Tests for antibodies against carbonic anhydrase I (CA-I) and carbonic anhydrase II (CA-II) are essential diagnostic tools for various autoimmune disorders. Carbonic anhydrase is, along with hemoglobin, the main protein component of erythrocytes. The enzyme occurs in at least six isoforms in the body, of which carbonic anhydrase I (low activity) and carbonic anhydrase II (high activity) occur in erythrocytes. Carbonic anhydrase, a zinc metalloenzyme, catalyzes the reversible reaction CO2 + H2O --> HCO3- + H+.
When the immune system malfunctions and generates antibodies targeting these enzymes, it can lead to several conditions, including autoimmune pancreatitis, Sjögren's syndrome, and certain kidney and brain pathologies. Specific autoantibodies against carbonic anhydrase can inhibit its enzymatic activity. Carbonic anhydrase II deficiency leads to osteoporosis, renal tubular acidosis, and cerebral calcifications.
Since carbonic anhydrase is expressed in many tissues, it’s crucial to interpret positive results in conjunction with the patient's clinical symptoms and other diagnostic findings. A high titer of anti-CA-II antibodies has been mainly linked to conditions such as autoimmune cholangitis, interstitial nephritis, and retinal degeneration. In contrast, antibodies against CA-I are less frequently associated with specific diseases but may indicate a broader autoimmune response.
In patients with systemic lupus erythematosus, antibodies against carbonic anhydrases I and II were found higher in patients with anti-U1 snRNP and anti-SS-A/Ro.
Antibodies to carbonic anhydrase are found in systemic lupus erythematosus (33%), scleroderma (12%), polymyositis (12%), dermatomyositis (25%), and primary Sjögren's syndrome (17%). Antibodies have also been described in endometriosis (35 - 69%), primary biliary cirrhosis (35%), autoimmune hepatitis (30%), chronic pancreatitis, and type 1 diabetes mellitus. The incidence of these autoantibodies in healthy individuals is 2 - 12%.
Because of their low specificity, autoantibodies have a relatively low diagnostic value. The hypothesis that they are marker antibodies for patients with mitochondrial antibody-negative primary biliary cirrhosis has not been confirmed.
See also: Carbonic Anhydrase II, Antibodies
