THE mechanism for the formation of CO32-, the reacting species in CaCO3 formation in biological systems which deposit CaCO3, is not known. The involvement of carbonic anhydrase in the biological formation of CaCO3 (refs. 1 and 2) suggests that CO32- arises from the dissociation of HCO3-. Hodges and Lörcher3,4 have summarized the major questions concerning the origin of CO32- in the avian eggshell-forming system and have outlined a model, originally proposed by Diamantstein5, in which the CO32- of eggshell CaCO3 arises in the shell gland lumen from HCO3- which is produced intracellularly by shell gland carbonic anhydrase. As emphasized by Hodges and Lörcher and also recently by Simkiss6, a major question remaining with the mechanism of CO32- formation is the fate of the proton formed by HCO3- dissociation. We propose that this proton may be neutralized by the reaction, NH3 + H+⇌NH4+. We also feel that NH3 acts in this manner in CaCO3 deposition by certain land snails7,8. The neutralization of protons by NH3 has precedence as a biological phenomenon. For example, protons are neutralized by NH3 in the mammalian kidney during metabolic acidosis9 and this reaction may also serve to regulate the acidity of sweat10. The effect of NH3 in the biological deposition of CaCO3 is summarized by the equation: This overall reaction has also been considered as a model for the geochemical deposition of CaCO3 in certain circumstances11.
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