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Coelenterate Cnidae Capsules: Disulfide Linkages Revealed by Silver Cytochemistry and Their Differential Responses to Thiol Reagents.

The sulfur cytochemistry of cnidae from the Portuguese man-of-war Physalia physalis, the scyphozoan Cassiopeia xamachana, and the black coral Cirrhipathes luetkeni was evaluated on the basis of electron microscopy, X-ray microanalysis, amino acid analysis, and response to disulfide reducing agents. The cnidae examined included large and small holotrichous isorhizas in P. physalis, another small isorhiza in C. xamachana, and both spirocysts and microbasic mastigophore nematocysts in C. leutkeni. A strong reaction with methenamine-silver reagent was characteristic of all cnidae capsules, but the pattern and extent of that argentophilia was dependent upon the type of cnida and its state of maturity. The large isorhizas of P. physalis reacted primarily in the outermost capsule layers, but in C. xamachana isorhizas, silver stained the entire capsule with the exception of the outermost region. The small isorhizas of P. physalis and the mastigophore capsules of C. leutkeni stained throughout, whereas the spirocyst capsules were outlined by silver, clearly delineating the inner and outer layers. All of these reactions were abolished with alkylation, but only after treatment with disulfide reducing agents; alkylation alone diminished silver staining only slightly, indicating that the argentophilic response was due primarily to disulfide linkages. The cystine content of these cnidae varied from 4.1 to 4.7 mole percent for a given species, but amino acid analyses did not separate components of the cnidom. Cnidae, both within and among species, exhibited differential responses to the disulfide reducing agent dithiothreitol (DTT). Isolated, unfixed, large isorhizas of P. physalis discharged and appeared to dissolve rapidly in the presence of this reagent, whereas small isorhizas from both P. physalis and C. xamachana discharged, but dissolved slowly if at all. The discharge and solution responses of the capsule coincided with the complete development of the tubule. Cnidae containing an undeveloped or partially developed tubule were resistant to DTT, displayed a weak capsular argentophilia, and contained background levels of sulfur; these results suggest that formation of disulfide linkages is one of the final steps in capsular maturation. In contrast, mature nematocyst and spirocyst capsules in C. leutkeni tentacles were resistant to DTT among other reagents, despite the presence of disulfides. This suggests that other types of covalent, intermolecular linkages could play a prominent role in the development of capsular stability in this species.