Purines

P. Köhler

All the parasitic protozoa and helminths studied so far are incapable of the de novo synthesis of purine nucleotides. These organisms therefore rely entirely on the salvage of preformed bases or nucleosides for their purine requirement. This contrasts with most other organisms which use both de novo biosynthesis and recycling pathways. As a consequence of the complete dependence on preformed purines, parasites are equipped with a variety of different purine salvage routes and possess elaborate mechanisms for uptake and interconversion of purines. An overview of the enzymatic steps involved in nucleoside and nucleotide salvage into the nucleotide pool of parasites is depicted in Fig. 1. The major part of these interconversions consists of a single reaction, in which the free purine reacts with 5-phosphoribosyl-1-pyrophosphate to yield the corresponding purine mononucleotide as catalyzed by phosphoribosyltransferases (PRTases). As an alternative, but often less preferred...