Ammonia-Forming Enzymes and Calcium-Carbonate Deposition in Terrestrial Pulmonates

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Abstract
The ammonia hypothesis of calcium-carbonate deposition was tested to determine its general applicability to all land snails of the subclass Pulmonata. Ten shelled (snails) and four shell-less (slugs) species were evaluated. The ammonia-forming enzyme adenosine deaminase, or urease, or both were demonstrated in the mantle tissue of all 14 species. Adenosine deaminase predominated in 10 species and urease in only one. A hypothesis is proposed whereby ammonia is generated catalytically by a modified version of the purine nucleotide cycle. In all cases, the activity level of the enzyme was theoretically able to account for known rates of calcium-carbonate deposition. The pH optimum of either urease or adenosine deaminase from the mantle tissue of shelled species was between 8.6 and 10.0. The mantle enzyme of slugs displayed either an acidic optimum or no optimum. Tissue ammonia levels in shelled, but not shell-less, species were at least two and a half times as high in the mantle as in the foot. The results of this study support the hypothesis that ammonia gas is generated enzymatically to enhance the rate of calcium-carbonate deposition in pulmonate land snails.

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