274 GERTRUDE E. CLOCK 



the chicken from CHsCi^OONa, C^^HaNHaCOOH, and HCi^COONa. The 

 isotope distribution patterns in glycogen and ribose isolated from pooled 

 internal organs were compared, but, since no five consecutive hexose 

 carbons showed the same C^^ pattern as the ribose isolated from purine 

 nucleotides, it was concluded that neither the direct oxidative pathw-ay 

 nor decarboxylation of uronic acids could account for synthesis of ribose 

 under the given experimental conditions. Although synthesis of ribose by 

 condensation of C3 and C2 units is consistent with these data, it is suggested 

 that some as yet unknown mechanism may be involved. Lanning and 

 Cohen^^'^ consider that the major pathway of ribose synthesis in E. coli 

 involves the "oxidative" pathway on account of the low isotope content of 

 the ribose moeity of PNA synthesised when glucose- l-C'* was the sole 

 source of carbon. 



The formation of ketopentoses from pentitols by DPN-linked polyol 

 dehydrogenases of rat liver and Acetohacter suboxydans has been investi- 

 gated by McCorkindale and Edson.^^^ The liver enzyme oxidises xylitol 

 (to D-xylulose) much more readily than ribitol (to D-ribulose) but does not 

 attack D- or L-arabitol, whereas the bacterial enzyme only oxidises D-arabi- 

 tol (to D-xylulose) and ribitol (to L-ribulose). 



Cell-free extracts of Pseudomonas hydrophila grown on xylose have been 

 shown to contain a specific xylose isomerase catalyzing the interconversion 

 of D-xylose and D-xylulose. ^"^^ As in the case of the adaptive xylose isomerase 

 of Lactobacillus pentosus,^''' approximately 16% of the keto-sugar is 

 present at equilibrium. On account of this active isomerase, Hochster and 

 Watson^^- now consider that the previously reported phosphorylation of 

 D-xylose"^ (see Fig. 7) may actually have been phosphorylation of D-xylu- 

 lose. Lampen and co-workers,'*^"* have continued their work on the mech- 

 anism of formation of ribose-5-P from D-xylose by extracts of Lactobacillis 

 pentosus, and the new data confirm their earlier suggestions^ •'"* that d- 

 xylose is converted to a ribose (or ribulose) phosphate before degradation 

 of the pentose chain occurs. Since intact cells ferment D-xylulose as rapidly 

 as D-xylose and cell-free extracts phosphorylate D-xylulose very rapidly 

 in the presence of ATP, it has been suggested that the formation of d- 

 xylulose may be the initial step in the fermentation of D-xylose by this 

 organism.'*^ 



A new enzyme, "phosphoribokinase," catalyzing the phosphorylation of 

 ribose-5-P by ATP to ribose- 1 ,5-diphosphate has been partially purified 



»» M. C. Lanning and S. S. Cohen, J. Biol. Chevi. 207, 193 (1954). 



"• J. McCorkindale and N. L. Edson, Biochern. J. 57, 518 (1954). 



'« R. M. Hochster and R. W. Watson, J. Am. Chem. Soc. 75, 3284 (1953). 



i« S. Mitsuhashi and J. O. Lampen, J. Biol. Chem. 204, 1011 (1953). 



1" J. O. Lampen, /. Biol. Chem. 204, 999 (1953). 



