Physiological preculiarities in the vitamin A metabolism of carnivores

摘要

Dogs and other carnivores differ from other species with respect to vitamin A metabolism. Carnivores transport vitamin A in blood plasma not only as retinol but predominantly as retinyl esters (vitamin A esters). In the plasma, retinol is bound to aspecific binding protein, the retinol-binding protein (RBP). Plasma retinyl esters, however, are nonspecifically bound to all three fractions of plasma lipoproteins. In dogs the concentration of vitamin A in plasma is dependent on dietary vitamin A intake. Although a high percentage of nonspecifically bound retinyl esters is accompanied with signs of a vitamin A intoxication in man, no clinical or clinical-chemical signs of vitamin A intoxication can be observed in carnivores investigated. In the liverand kidneys, species specific differences concerning the content of vitamin A can be observed. High vitamin A concentrations are detectable in the liver of domestic cats and dogs, whereas the kidneys of raccoon dogs and foxes contain high vitamin A concentrations which are many times higher compared to those found in the liver of these species. Based on these observations, the liver vitamin A concentration cannot be used to interpret the vitamin A status of carnivores. Canines, mustelids and to some extent domestic cats have the ability to excrete fat-soluble retinol and retinyl esters in the urine. In dogs the amount of urinary vitamin A excretion is dependent on dietary vitamin A intake as well. The hydrophobic vitamin A compounds are associated with the Tamm-Horsfall protein, which has a molecular mass of 100 kDa and should be characterized as a constituent of physiological proteinuria in carnivores.