Mitochondrial carbonic anhydrase VA and VB: Properties and roles in health and disease.

Carbonic anhydrase V (CA V), a mitochondrial enzyme, was first isolated from guinea pig liver and subsequently identified in mice and humans. Later, studies revealed that the mouse genome contains two mitochondrial CA sequences, named Car5A and Car5B. The CA VA enzyme is most highly expressed in the liver, whereas CA VB shows a broad tissue distribution. Car5A knockout mice showed a predominant role for CA VA in ammonia detoxification, while the roles of CA VB in ureagenesis and gluconeogenesis were evident only in the absence of CA VA. Previous studies have suggested that CA VA is mainly involved in the provision of HCO3 - for biosynthetic processes. In children, mutations in the CA5A gene led to reduced CA activity, and the enzyme was sensitive to increased temperature. The metabolite profiles of these children showed a reduced supply of HCO3 - to the enzymes that take part in intermediary metabolism: carbamoylphosphate synthetase, pyruvate carboxylase, propionyl-CoA carboxylase, and 3-methylcrotonyl-CoA carboxylase. Although the role of CA VB is still poorly understood, a recent study reported that it plays an essential role in human Sertoli cells (hSCs), which sustain spermatogenesis. Metabolic disease associated with CA VA appears to be more common than other inborn errors of metabolism (IEM) and responds well to treatment with N-carbamyl-l-glutamate (NCG). Therefore, early identification of hyperammonemia will allow specific treatment with NCG and prevent neurologic sequelae. CA VA deficiency should therefore be considered a treatable condition in the differential diagnosis of hyperammonemia in neonates and young children. Abstract figure legend Carbonic anhydrase isozymes VA and VB represent mitochondrial enzymes which contribute to several physiological functions mainly in intermediary metabolism. The liver hepatocytes are the main source of carbonic anhydrase VA with weaker signals in brain, testis, and muscle. The VB isozyme is more widely spread in several organs, such as brain, heart, liver, lung, kidney, spleen, intestine, testis, muscle, and pancreas. This article is protected by copyright. All rights reserved.