L-histidine = NH4(+) + trans-urocanate Amino-acid degradation; L-histidine degradation into L-glutamate; N-formimidoyl-L-glutamate from L-histidine: step 1/3. Contains an active site 4-methylidene-imidazol-5-one (MIO), which is formed autocatalytically by cyclization and dehydration of residues Cys-Ser-Gly. Defective histidine metabolism resulting in elevated histidine levels and resistance to nickel and zinc toxicity. Additionally, slightly resistant to copper, slightly sensitive to cobalt and iron and substantially sensitive to manganese. Belongs to the PAL/histidase family.