HEXOKINASES 



379 



competition might be seen if rates of inhibition in the presence of various 

 concentrations of glucose were determined, but once L-sorbose-1-P has 

 combined with the enzyme the inhibition is essentially irreversible. It is 

 possible that glucose forms an intermediary tightly bound complex with 

 the enzyme- ATP during the reaction and that L-sorbose-1-P forms a similar 

 complex that is stable. Actually the affinities of the brain enzyme for 

 D-glucose-6-P (Kj = 0.4 mM) and L-sorbose-1-P {K^ = 0.7 mM) are close 

 (Crane and Sols, 1954). Other hexokinases may not be so susceptible to 

 L-sorbose-1-P, since Taylor (1960) found only slight inhibition by 0.5 mM of 

 glucose uptake by Scenedesmus, the primary transfer site being hexokinase 

 on the outside of the membrane. 



A closely related inhibitor is l,5-anhydro-D-glucitol-6-P (1,5-D-sorbitan- 

 6-P), this lacking the 2-OH group in L-sorbose-1-P and binding somewhat 

 less tightly {K^ = 1 mM) to the brain hexokinase (Crane and Sols, 1954). 

 The nonphosphorylated compound is a very weak inhibitor (Sols, 1956). 

 Since there are very few potent and specific hexokinase inhibitors, Ferrari 

 et al. (1959) have recently investigated l,5-anhydro-D-glucitol-6-P in some 

 detail to see if it might be useful as a blocking agent of this enzyme in ho- 

 mogenates. It is stable to the enzymes attacking D-glucose-6-P, except for 

 hydrolysis by liver glucose-6-phosphatase. No inhibition of glucose-6-P de- 

 hydrogenase is evident, but it inhibits phosphoglucomutase variably (de- 

 pending on the concentrations of glucose-1-P, glucose-l,6-diP, and Mg++) 

 and phosphoglucose isomerase noncompetitively at higher concentrations. 

 At 6.25 mM it blocks glucose respiration in heart homogenates but has 

 no influence on the oxidation of glucose-6-P, indicating under these con- 

 ditions a rather specific inhibition of hexokinase. 



Brain hexokinase is inhibited by glucose-6-P whereas yeast hexokinase 

 is not (L-sorbose-1-P also does not inhibit the yeast enzyme), and the inhi- 

 bition has been found to be noncompetitive with respect to both glucose 

 and ATP (Weil-Malherbe and Bone, 1951). Inhibitions by various hexose 

 phosphates have been studied thorougly by Crane and Sols (1953, 1954); 

 the accompanying tabulation summarizes their data. The following are 

 noninhibitory: /?-D-glucose-l,6-diP, D-mannose-6-P, D-fructose-6-P, D-fruc- 

 tose-l,6diP, D-arabinose-5-P, D-ribose-5-P, D-galactose-6-P, a-glucose-l-P, 



