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THE cell-free haemolysin produced by growing certain strains of Escherichia coli on meat-broth medium was originally described by Lovell and Rees (1960) and has been termed a-haemolysin by Smith (1963). Since there have been a number of reports describing other filterable haemolysins (Snyder and Koch, 1966;Walton and Smith, 1969;Muranyi and Juhasz, 1971;Rennie and Arbuthnott, 1971) which might be confused with the a-haemolysin studied in these experiments, the properties that distinguish a-haemolysin from the others produced by E. coli are summarised here. A second type of filterable cell-free haemolysin, first described by Snyder and Koch (1966), differs from a-haemolysin in that it is produced on chemically defined media and is thermostable at 56°C. It is produced later in the growth phase than is a-haemolysin and its appearance is accompanied by a fall in the pH of the culture. A similar type of haemolysin, described by Rennie and Arbuthnott (1971), is produced on chemically defined medium in the presence of various carbohdyrates. It is possible that the haemolytic effects described in both of these reports may be due to the lytic effect of organic acids formed from the carbohydrates in the culture medium, as reported in related studies by Muranyi and Juhasz (1971).Walton and Smith (1969) have described a third type of filterable haemolysin, which they termed y. This haemolysin is found in mutants resistant to nalidixic acid and either its production or its release appears to be affected by nalidixic acid.The fourth haemolysin produced by E. coli is the cell-associated or /3-haemolysin. 8-Haemolytic activity has not been dissociated from intact cells and appears to be dependent on cellular metabolism (Smith, 1963; Short and Kurtz, 1971).It is not possible to determine which of these four types of haemolysin are produced by various strains of E. coli on the basis of the appearance of colonies on blood agar plates, since both a-and p-haemolysin-producing strains give rise to a clear zone of haemolysis (Smith, 1963). Smith found that 53-76% of the faecal samples from healthy pigs, sheep, and oxen contained haemolytic E. coli. He found that 63-92% of the haemolytic strains

It has been suggested that at physiological pH, the trypsin-catalyzed activation of the lipase cofactor, procolipase, to colipase has no consequence for intestinal lipolysis and serves primarily to release the N-terminal pentapeptide, enterostatin, a satiety factor (Larsson, A., and C. Erlanson-Albertsson 1991. The effect of pancreatic procolipase and colipase on pancreatic lipase activation. Biochim. Biophys. Acta 1083:283-288). This hypothesis was tested by measuring the adsorption of [14C]colipase to monolayers of 1-stearoyl-2-oleoyl-sn-3-glycerophosphocholine and 13, 16-cis, cis-docosadienoic acid in the presence and absence of procolipase. With saturating [14C]colipase in the subphase, the surface excess of [14C]colipase is 29% higher than that of procolipase, indicating that colipase packs more tightly in the interface. With [14C]colipase-procolipase mixtures, the proteins compete equally for occupancy of the argon-buffer interface. However, if a monolayer of either or both lipids is present, [14C]colipase dominates the adsorption process, even if bile salt is present in the subphase. If [14C]colipase and procolipase are premixed for > 12 h at pH approximately 8, this dominance is partial. If they are not premixed, procolipase is essentially excluded from the interface, even if procolipase is added before [14C]colipase. These results suggest that the tryptic cleavage of the N-terminal pentapeptide of procolipase may be of physiological consequence in the intestine.

Scanning electron micrographs of sheep erythrocytes showed that attachment of the alpha hemolysin produced by Escherichia coli resulted in the formation of spherocytes, with 10 to 20 small projections spaced relatively evenly over the surface of the erythrocyte membrane. This shape change was induced within 5 min after treatment. If the hemolysin concentration was reduced to a level which would lyse only a fraction of the total erythrocytes, the affected cells were easily identified against a background of normal, unaffected cells. Unlike sodium lauryl sulfate and other amphipathic agents which enter cell membranes and increase their flexibility, low concentrations of hemolysin did not provide protection against hypotonic hemolysis. These findings indicate that the surface projections were not the result of membrane expansion caused by incorporation of hemolysin into the outer portion of the lipid bilayer. The ability of a given amount of hemolysin to release a constant amount of hemoglobin in the presence of increasing concentrations of red cells confirmed that a single hit is sufficient for lysis. These results suggest that a single hemolysin molecule can bind to a sheep erythrocyte and trigger internal reactions which result in the derangement of membrane integrity at multiple sites on the surface. Confirmation of one-hit kinetics indicates that measurement of E. coli hemolysin activity should be carried out at low ratios of hemolysin to erythrocyte to decrease the possibility of multiple hits on a single cell.