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Notes: Abstract Small amounts of human leukocyte extracts (ENZ), inflammatory exudates, lysozyme (LYZ), and a variety of neutral proteases are capable of releasing (solubilizing) lipopolysaccharides (LPS) fromSalmonella typhi. LPS activity was determined by its capacity to sensitize red blood cells (RBC) passively to agglutination by antisalmonella serum. While the LPS-releasing capacity of ENZ, inflammatory exudates, and trypsin can be inhibited by phenyl-methyl-sulfonyl fluoride (PMSF, a known protease inhibitor), that of papain and LYZ is inhibited by heat treatment. On the other hand, excess amounts of ENZ, inflammatory exudate, and proteases failed to release substantial amounts of active LPS. The failure of large amounts of the releasing agents to yield LPS is due to the presence in the bacterial extracts of inhibitors to LPS which can, however, be removed by treatment with phenol. Thus the full amount of LPS can be recovered. The release of LPS by small amounts of ENZ is strongly inhibited by anionic and cationic polyelectrolytes (heparine, chondroitin sulfate, histone) as well as by normal serum and by antisalmonella serum. The inhibitors for LPS present in serum are not associated with antibodies to LPS but with other still unidentified serum proteins. Excess amounts of inflammatory exudates and anionic and cationic polyelectrolytes are also capable of blocking the receptors for LPS upon the RBC surface. The inhibition can however be overcome by trypsin and by anionic polyelectrolytes. The capacity of LPS to bind to RBC depends upon the state of activation of the LPS molecule and on the availability of unmasked receptor sites. While the activation of the LPS molecule can be achieved by pretreatment with small amounts of ENZ or by NaOH the unmasking of receptor sites upon RBC can be achieved by treatment with small amounts of ENZ or by neutral proteases. LPS released from the bacteria either by phenol or by ENZ can be separated into two fractions by gel filtration on Sephadex G-200. While the bulk of the LPS is associated with a fraction with a molecular weight similar to that of DNA, a smaller fraction which may represent deaggregated LPS has a molecular weight lower than hemoglobin. The role played by anionic and cationic polyelectrolytes by serum proteins and by activators of LPS and of LPS receptors, upon mammalian cells, in the initiation of passive immune cytolysis is discussed in relation to the pathogenesis of tissue damage induced in inflammatory sites.