[Aminopeptidase N, a marker of the apical pole in thyroid epithelium of swine and in cultured cells].

An aminopeptidase N has been detected by immunofluorescence in the apical plasma membrane of porcine thyroid cells, facing the follicular lumen. Freshly isolated cells obtained by tissue trypsinization have lost their polarity and exhibited a homogeneous enzyme distribution over the whole plasma membrane. In thyrotropin-stimulated cultured cells organized into follicles, the enzyme is localized on the apical cell pole as observed in tissue. In monolayer cells, the enzyme is localized on all the surface facing the medium. In both types of cultures, fluorescence is also observed in intracytoplasmic organelles, probably lysosomes. Aminopeptidase is a marker of the apical part of the thyroid plasma membrane. Its in vitro localization depends upon cell differentiation which is in relation to culture conditions.