Characterization, Modes of Synthesis, and Pleiotropic Effects of Hypocholesterolemic Compounds – A Review

Abstract: This study sought to investigate the possible mechanisms for the use of phenolic extracts from grapefruit peels in the management/prevention of cardiovascular complications. The effects of the phenolic extracts on key enzymes relevant to cardiovascular diseases [3-hydroxy-methyl-3-glutaryl coenzyme A reductase (HMG-CoA reductase) and angiotensin-I converting enzyme (ACE)], cellular antioxidant activity in human endothelial cells (EA.Hy 926) and radicals [1,1-diphenyl-2 picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS)] scavenging abilities were investigated. The phenolic contents of the extracts were investigated using HPLC–DAD. There was no significant ( P > 0.05) difference in the HMG-CoA reductase inhibitory ability of the two extracts, while the bound phenolic extracts had a stronger ACE inhibitory ability than the soluble free phenolics. The extracts also showed intracellular antioxidant activity in human endothelial (EA.Hy 926) cells. Furthermore, the bound phenolics had significantly higher radicals (DPPH* and ABTS*) scavenging abilities than the free phenolics. The HPLC analysis revealed the presence of flavonoids (quercetin and kaempferol), phenolics acids (resveratrol, gallic acid, ellagic acid and caffeic acid) and tannin (catechin). The cellular antioxidative properties and inhibition of enzymes relevant to the management of cardiovascular complications showed that grapefruit peels could be used as nutraceuticals for the management of such conditions.

Abstract: Citrus peels, which are an environmental menace in many developing countries have been in use in folk medicine for the management of some degenerative conditions, though there was limited information on the mechanism of such medicinal properties. These medicinal properties also promote the peels as functional foods; since they are generally regarded as safe and are consumed in some countries in forms of candies, wines, infusions and additives. Studies on the mechanisms for the antioxidant, anti-diabetic, cardioprotective, neuroprotective and anticancer activities of the peels have established the intereaction with some key enzymes relevant to the management of such diseases. The phenolic compounds and volatile compounds which have been linked with the biological activities of the peels have been characterized. This review looks at the bioactive compounds of citrus peels and the mechanisms for the biological activities of the peels.

Abstract: This study sought to characterize the soluble free and bound phenolic compounds from shaddock (Citrus maxima) peels and investigate their effect on 3-hydroxy-methyl-3-glutaryl coenzyme A reductase (HMG-CoA reductase) and glutathione-linked enzymes in colon (Caco-2) cells. The radicals scavenging ability and the protective ability of the phenolic extracts against pro-oxidant induced oxidative damage in Caco-2 cells and rat colon homogenates were also investigated. The free phenolics were extracted with 80% acetone (v/v), while bound phenolics were extracted from the alkaline (NaOH) and acid (HCl) hydrolyzed residue with ethyl acetate. HPLC fingerprinting revealed the presence of gallic acid, catechin, chlorogenic acid, caffeic acid, ellagic acid, epicatechin, rutin, quercitrin and quercetin in the extracts. The results revealed that the extracts inhibited HMG-CoA reductase activity and increased the activities of glutathione reductase and glutathione peroxidase in Caco-2 cells. The extracts inhibited peroxyl radical induced oxidation of membrane lipids in Caco-2 cells and malondialdehyde production in rat colon homogenates. Furthermore, the phenolic extracts scavenged radicals [2,2-diphenyl-1-picrylhydrazyl (DPPH) and hydroxyl (OH)] and chelated Fe2+ in a concentration-dependent manner. This study showed that shaddock peels could serve as a dietary means or nutraceutical source for protecting the colon from degeneration.

Abstract: Development of a novel method for the quantification of lovastatin is an interesting problem in the analytical field. In the literature, many reports use spectrophotometric method for the quantification of lovastatin. However, the analysis of fermentation broth containing lovastatin appears to be inaccurate using spectrophotometric method. Hence, the estimation of lovastatin produced by Monascus purpureus and pure lovastatin was attempted by UV-visible spectrophotometer as well as HPLC. It was observed that the analogues and/or intermediates of lovastatin synthesized in the fermentation broth and the products of fermentation caused superimposition effect on the absorption spectrum. Phosphate is a medium constituent for the production of lovastatin by the organism which contributed significantly to the superimposition of absorption spectrum. On the other hand, HPLC analysis consistently gave reliable results for the estimation of lovastatin under all the experimental conditions studied.

