Arginine-containing peptides from cells grown in ¹³C₆¹⁵N₄-arginine (Arg-10) exhibit lower-molecular-mass peaks that suggest partial loss of “heavy” atoms from a portion of the Arg-10 pool.

<p>Mass spectra of tryptic peptides (RGILTLK, VSTADNNLVLQELENLR, and VSTADNNLVLQELENLRER in (A), (B), and (C), respectively) from <i>S</i>. <i>pombe</i> (A) actin and (B, C) Mto2[17A]-GFP, isolated from <i>car2∆ arg1-230 lys3-37</i> cells grown in either (i) unlabeled arginine (Arg-0) and unlabeled lysine (Lys-0), or (ii) ¹³C₆¹⁵N₄-arginine (Arg-10) and ¹³C₆¹⁵N₂-lysine (Lys-8), as indicated. In peptides from cells grown in Arg-10, several “pre-peaks” are observed (indicated in blue), with molecular masses significantly lower than the expected monoisotopic masses for Arg-10-labeled peptides. The number of pre-peaks is proportional to the number of arginine residues in the peptide (compare (B) and (C)). Mass-to-charge (m/z) ratios of monoisotopic peaks for each growth condition (i.e., assuming no conversion of labeled arginine) and inferred peptide charge-states are indicated in magenta.