Malaria protection in hereditary ovalocytosis: relation to red cell deformability, red cell parameters and degree of ovalocytosis.

In the culture of red cells with Plasmodium falciparum, erythrocytes from both Thai patients and subjects (patient's parents) with hereditary ovalocytosis have a protective effect against malarial infection. High percentage of ovalocyte (75-100%) was found in patients whereas their parents had lower percentage (25-50%). Invasion index (II) and multiplication ratio (MR) of P. falciparum in these abnormal red cells from the patients were significantly decreased as compared to those in normal red cells (patients: II = 1.52 +/- 0.91, MR = 8.83 +/- 6.73; normal subjects: II = 4.45 +/- 1.51, MR = 25.23 +/- 6.25). This suggests that the red cells from these patients had significant degree of malaria protection. The significant protection was also shown in red cells from the parent group (II = 1.86 +/- 0.81, MR = 15.69 +/- 3.50). Although the parents had lower ovalocyte percentage, degree of protection against malaria parasite was as effective as those found in patients with high ovalocytic red cells. This has been confirmed by statistical analysis showing nonsignificant difference in II value between the two groups. In contrast, red cells of both groups had poor deformability (deformability index, DI) as compared to the normal group. No statistically different DI values were demonstrated between the two. This indicates that poorly deformable red cells, not their ovalocytic shape, make a significant contribution to limitation of malaria parasite invasion. The MR values in patients were less than those found in the parent group but statistical analysis showed no significant difference. Reduced MR values were found with increased numbers of microcytic, hyperchromic and hypochromic red cells in patients.

Biotechnology research in Thailand and applications to the study of animal parasites and their vectors.

Biotechnology research in Thailand owes its origins to the strength in biomedical and life sciences in the academia, and the importance of agriculture in the economy. With growing awareness of the impact of new biotechnology including genetic engineering, biotechnology R & D centres were set up in the universities, and the National Centre for Genetic Engineering and biotechnology was created in 1983. The National Center functions as the center for policy and planning in biotechnology, for support of important research, development and technology transfer projects in designated institutions, and serves to link these institutions with the private sector. The aim is to develop specific biotechnology areas from laboratory stages up to pilot-scale, with emphasis on transfer and utilization of genetic engineering and biotechnology in various fields including public health had on strengthening of basic infrastructure in relevant disciplines. The National Center has 4 affiliated laboratories, including pilot plants, and over 30 projects in 9 institutions in the network. Recently the Science and Technology for Development Program has also devoted a part of its substantial funding to support various biotechnology research projects. With regard to biotechnology research relevant to the study of animal parasites and their vectors, the work in Thailand has up to now concentrated more on the application of new techniques in clinical laboratory and field work than in industrial productions. Specific contributions from the Unit of Parasite Biochemistry, Mahidol University, were given as illustrative examples.

The interaction between the malaria parasite and the red cell membrane.

The malaria parasite intimately interacts with the host red cell membrane throughout the cycle of invasion and intracellular development. Direct interaction between the merozoite surface and the red cell membrane involves specific binding between the surface components of both cells, which leads to the subsequent endocytotic process still incompletely understood. Intracellular development of the parasite is accompanied by various changes in the structure and function of red cell membrane components. Some changes may benefit parasite survival while others trigger host immune response. An understanding of both the direct interaction between the surface components of the parasite and the red cell during invasion, and the subsequent changes in the red cell membrane following invasion, should lead to better ways of controlling malaria.

Biochemical aspects of drug action and resistance in malaria parasites.

Biochemical aspects of action of antifolates and 4-aminoquinolines and their resistance in the malaria parasites are reviewed, with emphasis on pyrimethamine and chloroquine respectively. Resistance to pyrimethamine has been shown to be associated with either an increase in the amount of parasite dihydrofolate reductase or a reduced affinity of the enzyme for drug binding, in line with the presence of a distinctive pathway for folate metabolism. The theories for drug synergism in the folate pathway are discussed with respect to resistance to pyrimethamine and its combination with sulpha drugs. The biochemical basis for chloroquine resistance is still unclear, reflecting incomplete understanding of its mechanism of action. Data implicating the role of haemozoin and other components as a putative chloroquine receptor of the parasites are reviewed, and possible explanations for resistance are discussed.

Nimbolide, a constituent of Azadirachta indica, inhibits Plasmodium falciparum in culture.

The terpenoid lactone nimbolide, the structure of which has been unambiguously established, was found to inhibit Plasmodium falciparum in culture with a moderate potency. The EC50 against the parasite line K1 from Thailand was approximately 2.0 microM (0.95 microgram/ml). The EC50 of crude aqueous extract of Azadirachta indica var. siamensis (Sadao tree), was 115 micrograms/ml, and of crude ethanol extract was 5.0 micrograms/ml. Since nimbolide is a major constituent in these extracts, it could account substantially for their inhibitory activity. However, neither the crude extracts nor nimbolide showed any activity in vivo against Plasmodium berghei in the mouse either through ingestion (746 mg aqueous extract, 62.5 mg ethanol extract or 12.5 mg nimbolide/kg/day), or subcutaneous injection (93 mg aqueous extract, 31 mg ethanol extract or 12.5 mg nimbolide/kg/day).