Abstract: Lovastatin is the first FDA-approved antihypercholesterolemic drug for the treatment of cardiovascular diseases with pleiotropic clinical applications. Monascus purpureus is one of the safest molds for the production of lovastatin that has been used in the preparation of Chinese medicine Red-Yeast Rice. This investigation dealt with the effect of both static and dynamic culture conditions on the morphological change and localization of lovastatin in Monascus purpureus. In dynamic culture condition, pellet morphology was observed and the maximum intra- and extra-cellular components of lovastatin including both β-hydroxy acid and lactone forms were found to be 1043.45 and 207.94 μg/l, respectively. Filamentous (mat) form of morphology was observed in the static culture condition and the intra- and extracellular concentration of lovastatin were 677.9 and 789.2 μg/l. Taguchi’s L12 (112) orthogonal arrays was employed to find optimal conditions for the submerged production of lovastatin and for the growth of Monascus purpureus. Three physical and five chemical variables were considered in the current experimental study. The maximum production of lovastatin was observed to be 3.66 mg/l. Among the chemical parameters, MnSO4 and MgSO4 were the most significant parameters for the production of lovastatin. Physical parameters, viz., agitation rate and temperature, were also equally significant to that of medium constituents for the production of lovastatin. The significance of the variables on the biomass growth was just opposite to that the results of lovastatin production. Ammonium chloride was the most significant parameter among the variables studied for the growth of M. purpureus, followed by glucose and phosphorous sources (KH2PO4). Statistical analysis showed that those parameters were significant with more than 99 % confidence (p<0.01).

Abstract: Flavonoids are a class of secondary plant phenolics with significant antioxidant and chelating properties. In the human diet, they are most concentrated in fruits, vegetables, wines, teas and cocoa. Their cardioprotective effects stem from the ability to inhibit lipid peroxidation, chelate redox-active metals, and attenuate other processes involving reactive oxygen species. Flavonoids occur in foods primarily as glycosides and polymers that are degraded to variable extents in the digestive tract. Although metabolism of these compounds remains elusive, enteric absorption occurs sufficiently to reduce plasma indices of oxidant status. The propensity of a flavonoid to inhibit free-radical mediated events is governed by its chemical structure. Since these compounds are based on the flavan nucleus, the number, positions, and types of substitutions influence radical scavenging and chelating activity. The diversity and multiple mechanisms of flavonoid action, together with the numerous methods of initiation, detection and measurement of oxidative processes in vitro and in vivo offer plausible explanations for existing discrepancies in structure-activity relationships. Despite some inconsistent lines of evidence, several structure-activity relationships are well established in vitro. Multiple hydroxyl groups confer upon the molecule substantial antioxidant, chelating and prooxidant activity. Methoxy groups introduce unfavorable steric effects and increase lipophilicity and membrane partitioning. A double bond and carbonyl function in the heterocycle or polymerization of the nuclear structure increases activity by affording a more stable flavonoid radical through conjugation and electron delocalization. Further investigation of the metabolism of these phytochemicals is justified to extend structure-activity relationships (SAR) to preventive and therapeutic nutritional strategies.

Abstract: HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase (HMGR) catalyzes the committed step in cholesterol biosynthesis. Statins are HMGR inhibitors with inhibition constant values in the nanomolar range that effectively lower serum cholesterol levels and are widely prescribed in the treatment of hypercholesterolemia. We have determined structures of the catalytic portion of human HMGR complexed with six different statins. The statins occupy a portion of the binding site of HMG-CoA, thus blocking access of this substrate to the active site. Near the carboxyl terminus of HMGR, several catalytically relevant residues are disordered in the enzyme-statin complexes. If these residues were not flexible, they would sterically hinder statin binding.

