272 GERTRUDE E. GLOCK 



"alimentary pentosuria" resulting from a pentose-rich diet, a rare inborn error of 

 metabolism known as "essential pentosuria" also occurs. In this apparently harmless 

 abnormality, pentose is excreted either as DL-arabinose'^" or more commonly as 

 L-xylulose.'^' Its metabolic significance is unknown although D-glucuronic acid has 

 been suggested as the source of the excreted L-xylulose.*' Pentosuria, often accom- 

 panied by high levels of free or combined pentose in the blood, has also been 

 reported in trauma and shock,'" in progressive muscular dystrophy, '^3 and in experi- 

 mental hyperthyroidism'^^ and is considered to arise from breakdown of tissue ribo- 

 nucleotides. 



Several workers have reported "pentolysis" of added ribose, xylose, and arabinose 

 by blood from humans and rats with malignant tumors, '^^ but this has recently been 

 disproved. '2^ 



VIII. Addendum 



The main reactions which participate in the metaboHsm of pentose phos- 

 phates by both animal and plant extracts have been recognised as trans- 

 aldolisations and transketolisations. Transketolase has been partially 

 purified from rat liver and spinach leaves^-'' and catalysis of the following 

 reaction shown to be reversible. 



Ribulose-5-P + ribose-5-P ;;=^ sedoheptulose-7-P + glyceraldehyde-3-P. 

 Pentose phosphate formation is favored at equilibrium. Transketolase is 

 not substrate specific and has already been shown to attack ribulose-5-P 

 sedoheptulose-7-P, L-erythrulose, hydroxypyruvate and fructose-6-P."' 

 127 . 128 jj^ ^\\ cases cleavage of ketol linkages occurs with the formation of 

 "active glycolaldehyde" which then condenses with an acceptor aldehyde. 

 Ribulose-5-P is formed when glyceraldehyde-3-P is the acceptor. 



The formation of hexosemonophosphate from pentose phosphate by rat 

 liver preparations has been studied with C^* labeled pentose phosphates.^29 

 The isotope data indicate that besides the transketolase — transaldolase 

 sequence of reactions involving sedoheptulose-7-P as an intermediate, 



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Howard, Brit. Med. J. ii, 777 (1920). 

 1" P. A. Levene and F. B. La Forge, J. Biol. Chem. 18, 319 (1914); I. Greenwald, 



ibid., 88, 1 (1930) ; 89, 501 (1930). 

 '22 H. N. Green, H. B. Stoner, and M. Bielschowsky J. Pathol. Bacteriol. 61, 101 (1949) ; 



W. H. McShan, V. R. Potter, A. Goldman, E. G. Shipley, and R. K. Meyer, Am. J. 



Physiol. 145,93 (1945). 

 '" A. S. Minot, H. Frank, and D. Dziewiatkowski, Arch. Biochem. 20, 394 (1949). 

 '" J. H. Roe and M. O. Coover, Proc. Soc. Exptl. Biol. Med. 75, 818 (1950). 

 i"S. N. Steen, Arch. Biochem. 26, 457 (1950). 

 1" S. N. Steen, J. Natl. Cancer hist. 12, 195 (1951) ; J. H. Roe, J. W. Cassidy, A. C. 



Tatum, and E. W. Rice, Cancer Research. 12, 238 (1952). 

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 1" E. Racker, G. de la Haba, and I. G. Leder, Arch. Biochem. and Biophys. 48, 238 



(1954). 

 129 B. L. Horecker, M. Gibbs, H. Klenow, and P. Z. Smyrniotis, /. Biol. Chem. 207, 



393 (1954). 



