A high-affinity hemoglobin is expressed in the notochord of amphioxus, Branchiostoma californiense

In the phylum Chordata, only members of the subphylum Vertebrata were thoug ht to express hemoglobin (Hb). Here we document the existence of intracellu lar Hb expressed;in members of the subphylum Cephalochordata. Hemoglobin is expressed in myotome tissue and in notochord cells within the body of amph ioxus. Both notochord and myotome tissue Hbs have a molecular size consiste nt with a dimeric molecule made up of two non-covalently linked monomers ea ch of approximately 19 kD. The notochord Hb has a relatively high oxygen-bi nding affinity, with an apparent P-50 Of 0.036 kPa (0.27 mm Hg), and it doe s not bind oxygen cooperatively. The notochord Hb may be involved in facili tating oxygen delivery and providing a short-term oxygen store within the n otochord cells in order to maintain a high level of aerobic metabolism in s upport of the sustained contraction necessary for notochord function.