A, Profiles of the 16 amino acids measured in culture medium samples. Relative levels are expressed as the background-subtracted log ratio of the normalized signal intensity in the infected RBC sample at each timepoint (XI, N) to the normalized signal intensity in the uninfected RBC time 0 sample (XU,0). Amino acids are ordered by the log ratio of the final timepoint, from highest (accumulating) (top) to lowest (depleting) (bottom). B, Arginine and ornithine concentrations in the culture medium of a 40-hour culture. A synchronized culture (6% initial parasitemia) was adjusted to 1% hematocrit in fresh RPMI 1640 culture medium at the trophozoite stage (approximately 24 hours post infection) and incubated 40 hours in parallel with an uninfected RBC control culture. Concentrations were determined by LC-MS/MS analysis using an internal standard solution containing known concentrations of isotopically labeled arginine and ornithine. The error is given as the standard deviation of n = 3 independent biological replicates.