Table 2.

¹³C serine labeling and model-optimized flux ratios with [1,2-¹³C] glucose

	Serine 13C isotopologue content (%)				Modeled flux ratios (%)
	M	M1	M2	M3	$\frac{J_{19}}{J_{19}+J_{20}}$ (R5)	$\frac{J_{21}}{J_{21}+J_{24}}$ (R6)	$\frac{J_{25}}{J_{19}+J_{20}}$ (R7)	Model error (%)
T47D	82.21±0.65 a (82.21±0.65)	14.16±0.50 a (14.16±0.50)	3.46±0.19 a (3.46±0.19)	0.17±0.02 a (0.17±0.02)	29.9±2.9 a	13.9±10.0 a	11.0±0.3 a	0.01±0.01
MDA231	95.25±0.37 b (nd)	3.88±0.29 b (nd)	0.84±0.07 b (nd)	0.03±0.04 b (nd)	0 b	nd	nd	nd
BoM	67.93±4.73 a,c (67.93±4.73)	20.53±2.91 a (20.50±2.89)	11.04±1.74 c (11.04±1.74)	0.49±0.08 c (0.53±0.11)	69.0±14.6 a	19.8±16.1 a	6.6±0.3 b	0.10±0.08
LM	66.58±2.69 c (66.58±2.69)	19.36±1.48 a (19.36±1.48)	13.43±1.16 c (13.43±1.16)	0.64±0.07 c (0.63±0.07)	63.9±10.4 a	36.6±20.1 a	5.6±0.2 c	0.01±0.01

Experiments with [1,2-¹³C] glucose were conducted as detailed in Table 1. ¹³C serine labeling (mean ± SEM, n=6) is expressed as percent contribution of each isotopologue to total extracellular serine (M, M1, M2, and M3 denote serine with none, one, two, two, or three ¹³C atoms, respectively). Data in parentheses are simulated serine labeling (mean ± SEM, n=6) from Model 2-optimized flux ratios R5, R6, and R7. Model error was calculated as the sum of the absolute values of the difference between the measured and calculated M-M3 serine. Within a column, means sharing common superscripts do not significantly differ. Background ¹³C abundance in serine standards averaged 95.17±0.38, 4.06±0.33, 0.76±0.05, and 0.01±0.00 % for M, M1, M2, and M3 isotopologues, respectively. Note that the table reports total serine labeling not corrected for background. Nd: not determined