In this work it is demonstrated that glucose constitutes the main substrate of energy metabolism of rabbit reticulocytes under aerobic conditions in the presence of 5 mM glucose. Amino acids and fatty acids are minor sources of energy. The shares of processes utilizing glucose in reticulocytes were estimated from tracer experiments. A new mathematical technique used permits the derivation of closed terms for the specific radioactivity of single positions of C atoms of the metabolites of the citrate cycle. By means of regression analysis, the undetermined flux rates in the citrate cycle were calculated.On the basis of the data an overall balance sheet of glucose utilization and of ATP generation is given. About 45 of the glucose of reticulocytes is catabolized via the citrate cycle, about the same percentage yields lactate. Only 2 of the glucose was oxidized in the oxidative pentose pathway whereas the remainder is used for the formation of serine and glycine required for hemoglobin synthesis.These results are related to knowledge about the main processes utilizing ATP in reticulocytes, i.e. the synthesis of hemoglobin and the energy-dependent proteolysis. Our approach to the investigation of metabolic relations in the reticulocytes can be applied to other tissues in which equilibria between large metabolite pools play a roleThe quantitative assessment of metabolic pathways in cells poses considerable difficulties. A clear-cut balance so far is only available for the simplest cell, the mature erythrocyte with its rudimentary metabolism [l]. It consumes only glucose which is utilized to about 90% by glycolysis and about 10% by the oxidative pentose pathway. In other cells several substrates are utilized both by anaerobic and aerobic pathways. Furthermore there are a multitude of synthetic processes. The complexities are compounded by compartmentation. Our knowledge about the distribution of energy production and utilization is also unsatisfactory for any type of cell. Even for the mature red cell there is no answer as yet concerning the apportionment of ATP-consuming processes, mostly located in the cell membrane [2], even though a good understanding exists with respect to the amount and regulation of ATP production [3 -51. Generally isotopic methods with 14C-labeled substrates have been used for the assessment of metabolic pathways. They involve a number of problems, such as isotopic exchange which may simulate metabolic flux, the occurrence of large pools of metabolites and, of course, compartmentation. The uneven distribution of the radioactivity among different C atoms of a metabolic is a further complication.For estimation of the distribution of glucose between the oxidative pentose and Embden-Meyerhof pathways the ratio between the radioactivities of the COz derived from [1-14C]-glucose and [6-14C]glucose has often been used. The calculation is based on the consideration that in the former pathway COZ is formed only from the C-I atom of glucose, whereas it is derived from both C-1 and C-6 atoms in the Embden...
