Transport of beta-lactoglobulin and alpha-lactalbumin in enterocyte-like Caco-2 cells.

The transport of [14C]-radiolabelled beta-lactoglobulin and alpha-lactalbumin through Caco-2 cell monolayers grown on permeable filters was studied in order to evaluate the different protein pathways through the intestinal epithelium. beta-Lactoglobulin or alpha-lactalbumin (0.25-3 mg/ml) was introduced on the apical side of the monolayer and both the transport and the release of labelled material from the cells were measured following different incubation times. The labelled material was analysed by either trichloroacetic acid precipitation or by high pressure liquid chromatography. Despite some differences between the 2 proteins, the overall mechanism followed approximately the same pattern. Part of the intact internalized protein was either recycled (10-15%) or transported via transcytosis (about 5%). Another pathway corresponded to the intracellular degradation of the protein. The calculation of the different routes followed by the proteins indicated that the main part of the degraded fraction (about 70%) was recycled whereas approximately 30% was transported to the other side. Moreover, 5-10% of the endocytosed material was retained intracellularly.