The capacity of certain strains of Streptococcus pyogenes to produce opacity in aged horse serum has been studied. Cells from all stages of the growth cycle are able to produce opacity. Maximal activity is reached towards the end of the exponential phase of growth.
Examination of cell fractions obtained by mechanical breakage and differential centrifugation suggested that the cell-bound activity is predominantly associated with the membrane fraction. Extraction with sodium deoxycholate yields a soluble fraction of high activity.
There is considerable strain variation in heat stability of the serum opacity factor. Cell-bound activity is often quite resistant to heat, whereas extracted activity is less stable.
Low concentrations of divalent cations have an activating effect, whereas high concentrations inhibit the serum opacity reaction. High concentrations of uni-valent cations are without effect on the cell-free enzyme but have an activating effect on the cell-bound enzyme.
For both the cell-bound and the cell-free enzyme the pH optimum was 5·8.
Although sensitive to trypsin and pepsin, the serum opacity factor appears to be resistant to streptococcal proteinase. Its activity is destroyed by formaldehyde and by periodate but is unaffected by a number of reducing agents.
Pre-heating of the serum or the addition of iodoacetate did not affect the serum opacity reaction. The enhanced cholesterol esterification previously described with fresh serum appears to be a secondary reaction. Even when isolated by relatively gentle methods, α-lipoprotein serves as a substrate only in the presence of crystalline serum albumin.