The finding that secretory component is not associated with gamma-glutamyltranspeptidase activity

The gamma-glutamyltranspeptidase activity of human milk was concentrated by ammonium sulphate precipitation. On gel chromatography of the dissolved precipitate, the activity was eluted in the high-molecular-weight fraction containing secretory IgA, while no activity appeared in the eluate at the position of free secretory component. Various antisera were added to portions of the pool of active fractions. No change of gamma-glutamyltranspeptidase activity appearedse activity was obtained with antisera against either IgA, secretory IgA or secretory component, while a large reduction of activity was seen with anti-human colostrum. Finally, purified free secretory component, secretory IgA and in vitro complexes between secretory component and IgA dimers were shown to be inactive in the gamma-glutamltranspeptidase assay, both in the absence and presence of zinc ions. Thus secretory component, either when free or bound to IgA, does not exhibit gamma-glutamyltranspeptidase activity, and therefore cannot function as such an enzyme in the transport of IgA across mucous membranes, as has been suggested previously.