IV PENTOSE FORMATION 49 



(c) Donors and acceptors in transaldolase reaction 

 Sedoheptulose-7-phosphate or fructose-6-phosphate function as "active 



dihydroxyacetone" donors in the transaldolase reaction. D-glyceraldehyde-3- 

 phosphate or D-erythrose-4-phosphate function as acceptors. 



[d) Aldolase reaction 



In the presence of the enzyme aldolase, dihydroxyacetone phosphate condenses 

 with D-glyceraldehyde-3-phosphate or with D-erythrose-4-phosphate. (Smyrniotis 

 and Horecker, 1956). In the first case, fructose diphosphate is the product while in 

 the second case, sedoheptulose diphosphate is formed (Ballou et al., 1955). Muscle 

 aldolase also catalyzes a condensation between dihydroxyacetone phosphate and 

 glycolaldehyde phosphate to give xylulose diphosphate (Byrne and Lardy, 1954). 

 The configuration of the newly formed secondary hydroxy groups are trans with 

 respect to one another in each case. Although L-gIyceraldehyde-3-phosphate can 

 substitute for D-glyceraldehyde phosphate in an aldolase catalyzed reaction, the 

 latter substrate reacts much more rapidly than the former (Tung et al., 1954). 

 In the presence of crystalline muscle aldolase d- or L-glyceraldehyde or glycolalde- 

 hyde may also condense with dihydroxyacetone phosphate to yield D-fructose-i- 

 phosphate, L-sorbose-i -phosphate, or D-xylulose-i -phosphate, respectively. 



J. Evaluation of the Embden-Meyerhof and phosphogluconate 

 pathways of glucose oxidation 



When glucose- 1 -''*C or glucose-6-''*C are incubated with tissue slices or cell 

 suspensions, it is observed that considerably more ^'^COj is derived from glucose- 

 i-''*C than from glucose-6-''*C. A preferential oxidation of glucose- i-^^'G over 

 uniformly labelled glucose- '"^C is also found. The preferential oxidation of the 

 first carbon of glucose to ^'^CO, constitutes evidence for the participation of the 

 phosphogluconate pathway in glucose metabolism. The ratio of the yield of ^''CO, 

 from glucose- 1 -'■*C to glucose-6-^'*C, {G — i)/(G — 6), depends upon the tissue, the 

 physiological state of the tissue, the concentration of glucose employed, the dura- 

 tion of the experiment, the ionic constitution of the medium, and the capacity of 

 a given tissue to oxidize to completion the pyruvate or acetyl-CoA formed from 

 glucose via glycolysis. It is to be anticipated that in neoplastic tissues in which the 

 aerobic formation of lactate is observed or in liver slices from diabetic animals 

 in which ketone bodies are formed, the oxidation of glucose-6-^''C to ^'*C02 

 will be retarded. Experimentally, the observed (G — i)/(G — 6) ratios have ranged 

 from I.I in mammary tissue at the end of pregnancy to 15.7 at 10-18 days of lac- 

 tation, and to 2.1 at the beginning of involution of the mammary tissue (Glock 

 etal., 1956a). The ratios for mammary carcinomas are much lower than for lactating 

 mammary tissue (Abraham ^f a/., 1956). {G — i)/(G — 6) ratios varying from 2-5 have 

 been observed with normal liver, regenerating liver, and hepatomas (Bloom and 

 Stetten, 1955; Abraham et al., 1955; AgranofFef al., 1954; Bloom et al., 1953 and 

 Katz et al., 1954, 1955), lymphatic tissues and lymphatic tumors (Kit, 1956), 

 various other tumors, (Emmelot et al., 1955), and yeast cells (Blumenthal et al., 

 1954). On the other hand, the ratios of (G — i )/(G — 6) approach one in rat diaphragm 



Literature p. 124 



