Protozoa depend on purine salvage for nucleic acid synthesis. An abundant salvage enzyme in Crithidia fasciculata is the inosine-uridine nucleoside hydrolase (IU-NH). The enzyme is homotetrameric (4 x 34 kDa subunits) and exhibits no known allosteric properties.
N-ribosyl hydrolase catalyses the hydrolysis of carbon-nitrogen bonds in all commonly occurring purine and pyrimidine nucleosides via an oxocarbenium-ion transition state. The hydrolysis forms ribose and the associated base. The enzyme has a preference for inosine and uridine as substrates. The substrate specificity and kinetic constants are consistent with His241 acting as a proton donor to activate the hypoxanthine leaving group. Although the enzyme is established as a member of the nonspecific nucleoside hydrolases, it has a unique substrate specificity. Its mechanism includes a tightly bound catalytic Ca(II).
The enzyme is involved in the purine salvage pathways of protozoan parasites and has not been found in mammals, since mammals possess an endogenous biochemical pathway which releases nucleosides by phospholysis, catalysed by nucleoside phosphorylase. Protozoan parasites lack de novo purine biosynthesis pathways and are dependent upon exogenous purines, salvaged from a host organism.
