Localized 13C NMR Spectroscopy in the Human Brain of Amino Acid Labeling from d‐[1‐13C]Glucose

Abstract: Cerebral metabolism of d[1‐13C]glucose was studied with localized 13C NMR spectroscopy during intravenous infusion of enriched [1‐13C]glucose in four healthy subjects. The use of three‐dimensional localization resulted in the complete elimination of triacylglycerol resonance that originated in scalp and subcutaneous fat. The sensitivity and resolution were sufficient to allow 4 min of time‐resolved observation of label incorporation into the C3 and C4 resonances of glutamate and C4 of glutamine, as well as C3 of aspartate with lower time resolution. [4‐13C]Glutamate labeled rapidly reaching close to maximum labeling at 60 min. The label flow into [3‐13C]glutamate clearly lagged behind that of [4‐13C]glutamate and peaked at t = 110–140 min. Multiplets due to homonuclear 13C‐13C coupling between the C3 and C4 peaks of the glutamate molecule were observed in vivo. Isotopomer analysis of spectra acquired between 120 and 180 min yielded a 13C isotopic fraction at C4 glutamate of 27 ± 2% (n = 4), which was slightly less than one‐half the enrichment of the C1 position of plasma glucose (63 ± 1%), p < 0.05. By comparison with an external standard the total amount of [4‐13C]glutamate was directly quantified to be 2.4 ± 0.1 µmol/ml‐brain. Together with the isotopomer data this gave a calculated brain glutamate concentration of 9.1 ± 0.7 µmol/ml, which agrees with previous estimates of total brain glutamate concentrations. The agreement suggests that essentially all of the brain glutamate is derived from glucose in healthy human brain.