Protein metalation by metal-based drugs: reactions of cytotoxic gold compounds with cytochrome c and lysozyme

Protein metalation processes are crucial for the
mechanism of action of several anticancer metallodrugs
and warrant deeper characterisation. We have explored the
reactions of three cytotoxic gold(III) compounds—namely
[(bipy2Me)2Au2(l-O)2][PF6]2 (where bipy2Me is 6,60-dimethyl-
2,20-bipyridine) (Auoxo6), [(phen2Me)2Au2(l-O)2]
[PF6]2 (where phen2Me is 2,9-dimethyl-1,10-phenanthroline)
(Au2phen) and [(bipydmb-H)Au(OH)][PF6] [where
bipydmb-H is deprotonated 6-(1,1-dimethylbenzyl)-2,20-
bipyridine] (Aubipyc)—with two representative model
proteins, i.e. horse heart cytochrome c and hen egg whitelysozyme, through UV–visible absorption spectroscopy
and electrospray ionisation mass spectrometry (ESI MS) to
characterise the inherent protein metalation processes.
Notably, Auoxo6 and Au2phen produced stable protein
adducts where one or more ‘‘naked’’ gold(I) ions are protein-
coordinated; very characteristic is the case of cytochrome
c, which upon reaction with Auoxo6 or Au2phen
preferentially forms ‘‘tetragold’’ adducts with four proteinbound
gold(I) ions. In turn, Aubipyc afforded monometalated
protein adducts where the structural core of the
gold(III) centre and its ?3 oxidation state are conserved.
Auranofin yielded protein derivatives containing the intact
auranofin molecule. Additional studies were performed to
assess the role played by a reducing environment in protein
metalation. Overall, the approach adopted provides
detailed insight into the formation of metallodrug–protein
derivatives and permits trends, peculiarities and mechanistic
details of the underlying processes to be highlighted.
In this respect, electrospray ionisation mass spectrometry is
a very straightforward and informative research tool. The
protein metalation processes investigated critically depend
on the nature of both the metal compound and the interacting
protein and also on the solution conditions used;
thus, predicting with accuracy the nature and the amounts
of the adducts formed for a given metallodrug–protein pair
is currently extremely difficult.