Some factors influencing the effect of cholesterol on streptolysin O activity.

Inhibition of streptolysin O activity by cholesterol depends on the latter being in free form. The normal esterified and protein-bound cholesterol fractions in serum do not influence streptolysin O activity. However, high cholesterol levels in rabbits fed cholesterol with cholic acid were associated with an increased antistreptolysin O effect. It is suggested that this occurs when all available protein sites are saturated and where a true ;free' cholesterol fraction is present. Splitting the esterified cholesterol fraction of human sera with raised cholesterol levels, by means of pancreatin, produced an increased antistreptolysin O effect, again presumably because of saturation of protein-binding sites. Similarly, removal of non-esterified cholesterol from sera of cholesterolfed rabbits, by means of digitonin, reduced antistreptolysin O activity of the sera. Evidence is presented that combination of bovine serum albumin and streptolysin O has a steric hindrance effect on attachment of cholesterol to streptolysin O. The method described for the estimation of free cholesterol is extremely sensitive, being capable of detecting concentrations of less than 1.0 mug/ml.