Comparative effects of β-carbolines on platelet aggregation and lipid membranes

Membrane Interactions of Phytochemicals as Their Molecular Mechanism Applicable to the Discovery of Drug Leads from Plants

In addition to interacting with functional proteins such as receptors, ion channels, and enzymes, a variety of drugs mechanistically act on membrane lipids to change the physicochemical properties of biomembranes as reported for anesthetic, adrenergic, cholinergic, non-steroidal anti-inflammatory, analgesic, antitumor, antiplatelet, antimicrobial, and antioxidant drugs. As well as these membrane-acting drugs, bioactive plant components, phytochemicals, with amphiphilic or hydrophobic structures, are presumed to interact with biological membranes and biomimetic membranes prepared with phospholipids and cholesterol, resulting in the modification of membrane fluidity, microviscosity, order, elasticity, and permeability with the potencies being consistent with their pharmacological effects. A novel mechanistic point of view of phytochemicals would lead to a better understanding of their bioactivities, an insight into their medicinal benefits, and a strategic implication for discovering drug leads from plants. This article reviews the membrane interactions of different classes of phytochemicals by highlighting their induced changes in membrane property. The phytochemicals to be reviewed include membrane-interactive flavonoids, terpenoids, stilbenoids, capsaicinoids, phloroglucinols, naphthodianthrones, organosulfur compounds, alkaloids, anthraquinonoids, ginsenosides, pentacyclic triterpene acids, and curcuminoids. The membrane interaction’s applicability to the discovery of phytochemical drug leads is also discussed while referring to previous screening and isolating studies.

Acetylsalicylic acid (ASA) increases the solubility of cholesterol when incorporated in lipid membranes

Cholesterol has been well established as a mediator of cell membrane fluidity. By interacting with lipid tails, cholesterol causes the membrane tails to be constrained thereby reducing membrane fluidity, well known as the condensation effect. Acetylsalicylic acid (ASA), the main ingredient in aspirin, has recently been shown to increase fluidity in lipid bilayers by primarily interacting with lipid head groups. We used high-resolution X-ray diffraction to study both ASA and cholesterol coexisting in model membranes of dimyristoylphosphatidylcholine (DMPC). While a high cholesterol concentration of 40 mol% cholesterol leads to the formation of immiscible cholesterol bilayers, as was reported previously, increasing the amount of ASA in the membranes between 0 to 12.5 mol% was found to significantly increase the fluidity of the bilayers and dissolve the cholesterol plaques. We, therefore, present experimental evidence for an interaction between cholesterol and ASA on the level of the cell membrane at elevated levels of cholesterol and ASA.

Identification and mode of action of 5-hydroxymethyl-2-furfural (5-hmf) and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA) as potent xanthine oxidase inhibitors in vinegars

Vinegars have been used as an alternative remedy for treating gout, but the scientific basis remains to be elucidated. In this study, seven commercial vinegars and one laboratory-prepared red-koji vinegar were evaluated for the inhibitory activity of xanthine oxidase (XO), a critical enzyme catalyzing uric acid formation. Red-koji vinegar exhibited potent xanthine oxidase inhibitory (XOI) activity and was used for isolating active compounds. The substances under two peaks with XOI activity from HPLC were identified as 5-hydroxymethyl-2-furfural (5-HMF) and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA), by LC-MS-MS and NMR. The XO half-maximal inhibitory concentrations (IC(50)) of 5-HMF and MTCA were 168 and 860 μg/mL, respectively. In further mode-of-action analysis, the inhibitory mechanism of each compound was elucidated at the IC(50) level in the presence of various concentrations of xanthine as the substrate. The following Michaelis-Menten kinetics analysis of XO inhibition revealed uncompetitive and competitive patterns for 5-HMF and MTCA, respectively.

Comparative Effects of α-, β-, and γ-Carbolines on Platelet Aggregation and Lipid Membranes

Cigarette smoking and alcohol consumption possibly affect platelet functions. To verify the hypothesis that some α-, β-, and γ-carboline components in cigarette smoke and alcoholic beverages may change platelet aggregability, their effects on human platelets were determined by aggregometry together with investigating their membrane effects by turbidimetry. Carbolines inhibited platelet aggregation induced by five agents with the potency being 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole > 3-amino-1-methyl-5H-pyrido[4,3-b]indole > 1-methyl-9H-pyrido[3,4-b]indole. The most potent 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole showed 50% aggregation-inhibitory concentrations of 6–172 μM. Both γ-carbolines interacted with phosphatidylcholine membranes to lower the lipid phase transition temperature with the potency correlating to the antiplatelet activity, suggesting that the interaction with platelet membranes to increase their fluidity underlies antiplatelet effects. Given their possible concentration and accumulation in platelets, γ- and β-carbolines would provide cigarette smokers and alcohol drinkers with reduced platelet aggregability, and they may be responsible for the occurrence of hemorrhagic diseases associated with heavy smoking and alcoholics.