46 



II. BIOSYNTHESIS 



rructose-l,6-diphosphate 



Zymohexase 

 (aldolase) 



i 



H2COPO(OH)2 



I Isomerase 



C=0 < y 



HsCOPOCOH), 

 HOCH 



H2COH HC=0 



Dihydroxyacetone 3-Phosphoglyceric 



Phosphate t aldehyde 



+ 2H Dehydrogenase -2H 



(DPN-2H) i (DPN + 2H) 



H2COPO(OH)2 H2COH 



I Phosphates | 



HCOH 



H2COH 

 a-Glycero- 

 phosphate 



ATP 



HCOH 



I 

 H2COH 



Glycerol 



The Breakdown of Fructose- 1,6-Diphosphate to Form Glycerol under Anaerobic 

 Conditions (DPN is diphosphopyridine nucleotide)*'^ 



mary gland itself. Thus, French and Popjak^^ reported that the specific 

 radioactivities of glycerol isolated from milk glycerides secreted six hours 

 after the administration of C^^-glucose and C^^-acetate, respectively, in 

 equiradioactive doses to lactating rabbits were much higher in the first 

 case. In fact, the specific activity of the glycerol was of the same order 

 as that of the lactose isolated from the same milk sample. An analysis of 

 these results indicated that between 50 and 100% of the glycerol excreted 

 in the triglyceride fraction of milk fat had been newly synthesized in the 

 udder.^* 



Although blood glucose is, in all probability, the main precursor of glyc- 

 erol in mammary tissue, acetate can likewise serve to a limited extent in a 

 similar capacity. Under these circumstances, acetate acts indirectly as a 

 source of lactose. It is known that acetate can be incorporated into the 

 lactose molecule.''^ Moreover, the glucose and lactose present in the 

 milk of the lactating rabbit after the administration of labeled acetate 

 were shown by French, Popjak, and Malpress^^^ to have identical specific 

 activities. Popjak et al^^^ found that the specific radioactivities of glyc- 

 erol and lactose, when plotted against time, indicate a relationship be- 

 tween the curve of a reaction product (glycerol) and that of its immediate 

 precursor (lactose). However, Folley^-^^^ is of the opinion that glycerol 



212 T. H. French, G. Popjdk, and F. H. Malpress, Nature, 169, 71 (1952). 



213 G. Popjdk, R. F. Glascock, and S. J. Folley, Biochem. J., 52, 472-475 (1952). 

 21* S. J. Folley, Biochem. J., 51, xv (1952). 



