Carbonic Anhydrase I and II as Biomarkers and Therapeutic Targets in Human Disease: From Physiology to Clinical Translation.

Carbonic anhydrases (CAs) are zinc-containing metalloenzymes that catalyze the reversible conversion of carbon dioxide and water into bicarbonate and protons, contributing to acid-base balance, pH regulation, and ion transport. Among human cytosolic isoforms, carbonic anhydrase I (CA I) and carbonic anhydrase II (CA II) are abundant and clinically relevant, yet their distinct roles are often obscured within broader discussions of the CA family. This narrative review evaluates CA I and CA II as biomarkers and therapeutic targets in glaucoma, atherosclerosis and vascular calcification, anemia, epilepsy, Alzheimer's disease, obstructive sleep apnea, obesity-related metabolic dysfunction, and selected cancers. CA II emerges as the more established pharmacological target, particularly in glaucoma, with acetazolamide and sultiame showing therapeutic potential in obstructive sleep apnea and possible contributions to epilepsy and neurodegeneration through pH regulation, bicarbonate-dependent signaling, and mitochondrial function. CA I instead appears more valuable as a disease-associated biomarker, especially in disorders involving erythrocyte turnover, inflammation, anemia, and malignancy, though circulating CA I may be confounded by hemolysis and altered erythrocyte dynamics. Clinical translation requires isoform-selective modulators, tissue-targeted delivery, standardized biomarker assays, and mechanistic models distinguishing primary CA involvement from secondary disease-related changes.