Margaret F. Roberts

Potentiation of the antimalarial activity of qinghaosu by methoxylated flavones.

Interaction between the flavones casticin and artemetin and the antimalarial activity of chloroquine and qinghaosu (QHS) was examined using an in vitro growth assay based on [3H]hypoxanthine incorporation in synchronized cultures of a cloned line of Plasmodium falciparum. Casticin, and to a lesser extent artemetin, selectively enhanced the inhibition of growth by QHS, but had little effect on the activity of chloroquine. The findings suggest that flavones indigenous to Artemisia annua, from which QHS is isolated, might significantly alter the clinical potential of this novel antimalarial drug in the treatment of chloroquine-resistant malaria.

Polyphenolic compounds from Croton lechleri

The blood-red sap of Croton lechleri was found to contain proanthocyanidins as major constituents which accounted for up to 90% of the dried weight. In addition to (+)-catechin, (−)-epicatechin, (+)-gallocatechin, (−)-epigallocatechin and dimeric procyanidins B-1 and B-4, five novel dimers and trimers were isolated and characterized as catechin-(4α→8)- epigallocatechin, gallocatechin-(4α→8)-epicatechin, gallocatechin-(4α→6)-epigallocatechin, catechin-(4α→8)- gallocatechin-(4α→8)-gallocatechin and gallocatechin-(4α→8)-gallocatechin-(4α→8)-epigallocatechin. Higher oligomers were also obtained. A new procedure combining chemical degradation with 1H NMR spectroscopy has been developed for determination of the composition and molecular size of oligomeric/polymeric proanthocyanidins. The oligomers of the sap were shown to have the mean degree of polymerization of 4,5–6 and 6–7, respectively, and Mr up to 2130. The heterogeneity of the oligomers was clearly indicated by the presence of a variety of flavan-3-ols as extension and terminal units. An exceptionally high content of gallocatechin and epigallocatechin in the oligomers was observed.

Antimalarial activity of Artemisia annua flavonoids from whole plants and cell cultures

SummaryCell suspension cultures developed from Artemisia annua exhibited antimalarial activity against Plasmodium faldparum in vitro both in the n-hexane extract of the plant cell culture medium and in the chloroform extract of the cells. Trace amounts of the antimalarial sesquiterpene lactone artemisinin may account for the activity of the n-hexane fraction but only the methoxylated flavonoids artemetin, chrysoplenetin, chrysosplenol-D and cirsilineol can account for the activity of the chloroform extract. These purified flavonoids were found to have IC50 values at 2.4 – 6.5 × 10−5M against P. falciparum in vitro compared with an IC50 value of about 3 × 10−8M for purified artimisinin. At concentrations of 5 × 10−6M these flavonoids were not active against P. falciparum but did have a marked and selective potentiating effect on the antiplasmodial activity of artemisinin.

Localization of enzymes and alkaloidal metabolites in Papaver latex

In continuing studies on the metabolic activity of Papaver somniferum, latex has been examined for its enzyme and alkaloidal metabolite content. After an initial centrifugation of latex at 1000g, the pellet which contained a heterogeneous population of dense organelles was further resolved on sucrose gradients. Of the enzymes monitored, acid phosphatase and L-3,4-dihydroxyphenylalanine decarboxylase were found to be in the latex 1000g supernatant, whereas catecholase (polyphenolase) was localized in two distinct organelles within the 1000g sediment. The lighter organelles, sedimenting at 30% sucrose, contained a soluble enzyme which was readily released on organelle plasmolysis, whereas the catecholase found within the heavier organelles, sedimenting at 55-60% sucrose, was membrane bound and showed significant activity only in the presence of Triton X-100. These latter organelles also contained the alkaloids, including morphine and thebaine, and were observed to readily accumulate [14CH3]morphine. The alkaloid precursor, dopamine, was localized in the same dense vesicle fraction as the alkaloids. The rate of uptake of [7-14C]dopamine into these fractions at room temperature, however, was markedly lower than that of morphine. Electron microscopic examination of the organelles of various densities revealed that they possessed different morphology. The results are consistent with the concept that both the 1000g and supernatant fractions of the latex are required for alkaloid biosynthesis and that a subpopulation of dense organelles found in the 1000g sediment have at least a function as a storage compartment for both alkaloids and their catecholamine precursor.

