The function of AMDHD1 (amidohydrolase domain containing 1, Ensembl gene identifier ENSG00000139344) is as follows. Catalyzes the hydrolytic cleavage of the carbon-nitrogen bond in imidazolone-5-propanoate to form N-formimidoyl-L-glutamate. This reaction represents the third step of the universal histidine degradation pathway. In addition to its metabolic role, functions as a tumor suppressor by promoting TGF-beta signaling. Mechanistically, inhibits ubiquitination and proteasomal degradation of SMAD4, thereby stabilizing the protein. Enhances SMAD2/3 phosphorylation and increases TGF-beta-dependent transcriptional responses. Consequently, promotes apoptosis and induces cell cycle arrest in tumor cells (PubMed:39143229). {ECO:0000250|UniProtKB:P42084, ECO:0000269|PubMed:39143229}.