Carbonic Anhydrase:isoenzymes. Properties. Distribution. And Functional Significance

Abstract

Summary 1. Carbonic anhydrases from vertebrates, plants and bacteria have molecular weights of 30,000 (or multiples thereof), contain one zinc atom per 30,000 molecular weight, and are inhibited by acetazolamide and related compounds. 2. In mammals, there are two major isoenzymes of carbonic anhydrase. The so‐called ‘high activity’ carbonic anhydrase possesses a carbon dioxide‐hydratase activity many times (in the case of the guinea‐pig isoenzymes, 18 times) that of the ‘low activity’ isoenzyme. 3. The mammalian isoenzymes differ from one another in their amino‐acid compositions (the difference in serine contents being a consistent finding in a number of species), in their physical properties (isoelectric pH, retention by ion‐exchange resins, electrophoretic mobility) and in their kinetic properties. 4. Mammalian carbonic anhydrases also catalyse the hydrolysis of some esters and the hydration of aldehydes. Their relative activities with these other substrates may be substantially different from their relative activities with carbon dioxide as substrate. The discovery that carbonic anhydrase may also catalyse other reactions raises the possibility that the enzyme may have other roles in metabolic pathways (e.g., in certain dehydrogenation reactions). 5. In plants, the role of carbonic anhydrase may be to catalyse the inter‐conversion of bicarbonate and carbon dioxide, to provide ‘carbon dioxide’ in the form appropriate for carbon‐fixing reactions. A similar role has been suggested for the carbonic anhydrase found in Neissariae. 6. The carbonic anhydrase of vertebrates (the ‘high activity’ isoenzyme of mammals) is found in many ion‐transporting epithelia, but its role in them is still uncertain. It occurs in acid‐transporting epithelia such as the stomach and kidney, and also in certain bicarbonate‐transporting epithelia like the large intestine. There is, however, a poor correlation between the presence of the enzyme and the occurrence of acid or bicarbonate secretion. Thus there is little or no carbonic anhydrase in some tissues noted for their ability to secrete bicarbonate ions, such as the pancreas and ileum. Conversely, carbonic anhydrase‐containing tissues like the avian salt gland and the elasmobranch rectal gland form concentrated sodium chloride solutions which are of nearly neutral pH. The correlation between the distribution of carbonic anhydrase and the occurrence of active chloride transport appears similar to that between carbonic anhydrase and bicarbonate ion transport. 7. While the red cells of most mammalian species so far studied contain both ‘high activity’ and ‘low activity’ isoenzymes, the latter is reported to be absent from the erythrocytes of sheep, ox and dog. Presumably the ‘low activity’ isoenzyme is not necessary for an adequate rate of carbon dioxide exchange to occur between tissues and lungs in these species. 8. The ‘low activity’ isoenzyme is present in tissues other than blood, for example, the colon, caecum and ox rumen, and probably also, the gall bladder and kidney medulla. Its...

Functional involvement of carbonic anhydrase in calcium transport of the chick chorioallantoic membrane

“…2). The I50 (Maren, 1967;Carter, 1972). Together, these results therefore indicate that a specific carbonic anhydrase activity is present in the chorioallantoic membrane of the chick embryo, and that it is expressed in an age-dependent manner during development.…”

“…(Maren, 1967;Carter, 1972;Bundy, 1977) found in the blood vessels, distinct dark cobaltous deposits were exclusively localized along the ectodermal layer of the chorioallantoic membrane, which has been previously shown to contain the Ca2+-transporting cells of the chorioallantoic membrane (Coleman & Terepka, 1972a;Armbrecht et al, 1976). At a higher magnification (Plate lb), the ectodermal stains appear to be cortically distributed in the columnar epithelial cells of the ectoderm and, in many cases, outlining these cells.…”

“…Sulphonamides strongly inhibit the carbonic anhydrase activity, but not the Ca2+-binding activity in chorioallantoic-membrane extracts. Moreover, the physicochemical properties of the Ca2+-binding protein (Tuan & Scott, 1977;Tuan et al, 1978a) differ significantly from the known properties of carbonic anhydrase isolated from various species (Maren, 1967;Carter, 1972).…”

“…The enzymic action of carbonic anhydrase in the reversible hydration of CO2 and as a regulator of physiological acid-base balance (Maren, 1967;Carter, 1972) suggests that it may be important in these aspects of the Ca2+-transport function of the chorioallantoic membrane. Indeed, carbonic anhydrase has been shown to be involved in active ion transport by various epithelia (see reviews by Maren, 1967;Carter, 1972;Bundy, 1977).…”

“…Indeed, carbonic anhydrase has been shown to be involved in active ion transport by various epithelia (see reviews by Maren, 1967;Carter, 1972;Bundy, 1977).…”