Endotoxins have been recognized for more than a century as microbial constituents capable of eliciting deleterious pathophysiologic responses when administered to experimental laboratory animals. Within the last several decades, evidence in support of the concept that endotoxins can serve as proinflammatory mediators in humans has derived primarily from studies carried out in healthy human volunteers using highly purified and well-characterized endotoxic lipopolysaccharides from Escherichia coli.<sup>10 Cannon J.G. Endotoxin and cytokine responses in human volunteers Immunopharmacology and Pathophysiology, vol II (Bacterial Endotoxic Lipopolysaccharides) Boca Raton, FL: CRC Press (1992).  311

Haga clic aquí para ir a la sección de Referencias</sup> Of considerable interest in this respect, however, is the finding that there is not at the present time strong and unequivocal evidence that would unambiguously support bacterial endotoxin as being among the dominant microbial factors that contributes substantially to morbidity and mortality in the septic patient.<sup>63 Morrison D.C., Dinarello C.A., Munford R.S. , y al. Bacterial endotoxins and pathogenesis of gram-negative infections: Current status and future direction J Endotoxin Research 1994 ;  1 : 71

Haga clic aquí para ir a la sección de Referencias</sup> Nevertheless, there is still sufficiently strong evidence from many sources to allow the conclusion that endotoxin is of considerable importance in sepsis, and it merits study to understand exactly how it contributes to the disease process.

Since the earliest days of endotoxin research, investigators have sought the answer to the question of the relationship between the structure of the endotoxin molecule and its function as a microbial toxin. From the very earliest studies, pioneered in large part by Andre Boivin, the Mesrobeanus, Walter Morgan, and others, it was recognized that endotoxin is indeed a complex chemical entity, consisting of protein, lipid, and carbohydrate.<sup>89 Rietschel E.T., Westphal O. Endotoxin: Historical Perspectives Endotoxin in Health and Disease New York: Marcel Dekker (1999). 

Haga clic aquí para ir a la sección de Referencias</sup> Experiments first carried out by the cancer researchers, Shear and his colleagues,<sup>33 Hartwell J.L., Shear M.J., Adams J.R. Chemical treatment of tumors, VII. Nature of the hemorrhage-producing fraction from Serratia marcescens (Bacillus prodigiosus) culture filtrates J Natl Cancer Inst 1943 ;  4 : 107

Haga clic aquí para ir a la sección de Referencias</sup> to test experimentally the concept that the endotoxin-containing mixed microbial toxins employed by William Coley to treat inoperable sarcomas were, in fact, endotoxin, first described these molecules as complex lipopolysaccharides (LPS). Strong support for this conclusion was provided by studies that allowed the conclusion that LPS was, in fact, identical to the O-antigen first described by Weil and Felix in 1917.<sup>114 Ulmer A.J., Feist W., Heine H. , y al. Modulation of endotoxin-induced monokine release in human monocytes by lipid A partial structures that inhibit binding of 125I-lipopolysaccharide Infect Immun 1992 ;  60 (12) : 5145

Haga clic aquí para ir a la sección de Referencias</sup> The seminal studies of Luderitz and Westphal in the 1950s provided the first insights into detailed structure-function relationships of endotoxic LPS when they demonstrated that heating of highly purified LPS with mild acid resulted in the generation of an insoluble lipid-enriched residue that they called lipid A (to distinguish it from the more readily removable lipids, termed lipid B, also present in the preparations).

By the late 1950s and early 1960s, the evidence was reasonably strong (but not without question) that the lipid component (and more precisely the lipid A) was the toxic component of endotoxin responsible for the lethal activity of endotoxin in experimental laboratory animals. Mounting evidence to further substantiate this claim derived from studies involving a diverse spectrum of experimental approaches, including (1) the isolation and characterization of the so-called deep rough Salmonella minnesota strain of bacteria<sup>70 Nikaido H. Galactose-sensitive mutants of Salmonella. I. Metabolism of galactose Biochim Biophys Acta 1961 ;  48 : 460

Haga clic aquí para ir a la sección de Referencias, 71 Nikaido H.  Washington, DC: Proc Natl Acad Sci U S A (1962).  1542

Haga clic aquí para ir a la sección de Referencias</sup>; (2) chemical treatment with mild alkali to modify selectively lipid A fatty acids<sup>85 Rietschel E.T., Gottert H., Luderitz O. , y al. Nature and linkages of the fatty acids present in the lipid A component of Salmonella lipopolysaccharides Eur J Biochem 1972 ;  28 : 166

Haga clic aquí para ir a la sección de Referencias</sup>; (3) experiments carried out in the presence of polymyxin B, an agent shown to bind exclusively to the lipid A region of LPS<sup>64 Morrison D.C., Jacobs D.M. Binding of polymyxin B to the lipid A portion of bacterial lipopolysaccharides Immunochemistry 1976 ;  13 : 813  [cross-ref]

Haga clic aquí para ir a la sección de Referencias</sup>; and (4) more comprehensive in vivo and in vitro studies comparing highly purified lipid A with intact LPS.<sup>11 Chiller J.M., Skidmore B.M., Morrison D.C. , y al. Relationship of lipopolysaccharide structure to its function in mitogenesis and adjuvanticity Proc Natl Acad Sci U S A 1973 ;  70 : 212

Haga clic aquí para ir a la sección de Referencias</sup> The final proof of the importance of lipid A awaited the precise determination of its chemical structure and subsequent complete chemical synthesis, the latter of which was finally achieved and published in the early 1980s by Tetsuo Shiba and his colleagues in Japan.<sup>106 Tanamoto K., Zahringer U., McKenzie G.R. , y al. Biological activities of synthetic lipid A analogs: Pyrogenicity, lethal toxicity, anticomplement activity and induction of gelation of Limulus amoebocyte lysate Infect Immun 1984 ;  44 : 421

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It is now universally recognized that bacterial endotoxins are complex chemical structures. The active component of endotoxin is the O-antigen–containing LPS structure. Within the LPS molecule, it is the common highly conserved lipid A domain that actually constitutes the toxic principle of endotoxin. Chemically synthesized polysaccharide-free preparations of lipid A will manifest biologic activities both in vitro and in vivo that are superimposible with those obtained using preparations of highly purified lipid A from natural microbial sources.

In spite of the now generally universally accepted concept that the toxic and inflammatory manifestations of endotoxin reside in the lipid A region, it is nevertheless important to appreciate that there exist credible data that would support the concept that other components of the endotoxin complex are also capable of eliciting host responses. These responses would be in addition to their ability to stimulate the host immune system in the induction of specific antibody. In this respect, there is evidence that the polysaccharide component of LPS (including both core oligosaccharide and O-antigen) can have biologic activity independent of lipid A. In addition, other microbial constituents that often associate with LPS during the isolation and purification process, primarily protein, can interact with host cells to produce inflammatory mediators, and such contributions should not be ignored, particularly when considering the pathophysiologic role of endotoxin in the septic patient.