REACTIVE SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 25

TABLE 1.7. Oxidizing Power of Some Biological Free Radicals (Midpoint Electrode
[Reduction] Potentials, E m of Radicals, pH 7.0-7.4 vs. Norman Hydrogen Electrode
[NHE]) (Values of the E m of the Couple Were Taken from Madej and Wardman
[236] with the Permission of Elsevier Inc.)
Radical Obtained from Oxidation of

Couple a

E m

(Bi)carbonate 1.74

CO •− , H /HCO

+ −

3

AdO • /Ado Adenosine 1.42

Met •+ /Met b Methionine ∼1.2-1.5

Guanosine G(-H) • , H + /G 1.29

Tryptophan Trp •+ , H + /TrpH ∼1.05

Nitrite ∼1.04 NO /NO

2

• −

2

Glutathione GS • , H + /GSH 0.92

Hydrogen sulfide S •− , H + /HS − 0.92

Tyrosine

TyrO • , H + /TyrOH ∼0.90

8-OHdG(-H) • , H + /8-OHdG

8-Oxo-7,8-dihydro-2′-
deoxyguanosine

0.74

5-Indoxyl • /5-hydroxyindole 5-Hydroxylindole 0.64

Urate UH − , H /UH 0.59

+ −

2

Ascorbate Asc •− , H + /AscH − 0.30

a Predominant forms of radical/reductant couples at pH 7-7.4 are shown.
b Irreversible process with rapid deprotonation from the radical cation.

radicals in chymotrypsin, pepsin, and β-lactoglobulin reacted with monohydro-
gen ascorbate with rate constants of 1.6 × 10 8 /M/s, 1.8 × 10 8 /M/s, and 2.2 ×
10 7 /M/s, respectively. Comparatively, the corresponding reaction of protein
tyrosyl radicals had rate constants an order of magnitude slower. The rate
constants of the reactions were not significantly affected by the location of
radicals in the protein chains [247]. A separate study estimated a rate constant
of 2.2 × 10 6 /M/s for the tyrosyl radical with GSH, which is about two orders
of magnitude slower than the reaction of the tyrosyl radical with ascorbate
[248]. This indicates GSH plays a minor role in radical “repair,” while the loss
of ascorbate may be due to its reactions with protein radicals under physiologi-
cal conditions.
Overall, the chemistry of reactive sulfur species is complex. In addition to
cysteine and GSH, a biological role for H 2 S is also being studied (Chapter 5)
[249-251]. Physiological levels of H 2 S are 50-160 μM and 10-100 μM in mam-
malian brain tissues and human plasma, respectively [252]. The comparison of
sulfahydryl radicals derived from H 2 S is compared with other thiyl radicals in
Chapter 5.

1.3.5 High-Valent Cr, Mn, and Fe Species

Transition metal ions play important roles in a number of biological processes
[35, 36, 253, 254]. Metal oxo and metal hydroxo moieties have been recognized