High-yield production of tropane alkaloids by hairy-root cultures of aDatura candida hybrid

Hairy root cultures were obtained following inoculation of the stems of sterile plantlets of aDatura candida hybrid withAgrobacterium rhizogenes. The scopolamine and hyoscyamine content was quantified by HPLC and compared with the non-transformed plants. The alkaloid yield (0.68% dry weight) obtained with the hairy roots was 1.6 and 2.6 times the amount found in the aerial parts and in the roots of the parent plants, respectively. Only a small proportion of alkaloids was released into the growth medium. Scopclamine was the principal alkaloid and the scopolamine/hyoscyamine ratio of ca. 5:1 makes these hairy roct cultures worthy of consideration as a source of scopolamine.

Chorismate mutase in microorganisms and plants

The enzymology of chorismate mutase in plants and microorganisms is reviewed to include occurrence, gene-enzyme relationships, reaction mechanisms and details of active site studies.

Anthranilate synthase in microorganisms and plants

The enzymology of anthranilate synthase (EC 5.4.99.6) in microorganisms and plants is reviewed. Aminoacid sequences of the enzyme subunits in different species are compared, and the mechanism of reaction is discussed.

Compartmentation of Alkaloid Synthesis, Transport, and Storage

The formation and sequestration of secondary metabolites is often regulated in space and time and correlated with differentiation (Wiermann, 1981). The importance of this correlation has been encountered in undifferentiated plant cell cultures, which either do not produce an alkaloid that is typically present in the intact plant or they accumulate the compounds in very low yields. The comparably few systems that produce high amounts of alkaloids are unfortunately rather the exception (Chapter 7). On the other hand, differentiated organ cultures, such as root or shoot cultures, are very reliable production systems, indicating that it is not the in vivo condition that is responsible for low yields in cell suspension cultures, but rather the degree of differentiation, i.e., differential gene expression of the enzymes involved in alkaloid biosynthesis and alkaloid storage.

Antiplasmodial activity of Artemisia annua plant cell cultures.

Extracts of Artemisia annua cultures have been assessed for in vitro activity against the malarial parasite Plasmodium falciparum. Callus and suspension cells and medium were analysed and examined for their activity at different stages of growth and development. Time-course experiments were carried out to investigate the influence of various basal media, plant growth regulators and light on both growth and possible artemisinin production. Two active fractions were obtained but artemisinin was not detected.

N-methyltransferases and 7-methyl-N9-nucleoside hydrolase activity in Coffea arabica and the biosynthesis of caffeine☆

Abstract The incorporation of radioactivity from L -[ 14 CH 3 ]-methionine into caffeine by coffee fruits was enhanced by additions of theobromine and paraxanthine but was reduced by additions of theophylline and caffeine. Cell-free extracts prepared from seedlings, partially ripe and unripe coffee fruits showed that only the unripe green fruits contained significant methyltransferase and 7-methyl- N 9 -nucleoside hydrolase activity. The cell-free extracts catalysed the transfer of methyl groups from S -adenosyl- L -[ 14 CH 3 ]-methionine to 7-methylxanthine, and 7-methylxanthosine, producing theobromine and to theobromine producing caffeine. The two enzymic methylations exhibited a sharp pH max at 8.5 and a similar pattern of effects with metal chelators, thiol reagents and Mg 2+ ions, which were slightly stimulating though not essential to enzyme activity. Paraxanthine (1,7-dimethylxanthine) was sh own to be the most active among methylxanthines as methyl acceptors; however its formation from 1-methylxanthine and 7-methylxanthine was not detectable, and biosynthesis from paraxanthine in the intact plant would therefore appear not to occur. The apparent K m values are as follows: 7-methylxanthine 0.2 mM, theobromine 0.2 mM, paraxanthine 0.07 mM and S -adenosyl- L -methionine with each substrate 0.01 mM. The results suggest the pathway for caffeine biosynthesis in Coffea arabica is: 7-methylxanthosine → 7-methylxanthine → theobromine → caffeine.