BIOSYNTHESIS OF PENTOSES 257 



TABLE I 



Levels of Glucose-6-phosphate and 6-Phosphogluconate Dehydrogeanse 



Activities in Adtjlt Mammalian Tissues 



Enzyme activity in units/g. tissue" 



G-6-P 6-PG dehydrogenase 



dehydrogenase 



Tissue (pH 7.6) (pH 9.0) (pH 7.6) 



Adrenal gland« 163 ± 25 350 ± 25 185 =b 9 



Adrenal cortex" 730 315 232 



Adrenal medulla"'' 67 114 67 



Spleen* 305 zb 31 96 db 7 64 ± 3 



Thymus^ 98 ± 1 87 ± 9 49 ± 5 



Liver (Female) 12 104 ± 12 290 db 16 130 ± 12 



Liver (Male)!^ 46 ± 3 147 ± 10 59 ± 8 



Lung^ 88 lb 6 106 dz 7 59 ± 3 



Kidney' 69 ± 7 62 ± 5 56 ± 3 



Brain^ 32 ± 3 22 ± 1 11 =b 1 



Cardiac muscle« 26 ± 13 34 ± 7 18 ± 3 



Skeletal muscle^ 8± 1 15 dz 0.5 8 dz 0.3 



Mammary gland 



20th day of pregnancy 86 92 50 



10th day of lactation 1 ,728 673 409 



21st day of lactation 5,452 1,734 883 



2nd day of involution 50 93 45 



" A unit of enzyme activity is defined as the quantity of enzyme which reduces 0.01 mM. TPN/min. at 20° 

 * Ox tissues (rat tissues otherwise used) . 



somewhat limited since only the maximum enzymic capacities are being 

 measured and these do not necessarily operate in the intact cell where 

 other regulatory factors probably limit or control the activity of this 

 pathway. 



Both glucose-6-phosphate and 6-phosphogluconate dehydrogenases are 

 located exclusively in the soluble fraction (cell sap) of tissue homogen- 

 ates.^"'^^ It is of interest that the glycolytic enzymes have the same intra- 

 cellular distribution.^^ 



2. As AN Alternative Pathway to Glycolysis 



Before the existence of the direct oxidative pathway was established 

 with certainty in animal tissues, considerable indirect evidence had ac- 

 cumulated indicating that although glycolysis is the principal mechanism 

 of carbohydrate breakdown under anaerobic conditions, other mechanisms 



"Gertrude E. Glock and P. McLean, Nature 170, 119 (1952). 



" G. A. LePage and W. C. Schneider, J. Biol. Chem. 176, 1021 (1948). 



