Partial characterisation of an inhibitor of streptolysin O produced by bacterial growth in serum.

An inhibitor of streptolysin O is generated in human and animal sera by the growth of certain organisms. The ability to do this occurs most often in Pseudomonas aeruginosa and Staphylococcus aureus (in 90% and 86% of strains respectively), but in only 32% of Staph. epidermidis strains. The inhibitor is not formed in broth. The effect appears slowly on incubation, with maximum activity after 4-7 days. Evidence suggests that two enzymes are involved, an esterase which splits ester-bound cholesterol and a proteolytic enzyme which partially hydrolyses lipoprotein, resulting in cholesterol remaining attached to protein or polypeptide fractions but with some alteration of its spatial configuration such that it is now capable of attaching to streptolysin O. The inhibitory factor appears to prevent streptolysin becoming attached to cholesterol receptor sites on the erythrocyte membrane. Removal of the precursor from serum with magnesium carbonate suggests that low-density lipoproteins may be the precursor of the inhibitor.