382 2. ANALOGS OF ENZYME REACTION COMPONENTS 



kinases of Schistosoma (Bueding and MacKinnon, 1955) and rat intestinal 

 mucosa (Sols, 1956) and the glucokinase of Spirochaeta recurrentis (P. J. C. 

 Smith, 1960 b) are moderately sensitive to glucosamine (50-65% inhibition 

 by 6-10 mM) and more sensitive to A^-acetylglucosamine (75% inhibition 

 by 1-2 mM). The iC, for iV-acetylglucosamine and the glucokinase of ascites 

 tumor cells is 0.074 mM (McComb and Yushok, 1959), indicating a binding 

 about 1 kcal/mole tighter than for glucose-6-P. Furthermore, both glucos- 

 amine and iV-acetylglucosamine inhibit the metabolism of glucose-C^* and 

 fructose-C^* to glycogen and COg in rat liver slices (Spiro, 1958), and the 

 A'^-(2?-nitrobenzoyl) and A^-(3,5-dinitrobenzoyl) derivatives inhabit glucose 

 uptake by Scenedesmus (Taylor, 1960). The phosphorylation of glucosamine 

 in liver extracts is competitively inhibited by glucose {K^ =0.11 mM), 

 fructose, A'^-acetylglucosamine, and hexose and glucosamine phosphates, 

 illustrating mutual interference by these substrates and products (McGar- 

 rahan and Maley, 1962). 



In all these instances the inhibitions are strictly competitive with glucose 

 or fructose. The question arises as to why the iV-acylated derivatives are 

 not phosphorylated. Maley and Lardy (1955) showed by molecular models 

 that the iV-acyl groups do not overlap the 6-position so that some other 

 explanation must be sought. It was suggested that the A^-acyl groups might 

 interfere with the binding of ATP to the enzyme, but it is also possible that 

 they shift the position of the pyranose or furanose rings sufficiently so 

 that the 6-position is not favorably oriented for phosphorylation. It may be 

 mentioned that no carcinostatic activity was noted with any of these sub- 

 stances when tested in sarcoma-bearing mice, perhaps due to the hydrolysis 

 of the iV-acyl compounds by tissue cathepsins. 



Kono and Quastel (1962) confirmed the glucosamine inhibition of glyco- 

 gen formation in rat liver slices (50% inhibition by around 0.8 mM) and 

 showed there to be no depression of the entry of glucose into the cells. Hexo- 

 kinase, phosphoglucomutase, and UDP-glucose pyrophosphorylase are in- 

 hibited quite weakly by glucosamine, significant effects being exerted only 

 at concentrations over 20 mM. UDP-glucose-glycogen glucosy transferase 

 in inhibited by glucosamine but not by iV-acetylglucosamine, which inhi- 

 bits glycogen synthesis as does glucosamine. The isolated phosphorylase 

 is also resistant to glucosamine. Thus the enzymes involved in glycogen 

 formation are not directly inhibited by glucosamine and iV-acetylglucosa- 

 mine. However, each of these substances stimulates phosphorylase activity 

 in slices when added with glucose. Thus the explanation for the reduced 

 glycogen formation may be an acceleration of glycogen breakdown and not 

 a true inhibition. Silverman (1963) found a significant reduction of glucose 

 oxidation by glucosamine in the rat epididymal fat pad in the presence of 

 insulin and felt that an inhibition of glucokinase could not entirely account 

 for the results, so that one must assume some action from nonmetabolized 



