LDL receptor related protein (LRP) is a ubiquitously expressed cell surface receptor that binds, at least in vitro, a plethora of ligands among them c~2-macroglobulin and lactoferrin (Lf). The function of LRP in internalisation and distribution of ligands within cellular metabolism is still unclear. We here investigated by combined ligand-and immunoblotting the participation of LRP/a2MR and its associated protein (RAP) in receptor mediated endocytosis of Lf into rat liver. We found LRP highly enriched in sucrose density gradient fractions around density I.I0 g/ml, previously characterised as endosomal fractions. RAP was concentrated in distinct fractions around density 1.14 g/ml. This separation of RAP from LRPIa2MR is physiologically meaningful as RAP avidly binds to LRPIa2MR and by that shuts off aH ligand binding function. In endosomal fractions we found one single binding protein for ~2Sl-labelled Lf. With a specific anti LRPla2MR antibody and ligand blotting with 125I-labelled RAP this endosomal Lf binding site was verified to be LRP/azMR. Endosomes did not bind labelled Lf when prepared from rats that received an intravenous injection of Lf (20 mg per animal) 20 rain prior to preparation. Surprisingly we immunodetected Lf in these endosomes at a position around 600 kDa, comigrating with LRP/ c~2MR. We determined Lf binding to be optimal at pH 5.8, what led us to suggest the existence of a very stable LF-LRP/c~2MR complex in endosomes. These data support the idea of effective binding of Lf at pH as found in inflamed tissue environment where Lf is reported to be involved in leukocyte mediated inflammation regulation.

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Fermentative production by the fungus Aspergillus niger is the world'sexclusive source ofcitric acid to be used for the food, pharmaceutical and detergents' industry. The extraordinary high yields and the delicate nutrient requirements of this process have attracted numerous investigations on its mechanism. This review highlights recent results on the regulation of its biosynthesis, emphazising especially a) the role of pH, b) the role of type and concentration of the carbon source, and c) the role of anaplerotic carbon dioxide fixation in the accumulation of citric acid.