Histochemical identification of the vascular endothelial isoenzyme of alkaline phosphatase

Alkaline phosphatase (AP) is a widely studied membrane bound ecto-enzyme with an extensive distribution in nature. Three major human isoenzymes have been defined and can be distinguished on the basis of their differential sensitivity to specific inhibitors. Despite the voluminous literature describing AP, the physiological role of this enzyme is unclear. Microvascular endothelium is strongly AP positive and may provide a convenient model for study of the role of AP in vitro. This report describes the use of freeze-substitution and high-resolution plastic embedding techniques to identify the isoenzyme of endothelial AP by quantitative analysis of the relative inhibition by specific inhibitors of AP, using human gingival tissues and a number of rat tissues. Endothelial AP is found to be the liver/bone/kidney isoenzyme, indicating kidney as a credible source of enzyme for further experimental work investigating the role of AP.