“Heavy”-isotope label from 13C615N4-arginine (Arg-10) is converted into other amino acids in fission yeast.

Mass spectra of tryptic peptides (TIFFKDDGNYK and GIDFKEDGNILGHK in (A) and (B), respectively) from S. pombe Mto2[17A]-GFP fusion protein isolated from car2∆ arg1-230 lys3-37 cells grown in either (i) unlabeled arginine (Arg-0) and unlabeled lysine (Lys-0), (ii) 13C6-arginine (Arg-6) and 13C15N2-lysine (Lys-8), or (iii) 13C615N4-arginine (Arg-10) and 13C615N2-lysine (Lys-8), as indicated. In peptides from cells grown in Arg-10, additional higher-molecular-mass peaks are observed (iii), indicating conversion of “heavy”-isotope label into other amino acids. Such peaks are not observed from cells grown in Arg-6. To simplify comparison, peptides shown here do not contain arginine residues, so the masses of monoisotopic peaks of peptides from cells grown in Arg-6 and Arg-10 are identical (see Fig 2). Mass-to-charge (m/z) ratios of monoisotopic peaks and inferred peptide charge-states are indicated in magenta.