E. coli has only one enzyme for both pathways. It is at a branchpoint. The next step is the best site to regulate these two pathways. In animals, the CPSI and II are in different compartments and so they do not compete with each other. It is an hypoxanthine analog that blocks xanthine oxidase and reduces uric acid concentrations.
Precursors are more soluble and can be excreted.
The enzyme is normally feedback inhibited by purine nucleotides. The mutation blocks this inhibitory mechanism and leaves the enzyme constitutively on. The disease is caused by superactivity rather than loss of activity. The histidine biosynthesis pathway is used to test for mutations. Various mutants in the pathway are plated on his minus media and a substance is added.