Structure--inhibition relationships of various phenolic compounds towards mitochondrial respiration

Final Report on the Safety Assessment of Pyrogallol

Pyrogallol, a benzenetriol, is used in oxidative hair dyes at concentrations ranging from ≤0.1 to 5.0%. The oral LD50's in rats ranged from 800 to 1270 mg/kg. Pyrogallol was not an ocular irritant when tested at a concentration of 1%. It was slightly irritating and induced sensitization reaction in the skin of guinea pigs. Sensitization reactions were noted in 3 of 25 patients patch tested with Pyrogallol.

Significant teratogenic effects were not observed in the offspring of female rats dosed with Pyrogallol. No treatment-related effects were observed in a multigeneration reproductive toxicity study in which rats received dermal applications of a hair dye containing 0.4% Pyrogallol.

Pyrogallol was mutagenic in almost all systems tested. However, in two carcinogenicity studies, the number of neoplasms in mice dermally treated with 50% Pyrogallol in acetone was not significantly different from that of controls. Similar results were reported in a carcinogenicity study in which a hair dye containing 0.49% Pyrogallol and H2O2 in aqueous solution was applied to the skins of mice.

On the basis of the available animal and clinical data presented in this report, it is concluded that Pyrogallol is safe as a cosmetic ingredient in the present practices of use and concentration.

Electrochemistry of flavonoids. Relationships between redox potentials, inhibition of mitochondrial respiration, and production of oxygen radicals by flavonoids

We have investigated the redox behavior of a series of structurally related flavonoids employing cyclic voltammetry under physiological conditions. The flavonoids that auto-oxidized and produced oxygen radicals had oxidation potentials (E 1/2) significantly lower [-30 to +60 mV vs (SCE)] than those that did not undergo auto-oxidation (+130 to +340 mV vs SCE). The range of E 1/2 values for the auto-oxidizable flavonoids was comparable to the E 1/2 range reported for the optimum quinone induced production of superoxide (O2 pi) in mitochondrial NADH-CoQ reductase (complex I). The most potent flavonoid inhibitors of mitochondrial succinate-CoQ reductase (complex II) possessed hydroxyl configurations capable of supporting redox reactions. For a series of 3,5,7-trihydroxyflavones that differed by b-ring hydroxylation it was found that decreasing E 1/2 of the flavonoids was associated with decreasing I50 values towards succinoxidase. These findings suggest that the electrochemical properties of the flavonoids may contribute to their biological activity.

Inhibitory effect of phenolic compounds on aflatoxin B1 metabolism and induced mutagenesis

The interaction between phenolic compounds and the food-borne carcinogenic mycotoxin, aflatoxin B1 (AFB1), was examined. 6 phenolic compounds (gallic acid, chlorogenic acid, caffeic acid, dopamine, p-hydroxybenzoic acid and salicylic acid) inhibited AFB1-induced mutagenesis in Salmonella typhimurium strain TA98 in a suspension assay in the presence of rat-liver microsomes (S9). The inhibitory effect was observed when the phenolic compound and the mutagen (AFB1 plus S9) were administered concurrently, but not when exposure to the mutagen was followed by the phenolic compound. The concentrations of the phenolic compounds used were not mutagenic to S. typhimurium strain TA98 and had no effect on the survival of the bacteria. The inhibition of AFB1 metabolism was studied using high-pressure liquid chromatography. Increasing the concentration of all 6 phenolic compounds resulted in a dose-dependent reduction of both major AFB1 metabolite peaks. The results are consistent with the hypothesis that the phenolic compounds do not react covalently with AFB1, and the inhibitory effect of phenolic compounds on AFB1-induced mutagenesis may be due to the inhibition of the activation enzymes.

Inhibition of mitochondrial respiration and production of toxic oxygen radicals by flavonoids. A structure-activity study

A series of fourteen flavonoids were employed in a systematic structure-activity study to assess their abilities to inhibit succinoxidase and generate toxic oxygen species in beef heart mitochondria. By comparing I50 values toward succinoxidase activity, flavonoids with a catechol moiety on the b ring exhibited the following general order of potency: chalcone greater than flavone greater than flavonol greater than dihydroflavonol greater than anthocyanidin. Catechins were inactive. In a series of 3,5,7-trihydroxyflavones containing various configurations of the b ring hydroxyl groups, it was found that the flavonoids possessing adjacent trihydroxy (pyrogallol) and b ring ortho-hydroxy(catechol) configurations were the most potent inhibitors of succinoxidase, followed by those with meta-hydroxyl, monohydroxyl and unhydroxylated configurations. Four of the fifteen flavonoids tested exhibited substrate-independent, KCN-insensitive respiration. Two flavonols with a pyrogallol configuration, myricetin and quercetagetin, produced the largest respiratory bursts and were found to auto-oxidize. Evidence is presented that the mitochondrial respiratory bursts induced by both flavonols and their auto-oxidation resulted in the generation of O-2 and H2O2.

Toxicology of various pesticides and their decomposition products on mitochondrial electron transport

The effect of various pesticides and derivatives on mitochondrial electron transport systems was assessed. DDT, DDE, TDE, Kelthane, chlorobenzilate, chloropropylate and Acarol were found to be inhibitory towards both heavy beef heart mitochondrial (HBHM) NADH-oxidase and succinoxidase enzyme systems. Dichlorobenzophenone and p-chlorophenol were less inhibitory towards the HBHM NADH-oxidase and did not inhibit the succinoxidase enzyme system. DDA did not inhibit either of the electron transport systems. Carbaryl was not inhibitory towards both HBHM oxidase systems, whereas its degradative product dihydroxynaphthalene was inhibitory at the same concentration. Furadan, Matacil, Baygon and Dimetilan were only slightly inhibitory towards themitochondrial NADH-oxidase system and did not inhibit the succinoxidase system. Zectran was inhibitory towards the NADH-oxidase system and was not inhibitory towards the succinoxidase system. DDT, DDE and TDE, dihydroxynaphthalene and 1-naphthol inhibited the NADH-oxidase enzyme system onthe substrate side of cytochrome c, whereas Kelthane inhibited on the oxygen side.