ABSTRACT
The metabolism of purines in protozoan parasites is significantly different from that in mammals. In humans, purines are synthesized de novo from non-nucleotide precursors (amino acids, carbon dioxide and ammonia) as well as from preformed purine bases via the salvage pathways. The general route for de novo biosynthesis is the same in those species of mammals, birds, yeasts, and bacteria that have been studied. This is a successive sequence of reactions by which the purine ring is assembled on C1 of ribose 5'-phosphate leading to the formation of inosinic acid (IMP) (see Figure 48.1). Neither free purines nor nucleosides appear as intermediates in this sequence. IMP is then converted to adenosine 5'-phosphate (AMP) or to guanosine 5'-phosphate (GMP) with subsequent conversion to nucleoside di-and tri-phosphates and nucleic acids. In contrast to mammals, de novo purine biosynthesis does not occur in any of the major parasitic protozoa.
