Hemoglobin Gun Hill is an unstable mutant
hemoglobin associated with mild compensated
hemolysis. This abnormal
protein has a deletion of five
amino acids in the beta-chains. The deletion includes the
heme-binding proximal
histidine at position 92. The beta-chains of
hemoglobin Gun Hill lack
heme groups. Approximately 32% of the circulating
hemoglobin in heterozygous subjects consists of the mutant
hemoglobin. When reticulocytes were incubated with radioactive
amino acid the specific activity of
hemoglobin Gun Hill was three to six times that of
hemoglobin A. Total incorporation of radioactivity into
hemoglobin Gun Hill was two to three times that into
hemoglobin A. There were 20-50% more total counts in beta-Gun Hill (beta(GH)) than in beta(A). These results indicate that in reticulocytes there was greater synthesis of the abnormal beta-chains than beta(A)-chains. The ratio of the specific activities of the alpha-chains of
hemoglobin Gun Hill to the alpha-chains of
hemoglobin A was 20: 1. There was evidence of a free pool of alpha-chains in the reticulocytes containing
hemoglobin Gun Hill. After 10 min of incubation approximately 40% of the total alpha-chain radioactivity was in the free pool. When
protein synthesis was blocked by incubation of reticulocytes with
puromycin, the specific activity of the alpha-chains of
hemoglobin Gun Hill continued to increase due to direct exchange of alpha-subunits between the free pool and preformed
hemoglobin Gun Hill. Studies of the assembly of beta(A) and beta(GH) revealed that the rates of translation of the two
polypeptide chains were equal and uniform. No evidence was obtained for the existence of "slow points" in the process of
globin chain assembly. The studies also suggest that lack of strong
heme-
globin binding does not hinder the synthesis of
globin chains.