Carbonic anhydrases in calcified tissues.

Abstract

Precipitation of calcium carbonate (CaCO3) and calcium phosphate, especially hydroxyapatite [Ca10(PO4)6(OH)2] has been an important physiological reaction through the evolution. Carbonic anhydrase activity has been associated with the calcification process regardless of precipitating calcium salt. Precipitation of calcium carbonate is a fundamental process in many marine invertebrates, particularly in corals. Lucas and Knapp (1997) have demonstrated that carbonic anhydrase plays a pivotal role in the formation of calcitic sclerites of octocoral (Leptogorgia virgulata) regardless of the carbon source. Interestingly and comparably to mammalian cartilage calcification, Giraud-Guille (1984) has demonstrated that in addition to intracellular carbonic anhydrases it is possible that an extra-cellular carbonic anhydrase may have a role in the initiation of calcification in the crab cuticle. Acetazolamide was found to be inhibitory also in the formation of calcified spicules of developing sea urchin embryos (Mitsunaga et al., 1986). Recent study of Pedrozo et al. (1996) showed that carbonic anhydrase activity regulates precipation of calcium carbonate and mineralization of statoconia in Aplysia californica. These examples clearly demonstrates the essential role of carbonic anhydrase in the regulation of calcium carbonate precipitation in various biological systems. However, most dramatic demonstration of the role of carbonic anhydrase in biological calcification comes from the experiments where egg shell formation in birds has totally been blocked by carbonic anhydrase inhibitors (Benesch, 1984).