ADA

ADA Full Name

adenosine deaminase

ADA Introduction

Adenosine deaminase (ADA) is a key enzyme in purine metabolism that catalyzes th
e irreversible hydrolytic deamination of adenosine and 2′-deoxyadenosine to inosine and 2′-deoxyinosine, respectively, with the release of ammonia. This reaction is critical for maintaining cellular purine homeostasis and for the breakdown of dietary purines as well as the turnover of nucleic acids in tissues. The enzyme is present in virtually all mammalian cells and plays a central role in immune system development and function. Today, ADA is recognized not only as a metabolic enzyme but also as a key regulator of both intracellular and extracellular adenosine signaling, with profound implications for immunology, neurology, and therapeutic development.

Figure 1. Strcuture of adenosine deaminase.

Two Isoforms of Adenosine Deaminase: ADA1 and ADA2
In humans, adenosine deaminase exists as two distinct isoenzymes: ADA1 and ADA2. ADA1 is the ubiquitously expressed intracellular isoform found in most body cells, with particularly high expression levels in lymphocytes, especially T lymphocytes and macrophages. ADA1 is present both in the cytosol and as an ecto-form attached to the cell membrane via binding to CD26 (dipeptidyl peptidase 4). In contrast, ADA2 is a secreted homodimer that is highly expressed in plasma and is found predominantly on monocytes and macrophages. ADA2 has a lower affinity for adenosine compared to ADA1 and is encoded by the CECR1 (ADA2) gene located on chromosome 22. Interestingly, ADA2 shares highest sequence similarity with ADA-related growth factors involved in tissue development, suggesting that ADA2 may have functions beyond its enzymatic activity, including roles in cell proliferation and differentiation. The existence of two isoforms with different tissue distributions and binding partners enables precise spatiotemporal regulation of adenosine levels in various physiological and pathological contexts.

Clinical Significance of ADA Deficiency
ADA deficiency is an autosomal recessive inherited disorder that primarily affects lymphocyte development, viability, and function. When ADA activity is absent or severely reduced, the substrate deoxyadenosine accumulates to toxic levels within cells, leading to early cell death. Immature lymphocytes in the thymus are particularly vulnerable to this toxic buildup. As a result, approximately 80% of individuals with ADA deficiency develop severe combined immunodeficiency (ADA-SCID), a condition in which patients lack virtually all immune protection from bacteria, viruses, and fungi. Infants with early-onset ADA-SCID typically present with failure to thrive, opportunistic infections, and marked depletion of T, B, and NK lymphocytes within the first six months of life; without treatment, most do not survive beyond age two. The estimated incidence of ADA deficiency is approximately 1 in 500,000 newborns worldwide, accounting for about 10–15% of all SCID cases. In addition to SCID, partial ADA deficiency and later-onset combined immunodeficiency (ADA-CID) have been described, characterized by reduced enzyme activity and milder clinical manifestations that may include recurrent respiratory infections, chronic lung damage, and autoimmune phenomena.

Alternate Names for ADA

ADA; adenosine deaminase; adenosine aminohydrolase;