TABLE 1.

Estimated values for ceramide rate of appearance (R_a), fractional turnover rate per unit time (k), and plateau isotopic enrichment (E_p) in HEK293 cells

N-acyl chain	16:0a	18:0	20:0	22:0	24:1	24:0	26:1	26:0
k	1.15b	1.24	1.30	0.652	0.296	0.066	0.296	0.479
Ep	0.548c	0.314	0.355	0.287	0.151	0.423	0.219	0.251
Q	103.2d	28.0	5.9	18.6	30.7	25.7	1.3	0.9
Ra	118.7e	34.7	7.6	12.1	9.1	1.7	0.4	0.4
R	0.999f	0.989	0.952	0.995	0.988	0.987	0.842	0.951

^aCeramide N-acyl chain lengths are designated x:y, where x = number of carbon atoms and y = number of double bonds.

^bFractional turnover rate per unit time (h⁻¹).

^cIsotopic enrichment at plateau/isotopic equilibrium (unitless).

^dUnlabeled ceramide (¹²C) quantities at 6 h (pmol/mg protein).

^eThe rate of appearance of ceramide calculated as R_a = k × Q (pmol/mg protein × h).