Inosine-uridine preferring nucleoside hydrolase (EC 3.2.2.1) (IU-nucleoside hydrolase or IUNH) is an enzyme first identified in protozoan [1] that catalyzes the hydrolysis of all of the commonly occuring purine and pyrimidine nucleosides into ribose and the associated base, but has a preference for inosine and uridine as substrates. This enzyme is important for these parasitic organisms, which are deficient in de novo synthsis of purines, to salvage the host purine nucleosides.

IUNH from Crithidia fasciculata has been sequenced and characterized, it is an homotetrameric enzyme of subunits of 34 Kd. An histidine has been shown to be important for the catalytic mechanism, it acts a proton donor to activate the hypoxanthine leaving group.

IUNH is evolutionary related to a number of uncharacterized proteins from various biological sources, notably:

• Escherichia coli hypothetical protein yaaF.
• Escherichia coli hypothetical protein ybeK.
• Escherichia coli hypothetical protein yeiK.
• Fission yeast hypothetical protein SpAC17G8.02.
• Yeast hypothetical protein YDR400w.
• An hypothetical protein from the archaebacteria Desulfurolobus ambivalens.

As a signature pattern for these proteins, we selected a highly conserved region located in the N-terminal extremity. This region contains four conserved aspartates that have been shown [2] to be located in the active site cavity.