A High-Affinity Hemoglobin Is Expressed in the Notochord of Amphioxus, Branchiostoma californiense

In the phylum Chordata, only members of the subphylum Vertebrata were thought to express hemoglobin (Hb). Here we document the existence of intracellular Hb expressed in members of the subphylum Cephalochordata. Hemoglobin is expressed in myotome tissue and in notochord cells within the body of amphioxus. Both notochord and myotome tissue Hbs have a molecular size consistent with a dimeric molecule made up of two non-covalently linked monomers each of approximately 19 kD. The notochord Hb has a relatively high oxygen-binding affinity, with an apparent P5O of 0.036 kPa (0.27mm Hg), and it does not bind oxygen cooperatively. The notochord Hb may be involved in facilitating oxygen delivery and providing a short-term oxygen store within the notochord cells in order to maintain a high level of aerobic metabolism in support of the sustained contraction necessary for notochord function.