Cell-bound and extracellular matrix-associated alkaline phosphatase activity in rat periodontal ligament

In previous studies it was noted that alkaline phosphatase (ALP) activity in periodontal ligament does not only seem to be related to cells but may also be associated with the extracellular matrix. In an attempt to clarify this we studied the distribution of the enzyme at the electron microscopic level. In addition, ALPactivity was assessed biochemically following extraction of the ligament with (i) agents dissolving the membrane or splitting the phosphatidylinositol anchor (Triton X-100 or phosphatidylinositol-phospholipase C, respectively), and (ii) a matrix-degrading enzyme cocktail (collagenase, hyaluronidase and elastase). Histochemical observations revealed (a) a heterogeneous distribution of ALP-activity, with highest activity adjacent to the alveolar bone and (b) two pools of activity; one bound to cells and one associated with the collagenous extracelluJar matrix. In line with this were the biochemical data indicating that approximately 10% of the enzyme activity was firmly bound to the extracellular matrix and 90% to plasma membranes. Isoelectric focusing did not reveal differences between the two fractions, both samples yielding a single broad band corresponding with an isoelectric point of about 4.4.