LIPOPOLYSACCHARIDES OF ACTINOBACILLUS-PLEUROPNEUMONIAE BIND PIG HEMOGLOBIN

A previous study indicated that lipopolysaccharides (LPS) extracted fr om Actinobacillus pleuropneumoniae bind two low-molecular-mass protein s, of approximately 10 and 11 kDa, present in porcine respiratory trac t secretions (M. Belanger, D. Dubreuil, and M. Jacques, Infect. Immun. 62:868-873, 1994). In the present study, we determined the N-terminal amino acid sequences of these two proteins, which revealed high homol ogy with the alpha and beta chains of pig hemoglobin, Some isolates of A. pleuropneumoniae were able to use hemoglobin from various animal s pecies as well as other heme compounds as sole sources of iron for gro wth, while other isolates were unable to use them, Immunoelectron micr oscopy showed binding of pig hemoglobin at the surface of all A. pleur opneumoniae isolates as well as labeling of outer membrane blebs, We o bserved, using Western blotting (immunoblotting), that the lipid A-cor e region of LPS of all isolates was binding pig hemoglobin, Furthermor e, lipid A obtained after acid hydrolysis of LPS extracted from A, ple uropneumoniae was able to bind pig hemoglobin and this binding was com pletely abolished by preincubation of lipid A with polymyxin B but was not inhibited by preincubation with glucosamines, Fatty acids constit uting the lipid A of A. pleuropneumoniae, namely, dodecanoic acid, tet radecanoic acid, 3-hydroxytetradecanoic acid, hexadecanoic acid, and o ctadecanoic acid, were also binding pig hemoglobin, Our results indica te that LPS of all A, pleuropneumoniae isolates tested bind pig hemogl obin and that lipid A is involved in this binding, Our results also in dicate that some A. pleuropneumoniae isolates are, in addition, able t o use hemoglobin for growth, Binding of hemoglobin to LPS might repres ent an important means by which A. pleuropneamoniae acquires iron in v ivo from hemoglobin released from erythrocytes lysed by the action of its hemolysins.