The Mechanism of Renal Excretion of Fructose and Galactose in Rabbit, Cat, Dog and Man: (with Special Reference to the Phosphorylation Theory)

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Abstract
Summary.: Fructose and galactose were demonstrated to be threshold substances, by which is meant that they are both reabsorbed actively by the renal tubular cells.A kidney threshold for fructose and galactose was demonstrated for man only (U/P ratio below 1).The U/P ratio for fructose and galactose, which at plasma concentrations between 15 and 20 mgm. per cent, did not exceed 1, rose very steeply at plasma concentrations above these values.In rabbits, cats and dogs the active reabsorption of fructose and galactose was demonstrated by: the indepence of fructose reabsorption of variations in the excretion of water. the sensitivity of the mechanism of reabsorption to poisoning of the tubular cells with phlorizin. experiments in saturating the mechanism of reabsorption for glucose (hyperglycaemia), which reduced the reabsorption of simultaneously filtered fructose and galactose (cat, man). The reabsorption of fructose and galactose was found to increase in proportion to increasing amounts filtered. In the experiments on the cat creatinine was employed for filtration determinations after the identity between creatinine and inulin clearances in this animal had been established.The results obtained appear to agree with the theory of an intermediate phosphorylation and dephosphorylation during the reabsorption of glucose, fructose and galactose in the kidneys. In regarding the adenylic acid system as a common phosphate donator to these three hexoses, the specific kidney threshold for each of the three hexoses is explained by the assumption of specific phosphorylating and dephosphorylating enzymic systems for each hexose in the cells of the tubules.