Aminoacylase-1 (ACY-1, N-acyl-L-amino-acid amidohydrolase) belongs to the M20 family of metalloproteases. It is a 408-amino-acid protein with a molecular weight of 45.8 KDa which catalyzes the hydrolysis of N-acetylated amino acids into the free amino acid and acetic acid. ACY-1 plays a general role in the cytosolic breakdown of acetylated amino acids generated during intracellular protein degradation. The gene encoding ACY-1 is evolutionarily conserved in fish, frog, mouse, rat and human.The expression of ACY-1 is highest in the kidney and brain. Aminoacylase-1 is a potential biomarker of long-term outcome after kidney transplantation. Significant differences in serum ACY-1 levels were apparent between delayed, slow and immediate graft function. ACY-1 was not detected in the majority of patients before transplantation and remained low in recipients of live-donor kidneys. Serum ACY-1 levels were increased significantly in the majority of patients with delayed graft function (DGF), but rose in only a minority of patients with immediate function. These analyses provide novel insights into the potential clinical utility of serum ACY-1 levels immediately post kidney transplation, enabling subdivision of patients with delayed graft function in terms of long-term outcome. Serum ACY-1 concentration was high in patients with tubular cell damage due to ischemia-reperfusion injury (IRI) and/ or acute tubular necrosis. ACY-1 also interacts with sphingosine kinase type 1, which is involved in promoting cell growth and inhibiting apoptosis of tumor cells. ACY-1 plays a role in regulating responses of the cell to oxidative stress. ACY-1 serves as a putative suppressor in renal cell carcinoma and small cell lung cancer. ACY-1 expression is also significantly correlated with serum alpha fetoprotein level and tumor invasiveness. It was also demonstrated that ACY-1 acts as a tumor suppresor in hepatocellular carcinoma. An enzymatic deficiency in ACY-1 may result in defects of brain metabolism and function, such as encephalopathy, unspecific psychomotor delay, psychomotor delay with atrophy of the vermis and syringomyelia, marked muscular hypotonia or normal clinical features (e.g. fever). Epileptic seizures are a frequent feature in these patients. ACY-1 deficiency has also been described as an inborn error of metabolism. This disorder was identified in children who have increased urinary excretion of N-acetylamino acids.