טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentElnekave Keren
SubjectCatabolism of L-arginine in Entamoeba histolytica:
Involvement in the Growth and in the Virulence
of the Parasite
DepartmentDepartment of Medicine
Supervisor Professor Serge Ankri


Abstract

Entamoeba histolytica is a protozoan parasite that causes intestinal disease in humans. The illness associated with this parasite, named amebiasis, accounts for 50 million intestinal infections worldwide each year with resultant 100 thousand deaths annually. Among the world parasitic diseases, Amebiasis is placed second to Malaria in mortality due to a protozoan parasite.  Entamoeba parasites are transmitted from human to human via fecal-oral contamination. The amoeba life cycle is reduced to two stages: The disease causing trophozoite form and the infectious cyst form.

The amoebic trophozoites normally reside in the human large intestine but occasionally they invade the intestinal mucosa, and are disseminated to other organs, mainly the liver. Understanding how the parasite evades the host response remains a critical issue in managing amoebiasis.


In this study we discuss the molecular cloning and expression of E.histolytica arginase, an enzyme that catalyses the hydrolysis of L-arginine to L-ornithine and urea. Nitric oxide (NO) is an antimicrobial agent that inhibits some key enzymes in the metabolism of E.histolytica.  NO is synthesized by activated macrophages from L-arginine, the substrate of NO synthase (iNOS). We show that E.histolytica inhibits the NO mediated amebicidal activity of activated macrophages by hydrolyzing L-arginine present in the media. By using a recombinant EhArg (rEhArg), we demonstrate that this enzyme is central to the inhibition mechanism of NO synthesis.

In addition, we show that the catabolism of L-arginine is required for the growth of the parasite. L-norvaline, a competitive inhibitor of E.histolytica L-arginase, inhibits the growth of the parasite, probably by reducing the synthesis of L-ornithine, a molecule used by the parasite as a precursor for the production of polyamines. Polyamines are ubiquitous polycations and have been implicated in pivotal cellular activities such as macromolecular synthesis, cell growth, and differentiation and previous studies had found they are necessary in the growth of number of parasites.  


The construction of strains that are downregulated of Eharg are needed to better understand the role played by this arginase in the virulence and the survival of the parasite.