Download DOES REPAIR OF PROTEIN RADICALS CAUSE THE LOSS OF

DOES REPAIR OF PROTEIN RADICALS CAUSE THE LOSS OF URATE
AND ASCORBATE UNDER OXIDATIVE STRESS?
Anastasia S. Domazou,1 Janusz M Gebicki2 and Willem H. Koppenol1
1
Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences,
Swiss Federal Institute of Technology, Zurich
2
Department of Biological Sciences, Macquarie University, Sydney, NSW 2109,
Australia
[email protected]
In living organisms, proteins are significant targets of partially reduced oxygen
species (PROS). Their oxidation plays a key role in ageing and age-related diseases.
Primary products of the PROS-mediated protein oxidation are protein radicals 
located on amino acid residues  that in the presence of oxygen may yield protein
peroxyl radicals (PrOO). All these species are considered hazardous. Oxidative
biological damage may be prevented, diminished or repaired by endogenous
antioxidants  such as urate and ascorbate. Reduced levels of these antioxidants,
measured under conditions of oxidative stress, have been related to their reactions
with protein radicals.
To elucidate whether urate and ascorbate are able to repair protein radicals and
whether such a process may be responsible for their loss in vivo, we studied their
reactions with a number of amino acids in model systems and in proteins.
The Trp, Tyr radicals and the peroxyl radicals (AAOO) of N-Ac-Gly-amide, N-AcAla-amide and N-Ac-Pro-amide are reduced rapidly by both antioxidants with the
formation of urate and ascorbate radicals [1].
The repair of C-centered amino acid radicals by the two antioxidants is by far slower
that the reduction of Trp, Tyr and AAOO. The radicals of -methylalanine, N-AcAla-amide and N-Ac-Pro-amide were repaired by ascorbate with rate constants of 105106 M–1 s–1. In contrast, repair of the radical of N-Ac-Gly-amide was not observed.
Preliminary results suggest an upper limit of 104 M–1 s–1 for the rate constants for the
repair of the C-radicals of these amino acids by urate.
Our results support the importance of ascorbate and urate as protective agents against
biological damage: they (a) repair Trp and Tyr in proteins, and (b) reduce PrOO to
the corresponding PrOOH. The loss of ascorbate and urate in living organisms under
oxidative stress may be due to their reactions with protein radicals and can be
regarded as a consequence of their protective role. Repair of C-centered amino acid
radicals by both antioxidants is presumably not a biologically important process.
[1] A.S. Domazou et al. Free Radic. Biol. Med. 2009, 46, 1049-1057; 2012, 52, 19291936; 2012, 53, 1565-1573.