Characterisation of opacity factor from group-A streptococci.

Extracellular opacity factor (OF) from group-A Streptococcus M-type 22 was purified by ammonium sulphate precipitation followed by ion-exchange on DE-52 cellulose and gel filtration on sephacryl S-400. OF was eluted near the void volume and shown to be heterogenous by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Antiserum to ammonium sulphate-purified OF from a cell-free culture supernate was prepared in rabbits. All preparations of OF from supernate and cell-extract were inhibited by the antiserum. No M protein was detected in the OF samples from various purification steps. The purified OF showed activity at a broad pH range with optimal activity at pH 6; it was inactivated considerably at high temperatures. Enzyme activity was inhibited by pepstatin A, but was unaffected by serine proteinase inhibitor, aprotinin, ethylene diamine trichloroacetic acid, N-ethylmaleimide, iodoacetamide and mercaptoethanol. This suggests that OF is an aspartic proteinase.