Comparative studies on glucose phosphorylating isoenzymes of vertebrates. Identification and characterization of amphibian liver hexokinases

Abstract

Glucose phosphorylating activities were measured in liver extracts from two urodeles and twenty-six anurans. Fractionation on diethylaminoethyl-cellulose columns of liver extracts from these amphibians permitted the recognition of four hexokinases which are called A, B, C, and D. However, any given amphibian displays only three liver hexokinases and the profiles so far observed are either of the type A-B-D or C-B-D. The distribution of the amphibians in either type of pattern does not show any simple taxonomic relationship. A wide generic and specific, but not individual, variation of the relative proportion of each isoenzyme was observed. Hexokinases A and B were shown to be low K,,,.,Ue,,w isoenzymes (0.06 and 0.15 rnr+r glucose, respectively) with normal hyperbolic kinetics. Hexokinase C, also a low K, isoenzyme (0.05 mM) was found to be inhibited by excess substrate at physiological levels of glucose. Hexokinases A, B, and C were able to phosphorylate fructose, mannose, and P-deoxyglucose at equal or higher rates than glucose when assayed at saturating sugar levels. Hexokinase D was found to be a high K, isoenzyme (K,., = 2 mM) with sigmoidal saturation curves for glucose (Hill coefficient =1.6). Fructose and mannose were also phosphorylated by this isoenzyme at about 70% of the glucose rate when studied at saturating sugar concentrations. The properties of the amphibian hexokinases are thus similar, although not identical, to those of mammalian hexokinases.