Table 2.

¹³C labeling strategies for studying the glutamate–glutamine cycle^*

GLUCOSE
[1-13C] or [1,6-13C2] glucose	Primary route of label transfer: glutamate C4 → glutamine C4
	Anaplerotic pathway: glutamine C2 → glutamate C2
[2-13C] or [2,5-13C2] glucose	Primary route of label transfer: glutamate C5 → glutamine C5
	Anaplerotic pathway: glutamine C3 → glutamate C3
ACETATE**
[2-13C]acetate	Primary route of label transfer: glutamine C4 → glutamate C4
[1-13C]acetate	Primary route of label transfer: glutamine C5 → glutamate C5
bHB AND LACTATE
[2,4-13C2]bHB, [3-13C] lactate	Primary route of label transfer: glutamate C4 → glutamine C4
[1,3-13C2]bHB, [ 2-13C] lactate	Primary route of label transfer: glutamate C5 → glutamine C5

^*For simplicity, only intercompartmental label transfer during the first turn of the TCA cycles is shown.

^**Isotope labels originating from ¹³C-ethanol enter the brain after being converted into acetate in the liver. The labeling pathway for ¹³C-labeled ethanol is the same as that of acetate (Xiang and Shen, 2011b).