Table 2. Peptides identified in human cells with LC-MS/MS.

cell line	substrate	number of peptides	number of trp peptides	number of 1-L-MT peptides
86HG39	L-tryptophan	238	9	0
86HG39	1-L-MT	245	3	0
HeLa	L-tryptophan	224	10	0
HeLa	1-L-MT	239	6	0

Total numbers and numbers for tryptophan (trp) containing peptides are given for protein samples from different human tumor cells grown either with L-tryptophan or 1-L-MT as the only available tryptophan source. For all analyses, tryptophan methylation was used as a variable modification, but none of the identified peptides contained methylated tryptophan.