Oxygenation properties of hemoglobin (Hb) from Oligobrachia mashikoi were extensively investigated. Compared to human Hb, Oligobrachia Hb showed a high oxygen affinity (P-50 = 1.4 mmHg), low cooperativity (n = 1.4), and a small Bohr effect (delta H+ = -0.28) at pH 7.4 in the presence of minimum salts. Addition of NaCl caused no change in the oxygenation properties of Oligobrachia Hb, indicating that Na+ and Cl-had no effect. Mg2+ and Ca2+ remarkably increased the oxygen affinity and cooperativity. The dependence of the oxygen affinity on Ca2+ concentration indicated that ca. 0.6 Ca2+ per heme is bound to the protein moiety upon oxygen binding. CO2 and a polyanion, inositol hexaphosphate, showed a null effect on the oxygenation properties. Thus, unlike the vertebrate Hbs, but like the annelid extracellular Hbs, the oxygen binding properties of Oligobrachia Hb are regulated by divalent cations which preferentially bind to the oxy form. (c) 2007 Elsevier Inc. All rights reserved.