CHARACTERIZATION OF THE EMBRYONIC GLOBIN CHAINS OF THE MARSUPIAL TAMMAR WALLABY, MACROPUS-EUGENII

The embryonic hemoglobins of the marsupial Tammar wallaby (Macropus eu genii) are known to aggregate, which was shown by the finding that the Hill coefficient, h, was greater than 4.0 in the upper part of the ox ygen equilibrium curve. Here, we have undertaken a detailed primary st ructure analysis of the Tammar wallaby pouch young hemoglobin compleme nt, which we hoped might provide clues into the residues that cause ag gregation and a high embryonic h. The Tammar wallaby embryonic hemoglo bin complement is principally four major hemoglobins each with a diffe rent isoelectric point. Two early expressed hemoglobins contain the sa me embryonic beta-like chain, epsilon (epsilon), but two separate alph a-like chains, termed zeta and zeta prime (zeta and zeta') both of whi ch are N-terminally blocked. The later two expressed hemoglobins conta in the same adult alpha-chain, but different beta-like chains. The lat est expressed hemoglobin contains the same beta-like chain, epsilon, a s the two early expressed forms, but the third expressed hemoglobin co ntains a unique beta-like chain which we have termed omega (omega). A protein database similarity search using the first 54 N-terminal amino acids of the omega-chain showed a range of sequence identities of 57- 72% to all known mammalian beta-like chains, including the other marsu pial epsilon-chains. The closest identity, reflected by both the highe st percentage identity and Smith-Waterman score, was with the embryoni c beta-chains of the aves. While the primary structures of the hemoglo bins reported here do not explain the low hemoglobin-oxygen affinity i n embryonic marsupial blood, the finding of the similarity with the bi rd globin-like sequence with one of the marsupial chains has implicati ons on mammalian globin evolution. How many other marsupials and place ntal mammals are harboring a bird-like globin in their embryos?