Cerebral metabolism of isotopic lipid and protein derivatives in normal human subjects.

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