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 Ala-Ser-Gly. Defects in Hal are the cause of histidinemia (His). Belongs to the PAL/histidase family.