What is the reason behind using heavy lysine and arginine often in SILAC (Stable isotope labeling by amino acid in cell culture) not other amino acids? And why is it on carbon 6?
To my knowledge, it is also possible the selection of other isotopically labeled residues, such as L or P. However, as you comment, the normal chose is K and R, since you ensure, at least, one heavy residue per tryptic peptide. If not, it is possible that one sequence does not contains the isotapically labeled residue, thus hampering its quantitation.