HEXOKINASES 383 



products formed from glucosamine, possibly glucosamine-P. Glucosamine 

 depresses the respiration and oxidation of pjTuvate in ascites cells, as 

 does glucose (Crabtree effect), and this is relieved by 2,4-dinitrophenol 

 (Ram et al., 1963). Since this is presumably related to the phosphorylation 

 of glucosamine and the effects on ADP-ATP levels, it constitutes another 

 mechanism whereby glucosamine can alter carbohydrate metabolism. 



Inhibition by Adenine Nucleotides and Polyphosphates 



The reaction rate of hexokinases falls with time due to the accumulation 

 not only of glucose-6-P but also of ADP (Sols and Crane, 1954). Phosphate, 

 pyrophosphate, and AMP do not inhibit. The nature of the ADP inhibition 

 appears to vary with the source of the hexokinase. With yeast hexokinase 

 the inhibition is noncompetitive with respect to ATP since around 50% 

 inhibition is produced by 0.5 mM ADP at all levels of ATP used (Gamble 

 and Najjar, 1955), and with Schistosoma glucokinase the inhibition actually 

 increases slightly with ATP concentration (Bueding and MacKinnon, 1955). 

 The inhibition of liver fructokinase by ADP is reduced slightly by increasing 

 the ATP concentration from 5 to 10 mM, but not enough to indicate pure 

 competitive inhibition; Parks et al. (1957) stated it is noncompetitive but it 

 might better be designated as mixed. Echinococcus fructokinase, on the 

 other hand, is inhibited competitively (Agosin and Aravena, 1959). 



Tripolyphosphate (PgOjo^") inhibits the fermentation of glucose by in- 

 tact yeast cells, glycolysis in cell-free extracts, and pure hexokinase (Vish- 

 niac, 1950). The inhibition of hexokinase is quite potent when (ATP) = 3.75 

 mM: 13% at 0.47 mM, 31% at 1.4 mM, 74% at 4.7 mM, and 93% at 14 

 mM tripolyphosphate. The inhibition is reversed by both ATP and Mg++. 

 Wheat germ hexokinase appears to be more resistant to tripolyphosphate, 

 only 8% inhibition being given by 5 mM (Saltman, 1953). 



Inhibition by Giucosone 



D-Glucosone may be formed from D-glucose by mild oxidation (e. g. 

 with Cu++) at C-2 but, although it has been known for over 75 years, its 

 structure is not completely understood (Becker and May, 1949). The 

 following forms are possible and it is difficult to choose between them: 



CH,OH 



O 



a) 



