Figure 3. Distribution of carbon from glutamine in melanoma cells and melanocytes emphasizes role of glutamate secretion.

(A) Yield of carbon from glutamine in total cellular amino acids (including protein), in secreted metabolites, and estimated fraction lost as CO₂ after culture of cells for 72 h in medium containing U-¹³C-glutamine. Abbreviations: Melcytes: melanocytes; Asp-asn: combined cellular aspartate and asparagine; Glu-gln: combined cellular glutamate and glutamine. (B) Metabolites associated with calculated CO₂ production (products of glutaminolysis). CO₂ production was estimated as the sum of “missing” ¹³C in partially labeled end-point metabolites plus CO₂ necessarily lost between glutamine and the measured metabolites. Metabolites associated with at least 5% of estimated CO₂ production are shown, with Asp-asn representing combined cellular and secreted aspartate and asparagine, and Glu-gln, combined cellular glutamate and glutamine and secreted glutamate. (C) UACC903 cells show net uptake of glutamate, unlike other melanoma cell lines, which secrete glutamate. (D) Fraction of secreted glutamate of TCA-cycle origin (partially ¹³C-labeled). For UACC903 cells, there was no net secretion, but there was exchange of labeled glutamate into the medium (Mean ± SEM of N = 3). Source data for parts (A, B) are shown in Supplementary Dataset 2.