Hemoglobin Structure and Function in the Rat-Tailed Sea Cucumber, Paracaudina chilensis

The rat-tailed sea cucumber, Paracaudina chilensis, has abundant hemoglobin-filled hemocytes in its perivisceral coelom, water vascular system, and hemal system. The perivisceral oxyhemoglobin consisted of 34 kDa dimers and molecules with an apparent molecular weight of ca. 50 kDa. The perivisceral hemoglobin had a high oxygen affinity with a P50 of 1.5 mm Hg at 15°C. It exhibited cooperative oxygen binding with a Hill coefficient of 1.26 to 1.86. Oxygen affinity appeared to be pH dependent, but the effect was not significant. The heat of oxygenation was -11.2 kcal mol-1. At high hemoglobin concentrations, the perivisceral hemoglobin oxygen affinity was lower and the apparent pH effect and cooperativity were increased. Perivisceral and water vascular hemoglobins had spectral characteristics similar to those of other invertebrate and vertebrate hemoglobins. The perivisceral hemoglobin appeared to be electrophoretically heterogeneous and was structurally distinguishable from water vascular hemoglobin. The oxygen affinity of water vascular hemoglobin was not different from that of the perivisceral hemoglobin in spite of the difference in structure and location in the animal. The exceptionally high oxygen affinity hemoglobin of P. chilensis, a burrowing sea cucumber, may be adaptive to this animal's oxygen-limited habitat.