br Materials and methods br Results

Materials and methods
Results and discussion The dose-dependent inhibition of HMG-CoA reductase activity by the extracts as presented in Fig. 1 and the IC50 values in Table 1 showed that there was no significant (P>0.05) difference in the inhibitory abilities of the two extracts. The control of elevated cholesterol levels has become imperative in recent times as hypercholesterolemia has been established as a risk factor for atherosclerosis and other cardiovascular complications. In most cases, the ipi-145 of cholesterol synthesis is achieved by inhibition of HMG-CoA reductase, which is the rate limiting enzyme in cholesterol synthesis. Therefore, the inhibition of HMG-CoA reductase by the phenolic extracts from grapefruit peels would be beneficial in the management of cardiovascular complications. The HMG-CoA reductase inhibition by the phenolic extracts could be the possible mechanism for the cholesterol lowering effect of the peels in experimental animals as observed by previous studies [18]. Although, as shown in this study the extracts favourably compared with pravastatin in inhibiting the HMG-CoA reductase especially at higher concentrations, it should be noted that phenolic compounds, especially flavonoids are not structurally similar to statins. Statins inhibit HMG-CoA reductase activity by competing with HMG-CoA for the enzyme active site [19]. The competitive inhibition of pravastatin is made possible by its structural similarity to HMG-CoA by the possession of a double cyclic ring structure [20]. However, in the case of phenolic compounds, flavonoids such as genistein and diadzein have been shown to inhibit HMG-CoA reductase activity both competitively with HMG-CoA and non-competitively with NADPH [21]. While statins are widely used in the management of cardiovascular complications [22], recent studies have supported the use of HMG-CoA reductase inhibitors from natural sources due to the side effects of statins [23]. Furthermore, the extracts inhibited ACE activity in a dose-dependent manner (Fig. 2) with the bound phenolics having a stronger inhibitory ability than the soluble free phenolics. Another mechanism for the management of cardiovascular complications such as hypertension is the use of ACE inhibitors. ACE inhibition prevents the enzyme from catalysing the conversion of angiotensin I to the powerful vasoconstrictor, angiotensin II [24]. Phenolic-rich extracts from natural products have been shown from previous studies to inhibit ACE activity [11,25,26]. The HPLC analysis of the extracts revealed the presence of kaempferol, rutin and quercetin which have been shown from previous studies to be ACE inhibitors [27]. Angiotensin II does not only stimulate vasoconstriction, but also oxidative stress. Angiotensin II-induced oxidative stress has been shown to play a major role in the development of hypertension [28,29]. More so, the role of angiotensin II-induced reactive oxygen species (ROS) formation in atherogenesis has been well established [30]. It is also noteworthy that the anti-oxidative effects of ACE inhibitors contribute to their therapeutic effects in patients suffering from cardiovascular complications [31]. Also, hypercholesterolemia has been shown to induce oxidative stress in experimental animals [5]. Generally, oxidative stress has been implicated in the development of major cardiovascular conditions such as atherosclerosis, ischaemic heart disease and hypertension [2]. Therefore, the antioxidative effects of phenolic compounds might also contribute to the medicinal effects of grapefruit peels in the management of cardiovascular complications. The anti-oxidative effects of the phenolic extracts from the grapefruit peels was tested in endothelial cells EA.Hy 926 and also in cell free system using DPPH and ABTS assays. The dose-dependent DPPH* scavenging ability (Fig. 3) revealed that the bound phenolics had significantly higher scavenging ability than the free phenolics. Table 1 showed that the bound phenolics had significantly higher ABTS* scavenging ability than the free soluble phenolic extracts. Fig. 4 showed the time courses for peroxyl radical-induced oxidation of DCFH to DCF in human endothelial cells (EA.Hy 926) cells and the inhibition of oxidation by grapefruit peel phenolic extracts. Fig. 5 showed that the CAA units for the extracts at the concentrations tested (0.1–100μg/L) were 6.5–31.42% and 8.88–46.42% for the free and bound phenolics respectively.