The conversion of IMP to XMP is the rate-limiting step in the de novo synthesis of guanine nucleotides and is catalyzed
by IMP dehydrogenase .
The Arabidopsis gene encoding this enzyme was recently cloned and found to be 69% similar to human Type II IMPDH after allowing for conservative substitutions Collart et al. (1996).
This enzyme is of particular interest due to its role in the production of ureides in legumes (see Purine nucleotide catabolism section).