Relation of Alkaline Phosphatase to Ultrastructure

The location of alkaline phosphatase in the cell and tissue is information basic to a full explanation of an increase in serum alkaline phosphatase. Modern techniques of enzyme histochemistry and of electron microscopy (R2) have succeeded in providing a clearer picture (GIO, M22). The cell wall is the location at the electron microscope level of 

Sites of alkaline phosphatase activity are frequently in endothelial cells of blood capillaries, mucous glandular cells (F3), microvilli of intestine (C6, CIO, D2, H21, P8, W7), bile canaliculi (D21, F27, W2), and placenta (W3), as well as in the brush border of the lumenal surface of epithelial cells of the proximal convoluted renal tubules (M22, Wl). The location of L-phenylalanine-sensitive alkaline phosphatase in human intestine and placenta is illustrated in Fig. 30. Electron micrographs (Fig. 31) show the details of the alkaline phosphatase, and illustrate the 

Site of Production of Alkaline Phosphatase IN Absorptive Cells 

There appears to be no research that traces the path of synthesis of alkaline phosphatase to its final location on the absorptive surfaces of cell-wall membranes. The usual sequence of specific enzyme synthesis envisions the collection of newly synthesized protein in secretion vacuoles that eventually leave the cell of origin as part of a secretion process. The sites of synthesis are the ribosomes of the rough-surfaced endoplasmic reticulum. 

On the other hand, the Golgi apparatus is implicated in the synthesis of cell-wall membranes. It is possible that the alkaline phosphatase is a catalytically inactive, structural protein of such cell membranes when they are still in the cytoplasm, which may explain many failures to demonstrate its presence in the Golgi apparatus. Upon becoming part of the outer surface, say of the brush border of the intestinal villus, the enzyme may be activated by the alkaline contents of the duodenal lumen. The concept of catalytically inactive membrane enzyme proteins is developed in a recent publication with regard to 8-glucuronidase (FIO). 

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