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Human intestinal alkaline phosphatase was immunohistochemically identified and localized in the pancreas, liver and kidney by use of a monoclonal antibody specific for intestinal alkaline phosphatase isozyme and by amplified biotin-streptavidin staining. In all the examined organs, the intestinal isozyme was found to be localized in the epithelial cells of ducts: bile ducts in the liver, distal convoluted tubules and collecting tubules in the kidney and ducts in the secretory epithelium in the pancreas. In the liver the antibody also stained some sinus-lining cells. In all the examined organs the endothelial cells of the capillaries and some vessels were stained. By use of immunoelectron microscopy, intestinal alkaline phosphatase was, as expected, found to be localized to the microvillar region of the small intestine. The isozyme was abundantly expressed in the apical area of the microvilli and in membrane remnants in the fuzzy coat. Capillaries and vessels in the submucosa were also stained, as well as small vesicles in the endothelial cells. The present investigation demonstrates the expression and localization of the intestinal alkaline phosphatase in several organs, though previously believed to be expressed only in the intestine.

By use of sensitive immunocatalytic assays, based on isozyme specific monoclonal antibodies, the activities of the three main human alkaline phosphatases were determined in serum. The activities were related to ABO blood groups and secretor phenotypes. The activity of intestinal alkaline phosphatase was found to be strongly correlated with ABO blood groups and secretor phenotypes, while neither the placenta alkaline phosphatase activity nor the tissue unspecific alkaline phosphatase activity demonstrated any dependence on blood groups or secretor phenotypes. Non-secretors, independent of ABO blood groups, demonstrated low activities of intestinal alkaline phosphatase in serum, amounting to approximately 20% of the activities in the secretor groups. Within the secretor group, the lowest activities were observed for blood group A (2.8 +/- 1.1 IU/l; mean +/- SEM) and the highest for blood groups B and O (14.1 +/- 1.1 IU/l and 19.0 +/- 2.5 IU/l, respectively). These results confirm that the activities of intestinal alkaline phosphatase in serum have to be related both to ABO blood groups and to secretor phenotypes in order to be informative in clinical contexts.

The three human isozymes of alkaline phosphatases were quantitatively determined in normal testis and seminoma tissues. The highly selective assays were based on isozyme specific monoclonal antibodies. In the normal testis approximately 90% of the catalytic activity originates from the tissue unspecific alkaline phosphatase, and the remaining activity was due to trace expression of both intestinal (approximately 5%) and placental alkaline phosphatase (PLAP) or PLAP-like isozyme (approximately 5%). In homogenates of seminoma tissues, highly increased levels of all three isozymes were identified. Both the tissue unspecific alkaline phosphatase and PLAP-like enzymes displayed relative increases of 10- to 100-fold and intestinal alkaline phosphatase 2- to 10-fold compared with normal testis. This finding indicates that the entire genome coding for alkaline phosphatases may be activated in seminomas. The PLAP-like enzyme from seminoma cells comprises a heterogenous population of molecules demonstrating partial heat sensitivity and microheterogeneity upon starch gel electrophoresis in contrast to the pregnancy related PLAP. These findings have implications for the different PLAP assays used in the clinical monitoring of seminoma patients.

Specific assays for human intestinal and liver alkaline phosphatases were developed by use of isozyme specific monoclonal antibodies bound to paper discs. The assays are fast, specific and convenient to use as demonstrated by determinations of alkaline phosphatase isozymes in sera and tissues. In sera from forty healthy individuals the activity of the tissue unspecific alkaline phosphatase was determined to 32 +/- 12 IU/l (mean +/- SD). The activity of the intestinal alkaline phosphatase was found to be ten-fold lower, 3.5 +/- 6.3 IU/l (mean +/- SD), and of the placental alkaline phosphatase another ten-fold lower, 0.3 +/- 0.2 IU/l (mean +/- SD), than that of the tissue unspecific alkaline phosphatase. Several normal tissues contained all three isozymes, the intestinal mucosa, for example, which besides intestinal alkaline phosphatase also expresses trace amounts of placental and liver-bone-kidney alkaline phosphatase. Seminomas, known to express eutopically placental-like alkaline phosphatase were demonstrated to contain increased levels of both intestinal and liver-bone-kidney alkaline phosphatases as compared to the normal testis.