Malaria is still a major health problem in many developing countries. The parasite responsible for the vast majority of fatal malarial infections is Plasmodium falciparum. The first effective antimalarial drug was quinine, which was isolated from the bark of Cinchona. Since then malaria has been treated with quinoline based drugs such as quinine, chloroquine, mefloquine and primaquine. Unfortunately, many Plasmodium strains have now become resistant to these drugs. Artemisinin, a sesquiterpene-lactone is a novel antimalarial drug isolated from Artemisia annua . It may meet the dual challenges posed by drug resistant parasites and rapid progression of malarial illness. The relatively low yield of artemisinin is, however, a serious limitation to the commercialization of this drug. Hence, optimization of artemisinin yield either in vivo or in vitro is highly desirable. One of the approaches to enhance the production of this compound in vivo is through the regulation of rate limiting enzyme(s) of its biosynthetic pathway employing both exogenous and endogenous factors.

It has recently been shown that isoprenoid compounds are produced via two pathways. Plastid isoprenoids are formed via the novel pathway or Rohmer pathway, while cytosolic isoprenoids like sesquiterpenes, triterpenes, polyterpenes and sterols produced via common biosynthetic pathway i.e. acetate-mevalonate. Henceforth, to confirm the role of alternate pathway in artemisinin biosynthesis, feeding experiments were performed by us with radioactive precursors. The results obtained in these studies strongly suggest that artemisinin is synthesized via acetate/mevalonate rather than Rohmer pathway. Isolation and assay protocols of HMG-CoA reductase, the key enzyme of acetate/mevalonate pathway, for A. annua L. leaves were standardized. The HMG-CoA reductase activity and artemisinin content were monitored in the leaves of A. annua L. at different phenological stages and a strong correlation was observed between the enzyme activity and artemisinin content throughout the growth period of the crop. The combined application of 100 ppm growth hormones (IAA+GA3) at the vegetative stage resulted into optimum activity of HMG-CoA reductase and the higher contents of artemisinin as well as its immediate precursors, viz. artemisinic acid and arteannuin B. The in vivo and in vitro regulations of HMG-CoA reductase activity were studied employing exogenous (light and temperature) and endogenous factors (precursors and products of isoprenoid pathway). Both these factors were significantly modulated the activity of this enzyme both in vivo as well as in vitro conditions. Finally, the enzyme was purified and characterized from the leaves of A. annua L. plants. One major band of molecular weight of 60 Kda was present in 45-60% (NH4 )2 SO4 fraction while absent in other (NH4 )2 SO4 fraction. This fraction also showed the maximum HMG-CoA reductase activity. This band, therefore, may represent the HMG-CoA reductase protein. Six fold purification of the enzyme was achieved by (NH4)2SO4 precipitation. The thiol compounds were found necessary for the long term stability of HMG-CoA reductase enzyme.