ABSTRACT
Two structurally related triterpenoids 1 and 2 from pink peppercorn (berries of Schinus terebinthifolius) are identified and characterized as active site-directed specific competitive inhibitors of the three classes of secreted 14 kDa phospholipase A2. The inhibitors not only protect the active site histidine from alkylation but also inhibit the action of secreted phospholipase A2 from pig pancreas, human synovial fluid, and bee venom. Detailed X-ray crystallographic results on the structures of the inhibitors are provided. By physical methods and X-ray crystallography the triterpenoids were identified as masticadienoic acid and masticadienolic acid (schinol). Several other triterpenoids were ineffective as inhibitors of phospholipase A2; however certain ganoderic acid derivatives showed noticeable inhibition. Results show that the side chain of these acidic tetracyclic terpenoids can access the catalytic-site region of phospholipase A2, whereas the acyclic nucleus is at the interfacial recognition region. The selectivity of the assay protocol used here is demonstrated by the fact that the original screen of ethyl acetate extracts of 60 commercially available spices and herbs was carried out with phospholipase A2 from pig pancreas, and only one extract showed inhibitory action on the hydrolytic activity in the scooting mode. Under such assay conditions the enzyme remains tightly bound to the surface of the substrate vesicles. In this way, nonspecific effects of additives that promote desorption of the enzyme from the substrate vesicle surface, under conditions in which the binding of the enzyme to the vesicle is weak, are avoided. The assay protocol is useful for the kinetic characterization of the inhibitors of phospholipase A2, and it does not give false positive results with amphiphilic and hydrophobic compounds, as is the case with virtually all assay systems in use.
Subject(s)
Phospholipases A/antagonists & inhibitors , Plants/chemistry , Triterpenes/pharmacology , Animals , Bee Venoms/enzymology , Binding Sites , Catalysis , Crystallography, X-Ray , Humans , Hydrogen Bonding , Models, Molecular , Pancreas/enzymology , Phospholipases A/metabolism , Phospholipases A2 , Spectrometry, Fluorescence , Spectrophotometry, Ultraviolet , Swine , Synovial Fluid/enzymology , Triterpenes/analysis , Triterpenes/chemistryABSTRACT
C26H41NO7, Mr = 479.61, triclinic, P1, a = 11.894 (4), b = 12.360 (2), c = 9.969 (2) A, alpha = 106.11 (2), beta = 100.68 (2), gamma = 99.02 (2) degrees, V = 1349.5 (6) A3, Z = 2, Dx = 1.180 Mg m-3, Mo K alpha, lambda = 0.71069 A, mu = 0.079 mm-1, F(000) = 520, T = 296 K, R = 0.041, wR = 0.047 for 3196 observed unique reflections. A new approach to the total synthesis of the nargenicin macrolide system employing a [2,3] Wittig rearrangement to control the remote C16-C17 stereochemistry required characterization of a pivotal intermediate by X-ray single-crystal methods. The structure is characterized by the formation of strong intermolecular alcohol to dihydrooxazole-N hydrogen bonds, an essentially strain-free tricyclic oxa-bridged octalin nucleus, and low-energy conformations of the methoxymethyl ether side chains.
Subject(s)
Anti-Bacterial Agents/chemistry , Macrolides , Lactones/chemistry , Models, Molecular , Molecular Structure , X-Ray DiffractionABSTRACT
Single-crystal x-ray diffraction methods have been used to directly observe and simultaneously determine the molecular structure of the UVA induced cis-syn photocycloaddition product in a partially photolyzed single crystal of a psoralen(pyrone ring side)-DNA(thymine) interaction model compound, 1'-(8-oxypsoralen)-8'(thym-1"yl)3',6'-dioxaoctane.
