50 



INTERMEDIARY METABOLISM AND GROWTH 



(Wenner and Weinhouse, 1956a), consistent with the low levels of the direct 

 oxidative pathway dehydrogenases in that tissue (Bloom and Stetten, 1953). The 

 ratio is also about one in brain, heart, or kidney slices (Wenner and Weinhouse^ 

 1956a) and in the respiratory CO2 collected after glucose labelled at various posi- 

 tions is injected into the intact rat (Bloom et al., 1953). 



The radioactivity of the fatty acids, acetoacetate, acetate, lactate, or alanine 

 has also been measured after tissues have been incubated with labelled glucose. 

 In this case, more radioactivity is observed in the metabolite after incubation with 

 glucose-6-^'*C than with glucose- i-^^'C. Radioactivity from glucose- i-^'*G can 

 reach the former compounds primarily as a result of the glycolytic pathway of 

 glucose metabolism since the label is lost when glucose is metabolized via the 

 direct oxidative pathway. On the other hand, radioactivity derived from glucose- 

 6-^'*C can reach the fatty acids, alanine, or lactate as a result of the metabolism 

 of glucose via the glycolytic pathway and also the 6-phosphogluconate pathway, 

 provided that pentose phosphate, formed from the shunt oxidations is further 

 degraded to the pyruvate level. A preferential labelling of triose phosphate derived 

 from glucose-6-^'*C compared with glucose- i-^'^C may also be anticipated to the 

 extent that the transketolase-transaldolase pathway of pentose formation is oper- 

 ating (Reactions 2 to 4; Section IV-A-2-a p. 48). The excess of radioactivity 

 fovmd in the three compounds or the fatty acids thus provides a means of estimating 

 the percentage of three carbon compounds derived via the shunt or via the glyco- 

 lytic pathway. On this basis, it has been estimated that 0-17% of the ethanol of 

 Saccharomyces cerevisiae (Blumenthal et al., 1954) and 13-23% of the alanine of 

 spleen and tumor cells are derived from glucose via the shunt (Kit, 1956). The 

 corresponding values for the fatty acids and lactate derived via the shunt were 

 25-58% for hepatocarcinomas and lactating mammary tissues (Abraham et al., 

 1956), and 12-25% ^'^'^ mouse ascites tumors (Wenner and Weinhouse, 1956b). In 

 liver slices or homogenates of the rat, approximately 25-33% of the acetoacetate 

 units were derived from glucose via the shunt pathway (Wenner and Weinhouse, 

 1956a; Bloom and Stetten, 1955; Ashmore et al., 1956). The conversion of glucose 

 to acetyl units via the shunt is increased in liver homogenates by adding TPN"" to 

 the experimental vessels. 



The injection of insulin into diabetic rats greatly increases the conversion of labelled 

 glucose to CO2 and to fatty acids by liver slices. Felts and co-workers have observed that 

 the effect of hormone treatment on the conversion of glucose-S-'-'C to fatty acids is 

 considerably more pronounced than the effect on glucose-i-'^C, while the opposite is true 

 of the effect of insulin on the oxidation of glucose- i-i'*C and glucose-S-'^C to "tCO: (Felts 

 et al., 1956). Glock and associates found an increase in the (G — i)/(G — 6) ratio after 

 insulin administration to alloxan diabetic rats (Glock et al., 1956b). After thyroxine 

 treatment, the formation of '^COz from glucose-6-i''C by liver slices is significantly increased 

 whereas that from glucose- i-i'*C is slightly increased. 



The difference between the micromoles of ''^CO, formed from glucose- i-^'^C 

 and glucose-6-''*C can be taken as a first approximation of the amount of glucose 

 not oxidized via the citric acid cycle. This value also approximates the total micro- 

 moles of pentose formed via the direct oxidative pathway since a pentose phos- 

 phate is the product of the decarboxylation of glucose- i-''*C to ^''COj. On the 



