256 GERTRUDE E. GLOCK 



Chlorella cells showed that there was practically no incorporation of C^^ 

 into ribulose diphosphate and much less into phosphoglyceric acid than in 

 the control cells. This was taken as an indication that the cleavage of hep- 

 tose and pentose phosphates might be dependent on sulfhydryl enzymes 

 (see also Sect. IV.2), A scheme for the cyclic conversion of phosphate 

 esters in photosynthesis is shown in Fig. 4. 



Although Calvin and his co-workers consider hexose phosphates to be 

 synthesized exclusively from triose phosphate by a reversal of the glyco- 

 lytic route, recent work of Horecker^^ indicates that the direct oxidative 

 pathway may serve as an additional source of hexose in photosynthesis. 

 He has shown that when energy is supplied in the form of reduced TPN, 

 pentose phosphate and CO2 can be reduced to glucose-6-phosphate. The 

 recent important observations of Vishniac and Ochoa^* indicate that photo- 

 synthetic mechanisms may be able to provide the reduced coenzyme for 

 these reactions. 



IV. Significance of the Hexosemonophosphate Oxidative Pathway 



1. Distribution 



The enzymes of this oxidative pathway of carbohydrate metabolism are 

 widely distributed throughout the animal and vegetable kingdom and have 

 been demonstrated in a variety of mammalian tissues and tumors,^^'^* in 

 lower animals,"*^ in higher plants and algae, ^"'^^ and in yeast and many 

 bacteria. ^2'*^ 



Recent quantitative results of Glock and McLean^- for glucose-6-phos- 

 phate and 6-phosphogluconate dehydrogenase activities of some normal 

 mammalian tissues are shown in Table I. The most interesting results are 

 the very high levels of activity of both dehydrogenases in adrenal cortex 

 and lactating mammary gland and the strikingly low levels in muscle. The 

 physiological significance of these findings is, however, at present obscure. 

 If one of the main functions of this pathway is to supply ribose-5-phosphate 

 for incorporating into ribonucleic acid, it would be expected to play an 

 important part in the metabolism of tumors and other rapidly dividing 

 cells. The levels of activity of both dehydrogenases in a variety of experi- 

 mental and spontaneous tumors were, however, found to fall within the 

 limits of activity of normal tissues. This method of approach is, however, 



«W. Vishniac and S. Ochoa, J. Biol. Chem. 195, 75 (1952). 



" S. S. Cohen, Biol. Bull. 99, 369 (1950); 101, 237 (1951). 



6»G. E. Glock and P. McLean, Biochem. J. 55, 440 (1953). 



^' P. K. Stumpf in "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 2, 



p. 46. Johns Hopkins Press, Baltimore, 1952. 

 " W. A. Wood and R. F. Schwerdt, J. Cellular Conip. Physiol. 41, Suppl. 1, 165 (1953) ; 



J. Biol. Chem. 231, 501 (1953). 

 " R. D. DeMoss, J. Cellular Comp. Physiol. 41, Suppl. 1, 207 (1953). 



