HB Shelby [β131(H9)GLN→LYS] in Association with HB S [β6(A3)GLU→VAL]: Characterization, Stability, and Effects on HB S Polymerization

Abstract

When first tested for abnormal hemoglobins, a 2-year-old boy, appeared to have Hb F, Hb S and Hb A₂. Confirmatory testing revealed a β chain variant inherited from his father and β^s from his mother. Analysis of tryptic peptides in conjunction with automated DNA sequence analysis showed that the variant hemoglobin was Hb Shelby [β131(H9)Gln→Lys (CAG→AAG)].

Heat and mechanical stabilities of various liganded Hb Shelby tetramers were compared to those of Hb A and Hb S. Oxy-Hb Shelby precipitated more readily than oxy-Hb A, but was much more stable than oxy-Hb S during mechanical agitation. In contrast, oxy-Hb Shelby was much less stable than oxy-Hb A and oxy-Hb S following heat treatment. Met-Hb Shelby was most unstable compared to other liganded forms of Hb Shelby, while deoxy- and carbonmonoxy-forms of Hb Shelby showed similar heat-induced precipitation rates. These data indicate that heat instability of Hb Shelby is accompanied by heme oxidation, and that denaturation by mechanical agitation occurs in the absence of heme oxidation.

Hb Shelby, like Hb A, can form hybrids with Hb S which participate in polymer formation in vitro. However, Hb S/Hb Shelby hybrids copolymerized with Hb S less than A/S hybrids. Since the patient's MCHC value is normal, this finding coupled with the elevated Hb A₂ and Hb F levels, both of which are known to inhibit polymerization of Hb S, may contribute to the patient's mild clinical presentation.