ORGANIC MATRIX SYNTHESIS IN THE SCLERACTINIAN CORAL STYLOPHORA-PISTILLATA - ROLE IN BIOMINERALIZATION AND POTENTIAL TARGET OF THE ORGANOTINTRIBUTYLTIN

The kinetics of organic matrix biosynthesis and incorporation into scl eractinian coral skeleton was studied using microcolonies of Stylophor a pistillata, [C-14]Aspartic acid was used to label the organic matrix since this acidic amino acid can represent up to 50 mol % of organic matrix proteins. External aspartate was rapidly incorporated into tiss ue protein without any detectable lag phase, suggesting either a small intracellular pool of aspartic acid or a pool with a fast turn-over r ate, The incorporation of C-14-labelled macromolecules into the skelet on was linear over time, after an initial delay of 20min, Rates of cal cification, measured by the incorporation of Ca-45 into the skeleton, and of organic matrix biosynthesis and incorporation into the skeleton were constant. Inhibition of calcification by the Ca2+ channel inhibi tor verapamil reduced the incorporation of organic matrix proteins int o the skeleton, Similarly, organic matrix incorporation into the skele ton, but not protein synthesis for incorporation into the tissue compa rtment, was dependent on the state of polymerization of both actin and tubulin, as shown by the sensitivity of this process to cytochalasin B and colchicin, These drugs may inhibit exocytosis of organic matrix proteins into the subcalicoblastic space. Finally, inhibition of prote in synthesis by emetin or cycloheximide and inhibition of N-glycosylat ion by tunicamycin reduced both the incorporation of macromolecules in to the skeleton and the rate of calcification. This suggests that orga nic matrix biosynthesis and its migration towards the site of calcific ation may be a prerequisite step in the calcification process. On the basis of these results, we investigated the effects of tributyltin (TB T), a component of antifouling painting known to interfere with biomin eralization processes. Our results have shown that this xenobiotic sig nificantly inhibits protein synthesis and the subsequent incorporation of protein into coral skeleton. This effect was correlated with a red uction in the rate of calcification, Protein synthesis was shown to be the parameter most sensitive to TBT (IC50=0.2 mu moll(-1)), followed by aspartic acid uptake by coral tissue (IC50=0.6 mu moll(-1)), skelet ogenesis (IC50=mu moll(-1)) and Ca2+ uptake by coral tissue (IC50=20 m u moll(-1)), These results suggest that the mode of action of TBT on c alcification may be the inhibition of organic matrix biosynthesis.