Potential mechanisms by which alpha-tocopherol enhances the stability of myoglobin

Cherry red oxymyoglobin (OxyMb) may be maintained in beef by delaying its oxidation to brown metmyoglobin (MetMb), or through MetMb reduction. Numerous studies demonstrated that dietary supplementation of α-tocopherol (α-Toc) to cattle significantly delays lipid oxidation and enhances OxyMb stability in beef, but the basis for this phenomenon is not understood. The effect of α-Toc on MetMb reduction was investigated using a bovine liver extract containing cytochrome b5 (LE) at pH 7.2, 6.2, and 5.6. The combination of α-Toc and LE reduced MetMb at pH 5.6 and pH 6.2 (P \u3c 0.05), but not pH 7.2. α-Toc-mediated reduction at pH 5.6 was further investigated using purified bovine liver cytochrome b5 (Cyt b5). α-Toc effectively reduced Cyt b 5 and the combination of α-Toc plus purified Cyt b5 resulted in a 15% reduction in MetMb at pH 5.6 (P \u3c 0.05) relative to controls. ^ Lipid oxidation and MetMb formation are positively correlated in vitro and in meat. OxyMb and MetMb are also known catalysts of lipid oxidation in model systems and in meat. The effects of aldehyde lipid oxidation products on myoglobin (Mb) redox stability and aldehyde-adduct formation were investigated at 37°C, pH 7.2. OxyMb oxidation increased in the presence of 4-hydroxynonenal (4-HNE) compared to controls (P \u3c 0.05). Preincubation of MetMb with aldehydes rendered the heme protein a poorer substrate for enzymatic MetMb reduction with the effect inversely proportional to preincubation time; unsaturated aldehydes were more effective than saturated aldehydes (P \u3c 0.05). 4-HNE histidine adducts were detected on OxyMb incorporated into liposomes while Western blot analysis showed increased 4-HNE adduct formation with incubation time. ^ These results demonstrate a potential mechanism whereby α-Toc maintains OxyMb via enhancement of Cyt b5 mediated reduction of MetMb. Aldehyde lipid oxidation products can alter Mb redox stability. 4-HNE produced during liposomal oxidation forms adducts with histidine residues of Mb and this formation is concomitant with enhanced OxyMb oxidation. The increased lipid and color stability in the presence of α-Toc may be due to α-Toc delaying the formation of aldehyde lipid oxidation products and potentially aldehyde-Mb adducts.