OxyFile #358

Inhibition of metmyoglobin/H2O2-dependent low density lipoprotein 
lipid peroxidation by naturally occurring phenolic acids.

Laranjinha J; Vierira O; Almeida L; Madeira V

Laboratório de Bioquímica, Faculdade de Farmácia, Universidade de 
Coimbra, Portugal.

Biochem Pharmacol, 51: 4, 1996 Feb 23, 395-402


The ferrylmyoglobin <==> metmyoglobin redox transitions promoted 
by hydrogen peroxide and dietary phenolic acids and their 
potential role in the oxidation of LDL were studied. The use of 
parinaric acid incorporated in LDL as a probe for radicals 
(detected by fluorescence quenching of the probe) revealed an 
oxidative stress inside LDL shortly ( < 1 min) after addition of 
hydrogen peroxide to metmyoglobin 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 kinetics of oxygen 
consumption. Triton X-100 decreases but does not suppress the lag 
phase of oxidation. Addition of metmyoglobin (without peroxide) to 
LDL was not followed by significant oxidation during the time of 
the experiment, unless Triton X-100 was present in the medium. 
When dietary phenolic acids were present in the medium before 
peroxide addition, an inhibition of parinaric acid fluorescence 
quenching and oxygen consumption was recorded as a function of 
concentration and substitution pattern on the phenol ring of the 
phenolic acids. This was associated with a conversion of 
ferrylmyoglobin to metmyoglobin. The results indicate that the 
naturally occurring phenolic acids prevent ferrylmyoglobin-
dependent LDL oxidation in a way strongly dependent on the 
substitution pattern on the phenol ring. Among the phenolic 
compounds studied, the o-dihydroxy derivatives of cinnamic and 
benzoic acids (caffeic, chlorogenic, and protocatechuic acids), in 
a molar ratio of 1 to metmyoglobin, efficiently blocked LDL 
oxidation initiated by ferrylmyoglobin. Replacement of one OH 
group from catecholic structure with an H (p-coumaric acid) or 
methoxy group (ferulic acid) decreased the antioxidant activity. 
Also, the catechol structure fused in heterocyclic rings 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. Therefore, the protection of LDL against oxidation 
is assigned to the reduction of the oxoferryl moiety of the 
hemoprotein to the ferric form. Additionally, it is suggested that 
an access constraint of oxidants plays a minor role in the 
ferrylmyoglobin-induced oxidation against LDL.