Abstract: Statins are the treatment of choice for the management of hypercholesterolaemia because of their proven efficacy and safety profile. They also have an increasing role in managing cardiovascular risk in patients with relatively normal levels of plasma cholesterol. Although all statins share a common mechanism of action, they differ in terms of their chemical structures, pharmacokinetic profiles, and lipid-modifying efficacy. The chemical structures of statins govern their water solubility, which in turn influences their absorption, distribution, metabolism and excretion. Lovastatin, pravastatin and simvastatin are derived from fungal metabolites and have elimination half-lives of 1-3 h. Atorvastatin, cerivastatin (withdrawn from clinical use in 2001), fluvastatin, pitavastatin and rosuvastatin are fully synthetic compounds, with elimination half-lives ranging from 1 h for fluvastatin to 19 h for rosuvastatin. Atorvastatin, simvastatin, lovastatin, fluvastatin, cerivastatin and pitavastatin are relatively lipophilic compounds. Lipophilic statins are more susceptible to metabolism by the cytochrome P(450) system, except for pitavastatin, which undergoes limited metabolism via this pathway. Pravastatin and rosuvastatin are relatively hydrophilic and not significantly metabolized by cytochrome P(450) enzymes. All statins are selective for effect in the liver, largely because of efficient first-pass uptake; passive diffusion through hepatocyte cell membranes is primarily responsible for hepatic uptake of lipophilic statins, while hydrophilic agents are taken up by active carrier-mediated processes. Pravastatin and rosuvastatin show greater hepatoselectivity than lipophilic agents, as well as a reduced potential for uptake by peripheral cells. The bioavailability of the statins differs greatly, from 5% for lovastatin and simvastatin to 60% or greater for cerivastatin and pitavastatin. Clinical studies have demonstrated rosuvastatin to be the most effective for reducing low-density lipoprotein cholesterol, followed by atorvastatin, simvastatin and pravastatin. As a class, statins are generally well tolerated and serious adverse events, including muscle toxicity leading to rhabdomyolysis, are rare. Consideration of the differences between the statins helps to provide a rational basis for their use in clinical practice.

Abstract: Cholesteryl ester transfer protein (CETP) promotes the transfer of cholesteryl esters from antiatherogenic HDLs to proatherogenic apolipoprotein B (apoB)-containing lipoproteins, including VLDLs, VLDL remnants, IDLs, and LDLs. A deficiency of CETP is associated with increased HDL levels and decreased LDL levels, a profile that is typically antiatherogenic. Studies in rabbits, a species with naturally high levels of CETP, support the therapeutic potential of CETP inhibition as an approach to retarding atherogenesis. Studies in mice, a species that lacks CETP activity, have provided mixed results. Human subjects with heterozygous CETP deficiency and an HDL cholesterol level >60 mg/dL have a reduced risk of coronary heart disease. Evidence that atherosclerosis may be increased in CETP-deficient subjects whose HDL levels are not increased is difficult to interpret and may reflect confounding or bias. Small-molecule inhibitors of CETP have now been tested in human subjects and shown to increase the concentration of HDL cholesterol while decreasing that of LDL cholesterol and apoB. Thus, it seems important and timely to test the hypothesis in randomized trials of humans that pharmacological inhibition of CETP retards the development of atherosclerosis.

Abstract: The beneficial effects of statins are the result of their capacity to reduce cholesterol biosyntesis, mainly in the liver, where they are selectively distributed, as well as to the modulation of lipid metabolism, derived from their effect of inhibition upon HMG-CoA reductase. Statins have antiatherosclerotic effects, that positively correlate with the percent decrease in LDL cholesterol. In addition, they can exert antiatherosclerotic effects independently of their hypolipidemic action. Because the mevalonate metabolism generates a series of isoprenoids vital for different cellular functions, from cholesterol synthesis to the control of cell growth and differentiation, HMG-CoA reductase inhibition has beneficial pleiotropic effects. Consequently, statins reduce significantly the incidence of coronary events, both in primary and secondary prevention, being the most efficient hypolipidemic compounds that have reduced the rate of mortality in coronary patients. Independent of their hypolipidemic properties, statins interfere with events involved in bone formation and impede tumor cell growth.