THE MINI-HEMOGLOBINS IN NEURAL AND BODY-WALL TISSUE OF THE NEMERTEAN WORM, CEREBRATULUS-LACTEUS

Hemoglobin (Hb) occurs in circulating red blood cells, neural tissue, and body wall muscle tissue of the nemertean worm, Cerebratubus lacteu s. The neural and body wall tissue each express single major Hb compon ents for which the amino acid sequences have been deduced from cDNA an d genomic DNA. These 109-residue globins form the smallest stable Hbs known. The globin genes have three exons and two introns with splice s ites in the highly conserved positions of most globin genes. Alignment of the sequences with those of other globins indicates that the A, B, and H helices are about one-half the typical length. Phylogenetic ana lysis indicates that shortening results in a small tendency of globins to group together regardless of their actual relationships. The neura l and body wall Hbs in situ are half-saturated with O-2 at 2.9 and 4.1 torr, respectively. The Hill coefficient for the neural Hb in situ, s imilar to 2.8, suggests that the neural Hb self-associates in the deox y state at least to tetramers at the 2-3 mM (heme) concentration estim ated in the cells. The Hb must dissociate upon oxygenation and dilutio n because the weight-average molecular mass of the HbO(2) in vitro is only about 18 kDa at 2-3 mu M heme concentration. Calculations suggest that the Hb can function as an O-2 store capable of extending neurona l activity in an anoxic environment for 5-30 min.