The Kinetics and Regulation of Rat Brain Hexokinase

Abstract

Abstract Initial rate kinetic studies of soluble rat brain hexokinase indicate that the kinetic mechanism is random Bi Bi. These studies also indicate that the ADP interaction with enzyme cannot be ascribed merely to action at a product site and suggest the presence of a separate inhibitory nucleotide-binding site. Rates of glucose phosphorylation were evaluated in the presence of a number of metabolites at or near their intracellular levels to determine their regulatory potential. Inhibition from ADP, GTP, and UTP was insensitive to fluctuations in the Pi level; whereas, glucose-6-P inhibition was partially relieved by Pi. Studies of the enzyme cumulatively inhibited by the above nucleotides and glucose-6-P indicate that the reaction rate was markedly depressed to a level corresponding to only 3 to 5% of the uninhibited rate. Under these conditions the rate responds to changes in glucose concentration and only large changes in Pi level. The soluble-particulate distribution of the brain enzyme was found to be relatively insensitive to changes in metabolite levels.