Cerebral Metabolism of Isotopic Glucose in Normal Human Subjects

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Abstract
Using a method previously described by the author, the total activity of labeled carbon dioxide in the blood entering and leaving the brain was determined following a single injection of C14-labeled glucose into normal human subjects. The variously labeled isotopes used as substrates were glucose-U-C14 (uniformly labeled glucose), glucose-1-C14, glucose-2-C14 and glucose-6-C14. An estimation of the fraction of the cumulative C14O2 resulting from the oxidation of labeled glucose, over a 90-minute time interval following injection, showed that an average of 14.6% of the injected C14 was converted into C14O2, by the brain, when glucose-U-C14 was the isotope injected. With glucose-1-C14 as the substrate, an average of 7.3% was found. When glucose-6-C14 was used, an average of 7.0% was found, and with glucose-2-C14 as the injected substrate, 10.8% of the injected C14 was converted into C14O2. Theoretical considerations of the data, using some equations currently employed for estimating the role of the hexose monophosphate shunt in glucose metabolism, suggested that practically all of the glucose-derived CO2 arose glycolytically. However, when ‘specific activities’ of the ‘added increment’ C14O2 (i.e. the ‘specific activity’ of the CO2 produced by the brain and added to the venous blood leaving the brain) were determined, the data seemed to warrant a serious consideration that there is a constant production, by the brain, of CO2 from some source(s) other than glucose. Submitted on June 20, 1956