Studies on Mutarotases: II. INVESTIGATIONS OF POSSIBLE RATE-LIMITING ANOMERIZATIONS IN GLUCOSE METABOLISM

Abstract

Abstract The possible involvement of mutarotases in glucose metabolism has been studied by examining the anomeric specificities of a number of glycolytic enzymes. Hexokinases from yeast or Ehrlich ascites tumor cells showed only a marginal preference for the α anomer of glucose. (All configurations, unless otherwise specified, refer to the d configuration of glucose and its derivatives.) The Km values for α- and β-glucose were 58 and 66 µm, respectively, for the yeast enzyme, 77 µm, and 82 µm for the tumor enzyme. Turnover numbers were also 10 to 20% higher with α-glucose. The anomeric composition and mutarotation rate of glucose 6-phosphate were measured under different conditions by utilizing the specificity of glucose 6-phosphate dehydrogenase for β-glucose 6-phosphate. The anomeric composition (39% α, 61% β) is similar to glucose, but the mutarotation rate (k = 0.157 min-1 at -0.5°) is approximately 270 times that of glucose. The anomeric composition of the glucose liberated from glucose 6-phosphate by acid or alkaline phosphatases and by glucose 6-phosphatase from rat liver was the same as that of the substrate, indicating a lack of anomeric specificity for these enzymes also. This lack of anomeric preference in the enzymatic phosphorylation or dephosphorylation of glucose, the extremely rapid spontaneous mutarotation of glucose 6-phosphate which is unaffected by mutarotase, and the low measured levels of mutarotase in certain tissues which nevertheless have high glycolytic rates suggest that this enzyme is not involved directly in glucose metabolism. It also appears that the existence of rate-limiting anomerizations in glucose metabolism is unlikely.