AIM: To evaluate the effects of frying oil and Houttuynia cordata Thunb (H. cordata), a vegetable traditionally consumed in Taiwan, on the xenobiotic-metabolizing enzyme system of rodents.

METHODS: Forty-eight Sprague-Dawley rats were fed with a diet containing 0%, 2% or 5% H. cordata powder and 15% fresh soybean oil or 24-h oxidized frying oil (OFO) for 28 d respectively. The level of microsomal protein, total cytochrome 450 content (CYP450) and enzyme activities including NADPH reductase, ethoxyresorufin O-deethylase (EROD), pentoxyresorufin O-dealkylase (PROD), aniline hydroxylase (ANH), aminopyrine demethylase (AMD), and quinone reductase (QR) were determined. QR represented phase II enzymes, the rest of the enzymes tested represented phase I enzymes.

RESULTS: The oxidized frying oil feeding produced a significant increase in phase I and II enzyme systems, including the content of CYP450 and microsomal protein, and the activities of NADPH reductase, EROD, PROD, ANH, AMD and QR in rats (P<0.05). In addition, the activities of EROD, ANH and AMD decreased and QR increased after feeding with H. cordata in OFO-fed group (P<0.05). The feeding with 2% H. cordata diet showed the most significant effect.

CONCLUSION: The OFO diet induces phases I and II enzyme activity, and the 2% H. cordata diet resulted in a better regulation of the xenobiotic-metabolizing enzyme system.

The liver is the primary site of drug metabolism and detoxification. Cytochrome P450 (CYP 450) enzymes are the predominant proteins in the hepatic endoplasmic reticulum representing about 5% to 25% of the total protein. They are responsible for the metabolism of most drugs, both endogenous and exogenous compounds, including steroid hormones and xenobiotics[1-3]. Cytochrome P450 enzymes, the major phase I components, have multiple roles in animals. They are important in bioactivation of chemical carcinogens, biotransformation of many endogenous vitamins and steroids, and detoxification of numerous xenobiotics[4,5]. Phase II components, which include transferases, dehydrogenases and glucuronidases, are involved in conjugation and detoxification reactions. Both phase I and phase II enzymes have important health implications for both animals and humans and have been reported to be influenced by various dietary factors[6] such as fried oil[7-9].

A high dietary intake of fat is prevalent in most developed countries, and fried foods are a principal fat source in the modern diet. According to a recent study[10], consumption of fried food is closely related to many chronic diseases, such as cancer and hyperlipidemia. One reason why the consumption of frying oil may influence the risk of chronic diseases is that it has a significant effect on CYP450. Polyphenols, which are plentiful in plants, also play an important role in CYP450 regulation[6]. Houttuynia cordata (H. cordata) Thunb, a traditional vegetable in Taiwan, which contains a high amount of polyphenol, has been showed to have both antioxidative and antimutagenic properties under OFO-feeding-induced oxidative stress[11]. This study was designed to assess the effect of frying oil and H. cordata thunb on CYP450 and xenobiotic-metabolizing enzyme system in rodents.

Chen et al[21] reported that the CYP450 content in OFO-fed rats was 1.6 times that of fresh oil-fed rats. After fried, a linolenic acid-containing oil could induce CYP450 by 4 and 6.5 folds relative to fresh soybean oil and linolenic oil respectively.

ANH is an indicator of CYP2C11. During oil frying, some chemicals with hydroxyl or carbonyl groups are produced, and these substrates will induce CYP 2C11. Chen et al[21] used soybean oil, red flower seed oil and shortening oil as study materials, and found that after fried, the amounts of carbonyl compounds in these three oils were 203 mEq/kg, 110 mEq/kg, and 3 mEq/kg, respectively. In addition, they also found that the ability of soybean oil to induce ANH activity was 2.5 times that of fresh soybean oil. Thus, it is suggested that these carbonyl compounds might induce the expression of CYP2C11.

We also compared the activities of ANH and AMD (an indicator of CYP2E1) and found that the increase in ANH was larger than that in AMD. This is because the aldehyde, acid and ketone that can induce AMD were easily vaporized and the accumulation of these in the OFO was not sufficient to strongly induce AMD[22].

PROD is an indicator of CYP2B1. Phenobarbital-type materials are the most common inducers of CYP 2B1[23]. The cyclic materials separated from the OFO resembled those of phenobarbital-type materials in structures[24]. Thus, the cyclic materials in OFO may be the major compounds to induce CYP 2B1. EROD is an indicator of CYP1A1. Polycyclic aromatic hydrocarbon (PAH) is an inducer. According to the Pon’s study[24], there are cyclic compounds but not polycyclic compounds in OFO. In fact, during the oil oxidation process, polycyclic hydrocarbon compounds will not be produced unless the temperature is higher than 300 °C and pyrolysis occurs. This can explain why the degree of induction by OFO of PROD is larger than that of EROD. In addition, CYP1A1 is the major enzyme that activates procarcinogens; thus OFO is considered to have the potential to cause tumors, as reported in a number of previous studies, by induction of this enzyme.

The enzymes measured in our study were all clinically relevant. AMD was related to hepatotoxicity induced by alcohol. EROD and PROD were related to the activation of carcinogens, such as polycyclic aromatic hydrocarbons, alfatoxin and nitrosamines. QR could promote the metabolism and excretion of mutagens and carcinogens[20]. Thus, we assume that the inhibition of the phase I enzymes and the induction of the phase II enzymes by polyphenols in H. cordata is helpful to animal cells. The reason why polyphenols are meaningful in the regulation of the xenobiotic-metabolizing enzyme system is unclear at present. We know that polyphenols could influence the combined processing of fat-soluble materials, which are activated by the phase I enzymes and DNA or protein, thus decreasing the toxicity and damage to cells caused by xenobiotic fat-soluble materials[25], but further study is required.

In conclusion, the OFO diet induces phase I and phase II enzyme activities, and the 2% H. cordata diet results in a better regulation of the xenobiotic-metabolizing enzyme system. Our findings suggest that polyphenol in H. cordata can be an important and necessary factor in the defense against CYP450-mediated cancers and other chronic diseases.