INHIBITION OF METMYOGLOBIN H2O2-DEPENDENT LOW-DENSITY-LIPOPROTEIN LIPID-PEROXIDATION BY NATURALLY-OCCURRING PHENOLIC-ACIDS/

The ferrylmyoglobin double left right arrow metmyoglobin redox transit ions promoted by hydrogen peroxide and dietary phenolic acids and thei r potential role in the oxidation of LDL were studied. The use of pari naric acid incorporated in LDL as a probe for radicals (detected by fl uorescence quenching of the probe) revealed an oxidative stress inside LDL shortly (<1 min) after addition of hydrogen peroxide to metmyoglo bin in the aqueous phase outside the particle, reflecting an efficient access of the oxidant to LDL lipids. However, the propagation step of peroxidation only occurs after a lag phase, as detected by the kineti cs of oxygen consumption. Triton X-100 decreases but does not suppress the lag phase of oxidation. Addition of metmyoglobin (without peroxid e) to LDL was not followed by significant oxidation during the time of the experiment, unless Triton X-100 was present in the medium. When d ietary phenolic acids were present in the medium before peroxide addit ion, an inhibition of parinaric acid fluorescence quenching and oxygen consumption was recorded as a function of concentration and substitut ion pattern on the phenol ring of the phenolic acids. This was associa ted with a conversion of ferrylmyoglobin to metmyoglobin. The results indicate that the naturally occurring phenolic acids prevent ferrylmyo globin-dependent LDL oxidation in a way strongly dependent on the subs titution pattern on the phenol ring. Among the phenolic compounds stud ied, the o-dihydroxy derivatives of cinnamic and benzoic acids (caffei c, chlorogenic, and protocatechuic acids), in a molar ratio of 1 to me tmyoglobin, efficiently blocked LDL oxidation initiated by ferrylmyogl obin. Replacement of one OH group from catecholic structure with an H (p-coumaric acid) or methoxy group (ferulic acid) decreased the antiox idant activity. Also, the catechol structure fused in heterocyclic rin gs with adjacent carbonyl groups (ellagic acid) resulted in decreased antioxidant activity. These observations correlate with the efficiency of phenolic acids to reduce ferrylmyoglobin to metmyoglobin. Therefor e, the protection of LDL against oxidation is assigned to the reductio n of the oxoferryl moiety of the hemoprotein to the ferric form. Addit ionally, it is suggested that an access constraint of oxidants plays a minor rule in the ferrylmyoglobin induced oxidation against LDL.