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Porphyrin/heme synthesis chemical pathway

ALA-hydratase (porphobilinogen synthase), the second step in this pathway (top of figure), is inhibited by lead as shown, but also by succinylacetone, an abnormal metabolite that accumulates in hepatorenal tyrosinemia. The result is an accumulation of δ-aminolevulinic acid (δ-ALA), which is excreted in the urine, and the development of porphyria-like neurologic crises. This process of secondary enzyme inhibition links a disorder of tyrosine degradation with a disorder of porphyrin/heme metabolism. Illustration by Ross Boswell and MrNerdy (2012). (Courtesy of Wikimedia Commons. Creative Commons Attribution-Share Alike 3.0 Unported License.)