolysate was analysed by ESI MS, and individual components purified by ion exchange and reverse phase chromatography. Peptide mapping was used to pinpoint the substitution and DNA sequencing to confirm the precise mutation. Oxygen affinity was measured and relative haptoglobin (Hp) binding estimated.

Intact protein analysis and peptide mapping suggested a mutation in peptide α13 and DNA sequencing confirmed a novel α127Lys→Glu substitution in the α 2 gene. The abnormal Hb had a significantly higher O(2) affinity (5.8 mmHg) than HbA (12.4 mmHg). In addition the mutation caused a small but significant decrease in Hp binding.

Molecular models show that the side chain of α127Lys stabilises the T structure of deoxy Hb and that mutation to Glu would favour conversion to the high affinity R state. Notwithstanding this and the demonstrated high affinity, there was only a small increase in RBCs, Hb concentration and PCV in other female carriers of the mutation. The absence of a significant phenotype of erythrocytosis is most probably due to the low level (19%) of the variant.