STUDIES ON SHELL FORMATION. IV. THE RESPIRATORY METABOLISM OF THE OYSTER MANTLE ,

Abstract

1. The oyster mantle has been studied with respect to endogenous respiration, activity of various oxidative enzymes and response to intermediates of the tricarboxylic acid cycle, and respiratory inhibitors. 2. The endogenous respiration has been found to be similar for different mantle regions, though small differences were present. Endogenous respiration was not significantly altered by changes in the pH of the sea water medium between 6.0 and 8.4 or by isolation of mantles in sea water without added substrate for periods of a few days. 3. Isocitric, succinic and malic dehydrogenases, fumarase, and cytochrome oxidase were present in the mantle. Aconitase could not be demonstrated. Oxaloacetic decarboxylase was found in very high concentration. Its presence may be of significance with respect to the formation of shell carbonate. 4. Isocitrate, succinate, malate, and oxaloacetate stimulated respiration. Enzyme assays and the effects of added substrates indicated the presence of the major portion of the tricarboxylic acid cycle in mantle tissue, although all steps have not been examined. Citrate and pyruvate were without effect on respiration over the range pH 6.0-8.4. 5. Malonate inhibited succinate oxidation partially but did not alter endogenous respiration. Selenite reduced respiration slightly at 10^-5M but brought about a mean increase of 49% at 10^-3M. Cyanide, 2 x 10^-4M, inhibited respiration almost completely. The inhibition was completely reversible after 1¼ hours but only partially reversible after longer periods. Methylene blue was ineffective in reversing cyanide inhibition. 6. Sodium diethyldithiocarbamate produced a maximum inhibition of 57% at 10^-3M. Higher and lower concentrations were less effective. Ethyl carbamate (urethane) was without effect at the concentrations employed with diethyldithiocarbamate. The results suggest a copper respiratory catalyst in mantle.