Detection of proteins by hyphenated techniques with endogenous metal tags and metal chemical labelling

The absolute and relative quantitation of proteins plays a fundamental role in modern proteomics, as it is the key to understand still unresolved biological questions in medical and pharmaceutical applications. Highly sensitive analytical methods are required to quantify proteins in biological samples and to correlate their concentration levels with several diseases. Enzyme-linked immunosorbent assay (ELISA) and Western blot represent specific strategies for protein quantitation. However, these approaches are impractical for quantitative studies: the availability of high quality ELISAs for biomarker candidates is limited, and the performance characteristics of many commercially marketed ELISAs are poorly documented or unknown. The development of ELISA or Western blot is also expensive and time-consuming. The limitations of these strategies, combined with the large numbers of biomarker candidates emerging from genomic and proteomic discovery studies, have created the need for alternative strategies for quantitation of targeted proteins. A widely explored approach to identify and quantify intact proteins is based on (i) the detection of endogenous metals covalently bound to the protein structure or (ii) the labelling of proteins with metallic probes. The development of several hyphenated analytical techniques for metal quantitation has led to new possibilities for the quantitative analysis of proteins. In the present review, we attempt to provide a full coverage of current methodologies for protein quantitation based on the detection of endogenous metal(loid)s or chemical labeling with metal(loid)s, highlighting, to the best of our knowledge, their merits and limits.