| Abstract
| - The pro-oxidative activity of trout hemoglobin was significantly increased at low pH (2.5−3.5) in awashed fish muscle (WFM) system. It was found that the more unfolded the hemoglobin was themore exposed its heme group was, which increased its pro-oxidative activity. The amount of oxidationproducts produced (TBARS) were, however, lower at low pH vs neutral pH. At pH 10.5−11, thepro-oxidative activity of hemoglobin was greatly suppressed. The conformation of hemoglobin wassignificantly more stable at high pH as compared to pH 7 as judged by its visible absorption spectrum.Hemoglobin readjusted from low pH to pH 7 had a higher pro-oxidative activity (i.e., more rapidoxidation) in WFM than native hemoglobin at pH 7, even though TBARS values were lower than inthe untreated sample at pH 7. The results suggest that the WFM becomes slightly more susceptibleto oxidation after low pH treatment but also produces less TBARS. The increased pro-oxidative activityafter pH readjustment correlated well with an incomplete recovery in the native structure on pHreadjustment. A longer unfolding time and a lower pH led to a less refolded hemoglobin with increasedpro-oxidative activity. Hemoglobin was less pro-oxidative at low pH in the presence of 500 mM NaCl.The presence of salt did, however, increase the pro-oxidative properties of hemoglobin afterreadjustment to pH 7. The treatment of washed fish muscle at alkaline pH followed by adjustment topH 7 led to a slight delay in hemoglobin-mediated lipid oxidation in WFM as compared to nativehemoglobin at pH 7. The results suggest that WFM becomes less susceptible toward oxidation afterpH readjustment from alkaline pH. These results clearly show that for muscle protein extraction/isolation processes requiring highly alkaline or acidic conditions, alkaline conditions are preferred ifthe lipid oxidation originating from hemoglobin is to be minimized. Keywords: Hemoglobin; lipid oxidation; trout; low pH; high pH; molten globule; conformation;unfolding; refolding
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