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Hb Arta [β45 (CD4) Phe→Cys]: a new unstable haemoglobin with reduced oxygen affinity in trans with β‐thalassaemia

Summary. The interaction of rare Hb variants with β°‐thalassaemia results in a quasihomozygous state where the erythrocytes contain the variant as the only major adult Hb component. Such a situation is a unique model that enables functional studies even in the case of a neutral variant that could not be isolated from Hb A. We report here an unusual patient carrying Hb Arta, a novel Hb variant [β45 (CD4) Phe→Cys], in trans with β°‐thalassaemia gene (β° 39). The aminoacid substitution at the critical CD corner of this Hb molecule renders the molecule unstable. In addition, haem is displaced in a position that favours the deoxy (T) conformation of the variant, but less than in Hb Cheverly [β45 (CD4) Phe‐Ser], and results in a p50 of 43 mmHg (pH 7‐4, 37°C) in the red cells with preservation of cooperativity. Solution studies of the almost pure Hb Arta show a 50% decrease in oxygen affinity and normal cooperativity; the Bohr effect and the interaction with organic phosphates are similar to those of Hb A. Hb Arta retains both normal homo‐ and hetero‐tropic effects allowing a well‐preserved oxygen transport in vivo despite a mild anaemia.