An electrophoretic study of crystalline preparations of a streptococcal proteinase and its precursor established their isoelectric points at pH values of 8.42 and 7.35 respectively (ionic strength 0.10).

Preparations of the proteinase appeared to be electrophoretically homogeneous over a pH range of 5 to 8.5. Precursor preparations contained a relatively low concentration of the active enzyme visible as a separate peak in electrophoretic patterns of sufficiently concentrated solutions.

Autocatalytic conversion of precursor to active enzyme was complete and resulted in a corresponding change in the electrophoretic pattern.

Treatment of precursor preparations with trypsin produced incomplete conversion to the active enzyme and resulted in the formation of a modified precursor protein. This differed from the parent substance in electrophoretic mobility and in susceptibility to trypsin, but resembled it in immunological specificity and, as previously shown, in susceptibility to conversion to active enzyme by autocatalysis.

Serological reactions of precursor and active enzyme components withdrawn from the cell after electrophoresis are described. It appears that the precursor protein may have two antigenic groups, one specific, the other shared by the active enzyme which behaves as a single antigen.

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