On the molecular mass of Lumbricus erythrocruorin

A critical examination of the published molecular mass of erythrocruorin (Ec) from Lumbricus and related earthworm species reveals that the results do cluster, not at one, but at two values of the molecular mass. One cluster corresponds to ∼3.6 MDa as predicted from the Vinogradov model for the hexagonal bilayer (HBL) assembly of Lumbricus terrestris EC and as estimated from the crystal structure of HBL at 5.5 A resolution. The other cluster corresponds to ∼4.4 MDa. However, in contrast to the controversy over the molecular mass, there is a consensus that the sedimentation coefficient of intact L. terrestris Ec is ∼60S. Drawing on the occurrence of central subunits in Ec of Oenone fulgida and few other annelid species, we propose for the 4.4 MDa molecule a model of HBL supplemented by a central mass. The proposed model abides by D₆ symmetry and is a suitable candidate to represent 60S Lumbricus terrestris Ec.