[The role of the kidneys in vitamin metabolism]

It is well established that the kidney plays an essential role in regulating the homeostasis of body fluids. Recent studies provided evidence for the kidney to be an important organ for the regulation in the metabolism of both fat (vitamin A, D) and water soluble vitamins (e.g. vitamin B12). This regulation is mediated by glomerular filtration as well as reabsorption and secretion processes of protein-bound vitamins. Vitamin transport proteins such as retinol-binding protein, vitamin D-binding protein and transcobalamin II are filtered in renal glomeruli and subsequently reabsorbed in the proximal tubules by endocytosis from the tubular fluid. Megalin, a scavenger receptor belonging to the LDL receptor family, is probably the most important receptor in this process in the proximal tubule cells. The carrier proteins are degraded in lysosomes whereas the vitamins are probably stored, may become coupled to newly synthesized carriers and secreted at the basolateral plasma membrane. A renal vitamin excretion is observed in dogs and other species of the family Canidae, in form of fat soluble retinol and retinyl esters bound to Tamm-Horsfall protein. This and the high vitamin A concentrations in renal tissue of canines suggest an important role of the kidneys in vitamin A metabolism of these species.