Tumors frequently have abnormal
L-methionine (Met) metabolism, the so-called Met-dependence phenotype that refers to the inability to proliferate in the absence of Met. However, the origin of this phenotype is still unknown and may arise from one of several pathways of Met metabolism. To help characterize the metabolic features of Met-dependent/independent phenotypes, the fate of the methyl
carbon of L-[methyl-13C]Met was chased in a murine model of
malignant melanoma (B16-F1) in vitro and in vivo. Growth curves under Met restriction showed that
melanoma cells in vitro were Met-independent, whereas implanted
melanoma tumors in vivo were Met-dependent. Label-assisted high-resolution magic angle spinning 1H-13C NMR spectroscopy metabolite profiling showed that, in vitro,
creatine and
phosphatidylcholine 13C-enrichments were poor, but S-adenosyl-Met and posttranslationally N-methylated
protein signals were strong. In contrast, in vivo,
creatine and
phosphatidylcholine enrichments were strong but S-adenosyl-Met and N-methylated
protein signals were poor. In addition, in vivo, transsulfuration was very efficient, consumed one-
carbon units originating from the methyl
carbon of Met, and yielded
taurine labeling. From these data, the Met-dependent/independent phenotypes appear closely related to the source of one-
carbon units. Thus, L-[methyl-13C]Met-assisted NMR spectroscopy metabolite profiling allowed the discrimination between Met-dependence and Met-independence and provided novel mechanistic information on their origin.