Rat-incisor phosphoprotein (RIP) has been subjected to isoelectric focusing at 4" in (a) an Ampholine pH gradient of 2.5-4 and (b) an acid pH gradient created by electrolysis of a system of acids and acidic ampholytes and covering the pH range OS-35 In the Ampholine gradient, the RIP unexpectedly formed several adjacent and strongly opalescent bands in the pH range 2.5-3.1. These bands, which migrated slowly toward the anode on prolonged focusing, are interpreted as being the result of an interaction between the amino groups of the Asnpholiie and the numerous phosphate groups of the protein. In the acid pH gradient, the RIP focused into one narrow zone corresponding to an isoelectric pH of 1.1 at 4". This value is consistent with the amino-acid composition and the phosphate content of the protein. INTRODUCITON Besides collagen, preparations of decalc%ed dentin of different species have proved to contain a class of proteins rich in aspartic acid, serine and organically bound phosphate1-5. These phosphoproteins are of special interest to workers in the field of calcified tissues, as it has been postulated k6 that they pIay an essential role in the mechanism by which hydroxyapatite crystals are deposited within the organic matrix.
However, from a methadologieal point of view, the dentin phosphoproteins are also very interesting to those working in the field of isoelectric focusing (IEF). Because of their high content of strongly acid phosphate groups, but low content of basic groups, these proteins would be expected to have isoelectric points too low