A chemical explanation has been provided for the production of opalescence in human and animal sera by extracts of certain Group A streptococci. Opalescence results from the selective liberation of specific lipids from α1-serum lipoprotein. The released lipids have been quantitatively separated through the relatively simple technique of ultracentrifugal flotation, and subsequently analyzed by chemical and chromatographic means. Esterified cholesterol constitutes by far the major low density reaction product, accounting for 85 to 90 per cent of the total lipid released. A small amount of phospholipid (apparently limited to lecithin) was the only other lipid consistently found in low density product fractions.
Comparative kinetics of opalescence development and cholesterol release under varying conditions of pH, extract concentration, and temperature reveal that the two effects do not run exactly parallel but are, nevertheless, probably closely related manifestations of a specific enzymic degradation of serum lipoprotein. It has been suggested that enzymic action may occur at a site within the lipoprotein remote from the cholesterol esters themselves. Although the over-all reaction appears to be mediated by an enzyme present in many Group A streptococci, it is uncertain whether this enzyme acts directly on lipoprotein or acts by causing the activation of a serum enzyme which in turn attacks the lipoprotein.