Nacre organic matrix has been conventionally classified as both Ôwater-solubleÕ and Ôwater-insolubleÕ, based on its solubility in aqueous solutions after decalcification with acid or EDTA. Some characteristics (aspartic acid-rich, silkfibroin-like content) were specifically attributed to either one or the other. The comparative study on the technique of extraction (extraction with water alone vs. demineralization with EDTA) presented here, seems to reveal that this generally accepted classification may need to be reconsidered. Actually, the nondecalcified soluble organic matrix, extracted in ultra-pure water, displays many of the characteristics of what until now has been called Ôinsoluble matrixÕ. We present the results obtained on this extract and on a conventional EDTA-soluble matrix, with various characterization methods: fractionation by size-exclusion and anion-exchange HPLC, amino acid analysis, glycosaminoglycan and calcium quantification, SDS/PAGE and FTIR spectroscopy. We propose that the model for the interlamellar matrix sheets of nacre given by Nakahara Keywords: nacre; undecalcified soluble matrix; EDTAsoluble matrix; hydrophobicity; silk-fibroin-like-proteins.In the biomineralization field, the mollusk shell is one of the best studied of all calcium carbonate biominerals. Particular attention has been given to the organic matrix [1][2][3][4][5]. The latter is thought to promote the nucleation of the mineral component, to direct the crystal growth and to act as glue, preventing fracture of the shell [6][7][8][9]. The main biopolymers present in the organic matrix are essentially proteins, either glycosylated or not, acidic polysaccharides and chitin. In nacre, they represent 1-5% (w/w) of the structure.From the earliest experiments, it was believed that the biochemical properties of matrix constituents depend of the use of a decalcification procedure for removing the mineral component, which is strongly associated with the organic matrix [1,3]. Therefore, all investigations up until now used either EDTA, acetic acid or hydrochloric acid for this demineralization step and, subsequently, two fractions of the organic matrix were separated, based on their solubility in aqueous solutions. Accordingly, a designation of matrix into two classes, the soluble matrix and the insoluble matrix, has evolved from this extraction [10][11][12][13][14]. This paper presents for the first time the results of a comparative study on the organic matrix extracted from the nacreous layer of the shell from the pearl oyster Pinctada maxima by two very different methods. The first is a nondecalcifying technique obtained by an extraction in ultra-pure water. This unconventional approach arises from previous in vivo and in vitro experiments where we showed that biochemical signals from nacre chips were able to diffuse in the surrounding media and to induce new bone formation [15][16][17][18][19][20][21][22]. In an attempt to identify these signal molecules, we have previously perfected this original method of extraction of the ...

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