| Abstrakt: |
Previous studies indicated that the function of mutarotase (aldose-1-epimerase) in glucose metabolism in primitive organisms should have become redundant with the evolution of the phosphorylitic pathways of glucose utilization and suggested that in higher organisms the enzyme may have evolved into a component of a glucose transport system. In order to obtain some information on its evolutionary background, the enzyme was purified to homogeneity from a variety of species, including some mammalian embryonic forms, and the physical and catalytic properties were compared. Mutarotase content is highest in mammalian kidneys, comprising from 0.15 to 0.29% of the total soluble protein of this tissue. The catalytic coefficients (moles of α-glucose converted to β-glucose per mole of enzyme per min) of the purified enzymes ranged from 2.8 x105for rabbit to 1.0 x106for the bovine kidney enzyme, the most active mutarotase so far isolated. On the basis of Kmvalues for glucose, mutarotases fall into two distinct groups: those with a Kmof 19 mm(bovine adult, sheep, sheep embryo, hog, hog embryo, mouse, gerbil, and dog) and those with a Kmof 12 mm(human, rabbit, toadfish, yellow perch, chicken, catfish, and newborn calf). In the ox, three different kinetic forms of the enzyme which have otherwise indistinguishable physical properties are present at different stages of development (embryo calf Km5 mm, calf (0 to 6 months) 12 mm, and adult 19 mm). All forms of the enzyme have the same substrate specificity and are inhibited equally by inhibitors of glucose transport such as phloridzin, phloretin, and diethylstilbestrol. |
