196 Essays in Biochemistry 



"oxidative pathway" of glucose dissimilation, in that in both the carbon 

 chain is split with the production of C0 2 and a 3-carbon compound. 

 However, the inositol pathway has the distinguishing characteristic 

 that the initial enzymatic attack is a dehydrogenation and not a phos- 

 phorylation. The degradation proceeds at least as far as the diketone 

 XII without the introduction of a phosphate group. It is not known 

 by what mechanism the energy generated in the dehydrogenation steps 

 (reactions 1 and 3) can be utilized by the cell. The observation that 

 the microorganism can grow on myoinositol in the absence of oxygen 

 suggests that a process other than oxidative phosphorylation can serve 

 to harness the reactions of inositol degradation to the production of 

 useful energy. 



The ability to initiate the degradation of a polyhydroxy compound 

 with the dehydrogenation of a secondary hydroxy 1 group seems to be 

 characteristic of certain microorganisms. A. suboxydans converts 

 mi/o-inositol to 2-keto-myo -inositol and glycerol to dihydroxyacetone, 

 but it does not possess the enzymes for the rapid degradation of these 

 keto compounds. Capsulated strains of A. aerogenes can carry out 

 not only the complete degradation of myoinositol but also that of 

 glycerol. 18 The direct dehydrogenation of glycerol by an enzyme whose 

 synthesis is specifically induced by glycerol yields dihydroxyacetone 

 which is rapidly dissimilated via pyruvic acid. On the other hand, 

 acapsulated strains of A. aerogenes cannot attack w?/o-inositol at all 

 and initiate the degradation of glycerol with its phosphorylation to 

 L-a-glycerophosphate. 



The pathways of dissimilation in which dehydrogenation is the 

 initial step of the enzymatic attack are as efficient as the classical 

 Embden-Meyerhoff pathway in providing capsulated A. aerogenes with 

 energy and building blocks for growth, since the growth rate and the 

 total cell crop are nearly the same whether glucose, glycerol, or myo- 

 inositol is the sole carbon source. However, the formation of adaptive 

 enzymes, such as the biosynthesis of histidase, which is induced by 

 histidine, proceeds during growth on m7/o-inositol or glycerol but not 

 during growth on glucose. 19 Thus the ultimate source of energy may 

 have a profound influence on the enzymatic constitution of the bac- 

 terial cell. 



References 



1. H. G. Fletcher, Jr., Advances in Carbohydrate Chemistry, 73, 2917 (1951). 



2. R. S. Harris et al., in W. H. Sebrell, Jr., and R. S. Harris, The Vitamins, II, 

 Academic Press, New York, 1954, p. 322 ff. 



