label: N-terminal L-glutamyl-[protein] + N-acetyl-L-cysteine => N-terminal N(alpha)-acetyl-L-cysteinyl-L-glutamyl-[protein] + H2O
label_lc: n-terminal l-glutamyl-[protein] + n-acetyl-l-cysteine => n-terminal n(alpha)-acetyl-l-cysteinyl-l-glutamyl-[protein] + h2o