Zonal rotor purification and characterization of "mannosomes": a tubular membrane system in gastropod mollusc digestive gland

Mannitol oxidase and polyol dehydrogenases in the digestive gland of gastropods: Correlations with phylogeny and diet

Mannitol oxidase and polyol dehydrogenases are enzymes that convert polyalcohols into sugars. Mannitol oxidase was previously investigated in terrestrial snails and slugs, being also present in a few aquatic gastropods. However, the overall distribution of this enzyme in the Gastropoda was not known. Polyol dehydrogenases are also poorly studied in gastropods and other mollusks. In this study, polyalcohol oxidase and dehydrogenase activities were assayed in the digestive gland of 26 species of gastropods, representing the clades Patellogastropoda, Neritimorpha, Vetigastropoda, Caenogastropoda and Heterobranchia. Marine, freshwater and terrestrial species, including herbivores and carnivores were analyzed. Ultrastructural observations were undertake in species possessing mannitol oxidase, in order to investigate the correlation between this enzyme and the presence of tubular structures known to be associated with it. Mannitol oxidase activity was detected in the digestive gland of herbivores from the clades Caenogastropoda and Heterobranchia, but not in any carnivores or in herbivores from the clades Patellogastropoda, Neritimorpha and Vetigastropoda. In most of the species used in this study, dehydrogenase activities were detected using both D-mannitol and D-sorbitol as substrates. Nevertheless, in some carnivores these activities were not detected with both polyalcohols. Ultrastructural observations revealed tubular structures in digestive gland cells of some species having mannitol oxidase activity, but they were not observed in others. Based on our results, we suggest that mannitol oxidase first occurred in a herbivorous or omnivorous ancestor of Apogastropoda, the clade formed by caenogastropods and heterobranchs, being subsequently lost in those species that shifted towards a carnivorous diet.

Mannosomes: a molluscan intracellular tubular membrane system related to heavy metal stress?

Amongst animals, several hydrogen peroxide-generating oxidases are apparently restricted to molluscs. One of these, D-mannitol oxidase, is concentrated in the alimentary system, where it is associated with its own subcellular membrane system of unique tubular morphology, most likely representing a structural modification of the ER. These structures can be purified by subcellular fractionation and have been termed 'mannosomes'. Little is known about the functions of mannitol oxidase or of mannosomes, but the previously reported molluscicide-induced increase in mannosomes implies their involvement in a general stress reaction. In this study, we examined the effects of heavy metal stress in the terrestrial gastropod Arion lusitanicus. The activity of mannitol oxidase and mannosome abundance were monitored, together with metal effects on heat-shock protein level, and these parameters were compared to heavy metal accumulation in the digestive gland. We found that mannitol oxidase is inhibited by heavy metals more than other oxidases. On the other hand, hsp70 levels and mannosomal protein were increased with enhanced heavy metal stress, the latter indicating a probable increase in the number of mannosome organelles. Thus, stress protein (hsp70) and mannosomal protein were positively correlated with heavy metal accumulation, whereas the enzyme activity showed a negative correlation with increasing heavy metal content of the slugs.

D-Aspartate oxidase and D-amino acid oxidase are localised in the peroxisomes of terrestrial gastropods

D-Aspartate oxidase and D-amino acid oxidase were found in high activity in the tissues of representative species of terrestrial gastropods. Analytical subcellular fractionation demonstrated that both of these oxidases co-localised with the peroxisome markers, acyl-CoA oxidase and catalase, in the digestive gland homogenate. Electron microscopy of peak peroxisome fractions showed particles of uniform size with generally well preserved variably electron-dense matrices bounded by an apparently single limiting membrane. Many of the particles exhibited a core region of enhanced electron density. Catalase cytochemistry of peak fractions confirmed the peroxisome identity of the organelles. Peroxisome-enriched subcellular fractions were used to investigate the properties of gastropod D-aspartate oxidase and D-amino acid oxidase activities. The substrate and inhibitor specificities of the two activities demonstrated that two distinct enzymes were present analogous to, but not identical to, the equivalent mammalian peroxisomal enzymes.

Gastropod mollusc aliphatic alcohol oxidase: subcellular localisation and properties

The digestive gland and other tissues of several species of terrestrial gastropod mollusc contain an aliphatic alcohol oxidase activity (EC1.1.3.13). The enzyme is FAD dependent, consumes oxygen and generates hydrogen peroxide and the corresponding aldehyde. Saturated primary alcohols are favoured as substrates with octanol preferred with an apparent Km of 3-4 microM. The activity is clearly distinguishable from previously reported molluscan aromatic alcohol oxidase (EC1.1.3.7) on the basis of FAD dependence, sensitivity to heat treatment and high salt concentration and with regard to substrate preferences. The aliphatic alcohol oxidase is membrane associated and most likely localised to the endoplasmic reticulum. Extraction of membranes with 1% Igipal solubilises the enzyme in active form. This enzyme is a further example of an oxidase apparently restricted to molluscs.