Abstract
Protein and lipid derivatives were employed as substrates for determining cerebral metabolism in humans in vivo. The isotopes used were dl-glutamate-1-C14, l-glutamate-U-C14 (uniformly labeled), dl-glutamate-2-C14, l-lysine-U-C14, octanoate-1-C14, l-aspartate-U-C14, dl-aspartate-4-C14, urea-C14, butyrate-1-C14, glycerol-1, (3)-C14, glycerol-U-C14 and glycerol-2-C14. Of these substrates, only labeled aspartate, butyrate and glycerol were oxidized to C14O2 by brain in vivo. An estimation of the fraction of the cumulative C14O2 resulting from the oxidation of l-aspartate-U-C14, over a 90-minute time interval following injection, showed that 1.7% of the injected C14 was converted into C14O2 by the brain. With butyrate-1-C14 an average of 3.9% was found. When glycerol-1, (3)-C14 was used, an average of 4.4% was found; with glycerol-2-C14, 7.2%; and with glycerol-U-C14 as the injected substrate, 9.9% of the injected C14 was converted into C14O2. However, much of the brain C14O2 in the case of the glycerol-C14 experiments could have been due to the oxidation of glucose-C14 which was synthesized by the body from the injected glycerol-C14. With aspartate-U-C14 and butyrate-1-C14 as substrates the formed glucose-C144 was insignificant. Comparison of ‘specific activities’ of ‘added increment’ C14O2 with brain venous blood substrate ‘specific activities’ indicated that about 9.8% of brain CO2 was derived from butyrate and about 2.3% from glycerol. The results offer evidence for cerebral metabolism of breakdown products of lipids and proteins in vivo. Submitted on October 7, 1957