Ultrastructural localization of alkaline phosphatases in rat incisor odontoblasts

The localization of alkaline phosphatases in dentinogenically active rat incisor odontoblasts was studied by means of subcellular fractionation and electron microscopical histochemistry. Subcellular fractionation revealed the predominant phosphatase activity to be present in the microsome fraction and to a lesser extent in the mitochondrial fraction. Adenosine triphosphate degrading enzyme activity was determined in the presence or absence of (+/-)-6(m-bromophenyl)-5, 6-dihydroimidazo(le) (2,1-b) thiazole oxalate (R 8231). Before the histochemical study, the effects on phosphatase activities by aldehyde fixation were studied by biochemical assay. A method of fixation for optimal preservation of phosphatase activity is presented. Phosphatase electron microscopic histochemistry was performed by using ATP as a substrate and with or without addition of the inhibitor R 82319 Precipitates were seen in the membranes of vesicles present in the odontoblast process and the Golgi region. When there were signs of insufficient fixation, precipitates were also seen in the outer membranes of mitochondria. No phosphatase activity was seen in the cell membrane. ATP degrading enzyme activities mediated by nonspecific alkaline phosphatase (APase) and Ca2+ -adenosine triphosphatase thus have the same morphological localization. This close association is consistent with earlier biochemical studies.