Sclerotized protein in the shell matrix of a bivalve mollusc

Abstract

The epsilon-amino groups of lysine and phenolic groups of tyrosine are most heavily concentrated in the newlydeposited organic matrix of the shell of the bivalve Mercenaria mercenaria. A phenoloxidase enzyme which oxidizes L-dihydroxyphenylalanine is present only in this “new” area of the shell matrix. Scanning electron micrographs of calcified secretions of the shell show that accretion lines, thought to be layers of organic matrix separating diurnal acceretions of calcium carbonate, are not developed until up to 4 d after deposition of shell material. These results suggest that the shell matrix is hardened by some kind of polymerization, and that lysine and tyrosine residues in the matrix are involved in the process. Accretion lines in polished and etched sections become visible only after complete hardening of the polymer occurs.