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Fig. 4.4 Heme protein (HP) oxidation and HP-mediated lipid oxidation can occur by
multiple pathways. Metals (M), oxyHP, and deoxyHP are sources of
·ÿ
O
2
and H
2
O
2
that
accelerate HP oxidation. MetHP readily releases hemin. Hemin decomposes LOOH
stimulating free radical-mediated lipid oxidation. H
2
O
2
and LOOH produce ferryl(+4)HP
forms that can initiate lipid oxidation. Reactions involving M are facilitated or prevented
depending on the type of chelator bound to the metal.
4.6.2 Relative ability of Hb and Mb to autooxidize and promote lipid
oxidation
Human Hb is known to release its ferric hemin moiety much more rapidly
compared to human Mb (Bunn and Jandl, 1968; Gattoni et al., 1998). In fact
apoMb can completely extract hemin from Hb (Banerjee, 1962).This is relevant
because released hemin is capable of stimulating extensive lipid oxidation
through decomposition of pre-formed lipid hydroperoxides (Tappel, 1955, Van
der Zee et al., 1996) (reactions 20 and 21). Release of hemin from trout Hb
occurred much more rapidly compared to trout Mb (Richards et al., 2005). On
the other hand, trout Mb autooxidized to metMb much more rapidly compared to
trout Hb (Richards et al., 2005). Ferrous, trout Hb was a more effective promoter
of lipid oxidation in washed fish muscle at pH 6.3 compared to ferrous, trout Mb
(Richards et al., 2005). This suggested that hemin loss from metHb was the
primary factor that promoted lipid oxidation in washed fish muscle at pH 6.3
while metMb and the rapid burst of superoxide that formed during Mb
autooxidation were relatively weak reactants. Hb promoted lipid oxidation more
effectively compared to Mb in lipoproteins which was also attributed to the
lower hemin affinity of Hb (Grinshtein et al., 2003).
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