Kidney Mutarotase

Despite the broad spectrum of distribution, the catalytic properties of the enzyme in the different species are remarkably similar. The same four sugars (D-glucose, D-galactose, D-xylose, and L-arabinose) are substrates for the enzyme from all sources (Table VI). Some changes, however, have taken place, and the marginal activity for maltose found in the lower species (67) has been lost in the mammals, whereas the relative activity towards the pentoses has become enhanced. The Km values and turnover numbers for the substrates of purified beef kidney mutarotase are given in Table V. 

Table V. Kinetic Parameters of Crystalline Beef Kidney Mutarotase...

Figure 5. Competitive inhibitors of bovine kidney mutarotase...

Diedrich has also measured by indirect means the content of the phloridzin-sensitive transport site in dog kidney and obtained a figure of 0.68 fimole per 100 grams of tissue (113). Dog kidney mutarotase has not yet been purified, but the content in those species where the turnover number is known (58) is calculated to be about 0.5 /xmole per 100 grams of cortex, in fair agreement with Diedrich s estimate for the transport site. 

Keston, A. S., Purification of Kidney Mutarotase, Evidence for Proposed... 

Keston30 reported a similar anomer-specificity for rat-kidney mutarotase. Chase and coworkers312,313 purified the mutarotase from... 

Figure 5. Mutarotase assay by the GLC-TMSi method. The enzyme preparation was obtained from hog kidney, and the conditions are those described previously (26).

Mutarotation of 0.3% solutions of the freshly dissolved sugars in 12 ml of 5 mM EDTA, pH 7.4 was followed. Significant differences in mutarotation rates (AK) in the presence and absence of 100 units of bovine kidney enzyme were expressed as the ratio AK/Ksp. Differences of less than 5% in these rate constants were not considered significant. Of the 18 sugars listed, nine have been tested previously as substrates for other mammalian mutarotases with essentially the same pattern as described here. The pattern of specificity indicates that a 3-point attachment of enzyme and substrate is necessary for catalysis of mutarotation. b Data from 72). 

Embryological Development of Mutarotase and Sugar Transport in Kidney and Intestine... 

In the kidney the number of nephrons is more or less well established at birth, and the increase in cortex size is predominantly a result of increase in tubular volume. The data of Arataki (84) on the number of rat nephrons per kidney, taken along with the observed increase in size of the kidney, have been used to calculate the average increase in the volume of the individual tubule. This increase in tubular volume results primarily from growth of the proximal segment of the tubule, which is the site of sugar reabsorption. A close correlation between tubular volume and mutarotase content was observed over the entire postnatal growth period (102). 

The relative amounts of the enzyme in kidneys of fish from different habitats have been measured (105) (Table IX). Particularly interesting is the finding that the mutarotase levels in kidneys of freshwater fish are always higher than those in saltwater fish, averaging six times greater. This extends an earlier observation by Keston that the enzyme was present in low levels in the aglomerular toadfish kidney (68). 

Table IX. Mutarotase Content of Marine vs. Freshwater Fish Kidneys...

However, if these limitations are remembered, it can be instructive to compare the patterns of interaction which have been variously reported for substrates and inhibitors of mutarotase, with similar patterns for the kidney and intestinal sugar transport processes. In Table XIII the reported specificities for mammalian intestine are compared. In cases where comparative data are available, all sugars which are actively transported or which are passively transported but share the same carrier as glucose also interact with the active center of mutarotase. Particularly interesting is the observation that L-fucose, the most potent sugar inhibi-... 

Whatever the site of the enzyme may be, Keston et al. have recently produced fairly conclusive evidence that glucose, which is reabsorbed by the kidney, is exposed to mutarotase at some stage of the process (117). Glucose infused into the renal artery spills into urine when the renal threshold is exceeded in the same anomeric form as that administered, whereas reabsorbed glucose in the renal vein is mutarotated. Hill has also shown that the anomer infused in excess is excreted in excess (73). 

Other Properties of a Mutarotase (Aldose- 1-Epimerase) from Hog Kidney, Proc. VHth Int. Congr. Biochem. (1967) F, 141. 

Isolation and Characterisation of a Mutarotase from Bovine Kidney Cortex, J. Biol. Chem. (1969) 244, 781. 

Mutarotase in Kidneys of Saltwater versus Freshwater Fish, Proc. Soc. Exper. Biol. Med. (1969) 131,861. 

Keston305-307 showed that the kidney, liver, and lens tissues of many animals contain an enzyme that catalyzes the mutarotation of a- and /3-D-glucose and other sugars it was called mutarotase.3 8,309 This, or a similar, enzyme had earlier been found in preparations of D-glucose oxidase from Penicillium notatum by Bentley and Neu-berger310 and Keilin and Hartree.309 The enzyme was extensively purified by Bentley and Bhate.140 The enzyme is also widely distributed in plant tissues.311... 

Phlorhizin, pUoridziK a dihydrochalcone found in the root bark of pears, apples and other members of the Rosaceae. P specific y blocks resorption of glucose by kidney tubules, thus inducing ucosuria. It therefore finds use in experimental physiology. Its activity may be due to inhibition of mutarotase. 

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