Assay and Properties of IgA Protease of Streptococcus Sanguis

Abstract

IgA proteases are proteolytic enzymes elaborated by Streptococcus sanguis, Neisseria gonorrhoeae and Neisseria meningitidis which cleave human immunoglobulin A proteins into intact Fab α and Fc α fragments (1). The enzymes attack prolythreonyl bonds in the “hinge” peptide of the heavy chain of human IgAl subclass proteins, but proteins of the IgA2 subclass, which lack the susceptible peptide bonds, are IgA protease resistant (2). While the biological significance of these enzymes has not been determined, they have the capacity to markedly reduce the titer of agglutinating antibodies of the IgA isotype, as has been determined by their effect on human IgA myeloma paraproteins which have defineable antibody activity (3). The specificity of the IgA proteases is of considerable interest but forms an obstacle in the assay of the enzyme because it requires IgAl protein to be used as substrate. In general, macromolecular substrates for enzyme assays create problems, e.g., difficulty in reaching molar concentrations sufficient for enzyme kinetic studies, insolubility of substrate, instability of macromolecules at extremes of pH, and confusing protein-protein interactions independent of those directly concerned with enzyme catalysis. The use of a macromolecular substrates can be aided if the substance has a biological property which is altered progressively as enzyme action proceeds. In the case of IgA, the biological properties are not yet characterized to the point where they can be used to monitor enzymatic cleavage. The loss of antibody activity in IgA paraproteins could serve such a function, but such proteins are not readily available and thus would be inappropriate for use as a general substrate.