Comparison of the molecular size and shape of the extracellular hemoglobins of Tubifex tubifex and Lumbricus terrestris

Abstract

The extracellular hemoglobins of Tubifex tubifex and Lumbricus terrestris have been examined by gel filtration and scanning transmission electron microscopy and a detailed comparison of their morphologies has been performed using digital image processing techniques. The hemoglobins possess identical elution volumes on analytical columns of Sepharose CL-6B and CL-2B. In electron micrographs the overall shape of negatively stained Tubifex and Lumbricus hemoglobins is similar to the two-tiered hexagonal appearance of other annelid extracellular hemoglobins. Tubifex hemoglobin possesses a vertex to vertex diameter of 31.3 nm and a height of 21.1 nm, while the corresponding dimensions of Lumbricus hemoglobin are 31.2 nm and 21.5 nm, respectively. These dimensions are about 25% larger than those obtained by conventional transmission electron microscopy but are consistent with other values obtained by scanning transmission EM (Kapp, O.H., Vinogradov, S.N., Ohtsuki, M. and Crewe, A.V. (1982) Biochim. Biophys. Acta 704, 546–548) and by X-ray diffraction of wet crystals of Lumbricus hemoglobin (Royer, W.E., Broden B.C., Jacobs, H.C. and Loves, W.E. (1981) in Invertebrate Oxygen-Binding Proteins (Lamy, J. and Lamy, J., eds.), 337–341, Marcel Dekker, New York). Comparison of the morphologies of the two hemoglobins by examination of individual pixel lines through their digital projections suggests that the molecules are very similar in overall structure.