The use of biotin hydrazide was crucial in the discovery of patterns of
protein damagewithin the proteome. However, these studies were limited
by the presence of high background due to the lack of an efficient way
to elute adduction proteins from affinity columns as well as to crossreactivity
of biotin hydrazide with endogenous carbonyls [43,44]. Additionally,
the inability to remove the biotin moiety from target protein adducts
makes mass spectrometry analyses of isolated proteins difficult. This occurs
because the biotin appendage alters MS/MS fragmentation patterns
and prevents the identification of peptide adduction sites [44,49]. In
order to reduce high background proteins in proteomic adduct inventories,
it was necessary to synthesize a biotin linker that could be easily released
from target protein adducts to improve MS/MS identification of
adducted proteins. A newly synthesized click reagent containing azido
and biotin groups separated by a photocleavable linker improves upon
these preliminary affinity purification studies and allows the isolation of
a protein mixture enriched in adducted proteins (Fig. 4) [50]. Importantly,
by removing the biotinmoiety, this approach allowed the LC–MS/MS detection
of specific modification sites on proteins. Thus, such affinity
purification-photorelease strategies improve the capability to isolate
and identify specific lipid electrophile modifications to proteins from
complex biological mixtures.