Extracellular parasites, metabolism

Finally, perhaps an explanation for the beneficial effects of coenzyme A (CoA), malate and pyruvate for the extracellular in vitro growth of P. lophurae found by Trager (1952) and interpreted by Moulder (1962) to neatly explain the shift in pattern of carbohydrate metabolism accompanying liberation of parasites from the host cell. .. (The) lack of CoA in free parasites logically explains the lessened rate of pyruvic acid oxidation via the Krebs cycle. It is difficult to escape the conclusion that the inability of plasmodia to synthesize CoA extracellularly results in extensive dislocations in glucose metabolism, which in turn contribute heavily to the restriction of the malarial parasite to an intracellular habitat is this malate and pyruvate could be linked to the generation of dihydronicotinamide adenine dinucleotide (NADH) for glycolysis, and a CoA deficiency could limit activity in pathways other than the TCA cycle. 

In many respects the basic features of amino acid and protein metabolism of parasitic protozoa and helminths resemble those of their mammalian hosts. Proteins can be broken down extracellularly or within lysosomes, and amino acids taken up and used for biosynthesis or energy metabolism. The pathways of amino acid metabolism are mostly the same as those used by animal cells. There are, nevertheless, some significant differences. Although no unique class of proteinase has been found in either protozoa or helminths, differences in specificity between parasite and host enzymes mean that inhibitors can be designed which selectively inactivate parasite enzymes. Because proteolysis is so important to many parasites at various stages of their life cycle, proteinase inhibitors are being studied and tested as potential antiparasitic agents... 

Purine nucleotides are required by the rapidly proliferating malaria parasite primarily for nucleic acid synthesis and energy metabolism. The malaria parasite cannot synthesize purines novo and depends for Its Intraerythrocytlc (IE) growth and development on salvage of purine bases from the host RBC and extracellular environment (2). We have shown with falciparum. In vitro, that hypoxanthlne is an essential purine base precursor for parasite Synthesis of adenosine and guanoslne nucleotides (3). Whetiier hypoxanthlne Is the malaria parasites preferred substrate vivo Is not known. 

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