As shown in Table the extract

As shown in Table 3, the extract also inhibited Fe2+-induced lipid peroxidation in the kidney, liver and lungs tissues homogenates of rats. Its IC50 values varied significantly (P<0.05) such that that of the liver (36.97±2.06μg/mL)>lungs (26.65±1.82μg/mL)>kidney (20.39±1.19μg/mL); and its pattern of inhibition was also dose-dependent as shown in Fig. 3.
Discussion Dietary spices are recognized as natural antioxidant agents and inhibitors of pro-oxidant enzymes such as XO. These beneficial activities are attributable to the flavonoids and other polyphenols present in the spices [33]. Hence, we evaluated the inhibitory effect of the phenolic extract of T. tetraptera fruits, a dietary spice, on XO and Fe2+-induced lipid peroxidation in the kidney, liver, and lungs tissues of rats in vitro. The important roles the kidney, liver and lungs play as organs of metabolism and excretion in mammals, and the previously reported XO activity in these organs [6,13], informed the choice of their homogenates as the source of the enzyme. The chromatographic analysis of phenolic compounds (Table 1) revealed that T. tetraptera fruits are rich in flavonoids (apigenin, quercetin, rutin, epicatechin, luteolin and catechin) and phenolic acids (caffeic acid, ellagic acid, gallic AICAR and chlorogenic acid). These two classes of natural phenolic compounds are regarded to be of pharmacological importance [34], as they possess diverse health benefits, including anti-inflammatory activity [35]. They can modulate the expression of pro-inflammatory signals and ameliorate inflammatory diseases such as arthritis like other natural polyphenolics [19,20]. As part of their anti-inflammatory activity, polyphenolics including the flavonoids and phenolic acids have been reported to inhibit XO activity by recent studies [33,36,37]. Probably, like allopurinol, they inhibit XO by binding at its purine binding site [38], thereby blocking the ultimate formation of uric acid. In addition to their ability to inhibit XO, phenolics are well-known for their antioxidant activities [39], and ability to inhibit Fe2+-induced lipid peroxidation [34]. The flavonoids exhibit antioxidant activity by acting as both electrons donors and terminators of chain reactions; and this is due the hydroxyl groups they possess, particularly at the 3′OH and 4′OH of their three-carbon chain [40,41]. Phenolic acids, on the other hand, have a phenolic ring and an organic carboxylic acid function [42], which make them capable of stabilizing and delocalizing unpaired electrons [43]. These distinguishing structural features make flavonoids and phenolic acids prominent antioxidant phenolic compounds. Inhibitors of XO are used clinically for the treatment of hyperuricaemia and gouty arthritis; as they help in reducing the levels of uric acid in circulation, and vascular oxidative stress [44]. In this regard, the efficacy of plant-derived polyphenolics in inhibiting XO and alleviating the resultant hyperuricaemia has been reported; and as natural components of food, they are regarded to be safer than synthetic XO inhibitors such as allopurinol [33]. Their inhibitory effect against XO has been attributed to the possibility of the C-5 and C-7 hydroxyl groups of flavones and flavonols to replace the C-2 and C-6 ones of xanthine in the XO active site [45,46], due to the mutual inter-convertibility of the carboxyl structures of xanthine to hydroxyl groups [47]. In this study, the phenolic extract of T. tetraptera fruits effectively inhibited XO in the kidney, liver and lungs tissues homogenates of rats. Interestingly, the IC50 values of the extract on the XO from these three tissues (kidney: 39.53±1.02μg/mL; liver: 45.71±1.44μg/mL; lungs: 33.87±0.96μg/mL) (Table 2) are in the range of the IC50 values earlier reported for Erythrina stricta leaves extract (21.2μg/mL) [17], and Olea europaea leaf extract (42μg/mL) [36] on XO. Furthermore, the order of the IC50 values of the extract on the XO from these three tissues (liver>kidney>lungs) may suggest a higher activity of the XO in the liver homogenate than in the kidney and the lungs homogenates. This supports an earlier report that the distribution XO activity is highest in the liver followed by the kidney and other tissues, as demonstrated by Carro et al. [14]. Similarly, Liu et al. [48] reported that XO is present in significant concentration in the liver. It is noteworthy that apigenin, the most abundant of the phenolics in T. tetraptera fruit from our HPLC-DAD result, was earlier reported to possess the strongest XO inhibitory activity compared with other phenolics in O. europaea leaf extract [36].