The pentose phosphate pathway in rabbit liver. Studies on the metabolic sequence and quantitative role of the pentose phosphate cycle by using a system in situ

1. The reactions of the pentose phosphate cycle were investigated by the intraportal infusion of specifically labelled [¹⁴C]glucose or [¹⁴C]ribose into the liver of the anaesthetized rabbit. The sugars were confined in the liver by haemostasis and metabolism was allowed to proceed for periods up to 5min. Metabolism was assessed by measuring the rate of change of the specific radioactivity of CO₂, the carbon atoms of glucose 6-phosphate, fructose 6-phosphate and tissue glucose. 2. The quotient oxidation of [1-¹⁴C]glucose/oxidation of [6-¹⁴C]glucose as measured by the incorporation into respiratory CO₂ was greater than 1.0 during most of the time-course and increased to a maximum of 3.1 but was found to decrease markedly upon application of a glucose load. 3. The estimate of the pentose phosphate cycle from C-1/C-2 ratios generally increased during the time-course, whereas the estimate of the pentose phosphate cycle from C-3/C-2 ratios varied depending on whether the ratios were measured in glucose or hexose 6-phosphates. 4. The distribution of ¹⁴C in hexose 6-phosphate after the metabolism of [1-¹⁴C]ribose showed that 65–95% of the label was in C-1 and was concluded to have been the result of a rapidly acting transketolase exchange reaction. 5. Transaldolase exchange reactions catalysed extensive transfer of ¹⁴C from [2-¹⁴C]glucose into C-5 of the hexose 6-phosphates during the entire time-course. The high concentration of label in C-4, C-5 and C-6 of the hexose 6-phosphates was not seen in tissue glucose in spite of an unchanging rate of glucose production during the time-course. 6. It is concluded that the reaction sequences catalysed by the pentose phosphate pathway enzymes do not constitute a formal metabolic cycle in intact liver, neither do they allow the definition of a fixed stoicheiometry for the dissimilation of glucose.