Multiple Hexokinases of Rat Tissues

Abstract

Abstract Rat tissues contain four hexokinases including both the liver-specific high Km glucokinase and three low Km hexo-kinases present in differing proportions in the various tissues. Each of the low Km hexokinase types designated I, II, and III has been purified approximately 150- to 400-fold from one or more rat tissues (Type I from brain and kidney, Type II from skeletal muscle and epididymal fat pad, and Type III from liver). Each purified enzyme type, regardless of tissue source, has certain unique properties that distinguish it from the other hexokinase types. The purified types retain their different electrophoretic and chromatographic properties as found in crude extracts. They also differ with respect to apparent Km values for glucose and probably for adenosine triphosphate, apparent Ki values for adenosine diphosphate and glucose 6-phosphate, and stability to heat and proteolytic inactivation. The different enzyme types are similar with respect to pH optimum, molecular weight, hexose and nucleotide specificities, Km for fructose, and the qualitative nature of inhibition by ADP and glucose-6-P. The available evidence indicates that these hexokinase types represent different molecular forms and are not artifacts of preparation.