Heme proteins and oxidation in fresh and processed meats - Oxidation in Foods and Beverages and Antioxidant Applications - page 91

and peroxyl radicals that propagate lipid oxidation (Van der Zee et al., 1996)
(reactions 20 and 21).
L29F/H64Q is susceptible to porphyrin destruction in the presence of
hydrogen peroxide compared to WT Mb (Alayash et al., 1999). Porphyrin
destruction releases the iron in the heme ring and produces biliverdin. L29F/
H64Q was a weaker promoter of lipid oxidation in washed cod muscle compared
to WT Mb (Grunwald and Richards, 2006b). This may be partly due to the
antioxidant action of biliverdin (Baranano et al., 2002). It also indicates that
liberated iron atoms from the porphyrin were not pro-oxidative. It should be
noted hydrogen peroxide is water soluble so that heme destruction likely
occurred in the aqueous phase. Iron atoms in the aqueous phase may be less
reactive than iron atoms that incorporate into the membrane. Hemin is noted as a
molecule that transports reactive iron atoms to lipid sites (Balla et al., 1991;
Grinshtein et al., 2003).
Ferrous V68T and ferrous WT Mb were compared to assess the ability of
autooxidation relative to hemin affinity to promote lipid oxidation. V68T
autooxidizes rapidly compared to WT while WT has lower hemin affinity.
Ferrous WT Mb was a better promoter of lipid oxidation in washed cod
compared to ferrous V68T (Grunwald and Richards, 2006a). This suggested that
hemin affinity was more critical in promoting lipid oxidation compared to Mb
autooxidation rate in washed cod.
The fact that animal tissues contain hemopexin and heme oxygenase further
implicates hemin as an oxidant that must be removed for cells to avoid oxidative
effects associated with unbound hemin. Hemopexin binds hemin and is then
detoxified in the liver (Paoli et al., 1999). Heme oxygenase degrades the heme
ring releasing iron atoms and forming biliverdin that is reduced by biliverdin
reductase to bilirubin, a potent antioxidant (Halliwell and Gutteridge, 1990;
Kumar and Bandyopadhyay, 2005)
Tyrosine at site G4 is considered a critical residue that facilitates ferryl Mb
formation from hydrogen peroxide and metMb. Ferryl Mb can stimulate lipid
oxidation (Baron et al., 1997). Substituting Tyr with Phe decreased ferryl Mb
radical formation 1.4-fold (Witting et al., 2002). The Tyr(G4)Phe human Mb
mutant promoted lipid oxidation about half as effectively as wild-type Mb when
examining linoleic acid with added hydrogen peroxide at pH 7.4 (Rayner et al.,
2004). This suggested that decreasing ferrylMb radical formation through
mutagenesis decreased the ability of Mb to promote lipid oxidation in linoleic
acid.
Hydrogen peroxide caused the heme moiety of horse heart Mb to be
covalently cross-linked to the globin at pH values near 7 (Vuletich et al., 2000).
This cross-linked Mb promoted lipid oxidation in low density lipoproteins more
readily compared to native Mb. Thus under certain conditions, cross-linked Mb
promotes lipid oxidation more effectively than Mb that is not cross-linked.
There is also evidence that cross-linked Mb formation is favored at acidic pH
values (Reeder et al., 2002). Hydrogen peroxide was also found to degrade the
heme ring of Hb (Nagababu and Rifkind, 1998). This indicates hydrogen