Regenerated and denatured peroxidase as potential lipid oxidation catalysts

Abstract

The aim of this study was to determine the relative lipid oxidation activities of regenerated and denatured peroxidase using horseradish peroxidase (HRP) as a model. Poor correlation between peroxidase activity regeneration and enhancement of lipid oxidation activity suggested that the regenerated enzyme did not possess increased potential to catalyse lipid oxidation. Heat denatured peroxidase, not containing any regenerated enzyme, was shown to have a greater lipid oxidation activity than the native enzyme. The denatured peroxidase comprised three haemoproteins with similar isoelectric points that were significantly lower than that of the major native isoenzyme. Two of the proteins, present in relatively minor amounts, possessed most of the lipid oxidation activity. Evidence for the release of haem on heating HRP under conditions that led to regeneration of activity of the cooled enzyme was provided by circular dichroism (CD) spectroscopy. The CD study also suggested that irreversible polymerisation of haem occurred on heating, which may have restricted subsequent activity regeneration. The freed haem appeared to have only a low lipid oxidation activity compared with the denatured holoenzyme. Nuclear magnetic resonance (NMR) spectroscopy detected ‘structural regeneration’ of the haem pocket under conditions that did not lead to regeneration of functional enzyme. ‘Structurally regenerated’ forms could potentially enhance lipid oxidation on storage of heat processed foods.