Segment-specific localization of intestinal-type alkaline phosphatase in human kidney

Specific monoclonal antibodies raised against human intestinal and human tissue-unspecific alkaline phosphatase (AP) were developed and were used to study the expression of these two isoenzymes in human renal tissue and their release into urine. Approximately 25% of the total AP content of renal tissue at the transition between cortex and medulla was of the intestinal type; the remainder was of the tissue-unspecific type (liver, bone, kidney AP). Immunoperoxidase staining using specific monoclonal antibodies against liver and intestinal AP revealed that the tissue-unspecific AP isoenzyme is present through-out the different segments of the proximal tubule, whereas intestinal-type AP is found exclusively in tubuloepithelial cells of the S3-segment of the proximal tubule. The intestinal-type enzyme obtained from the kidney had a similar heat stability and Km value, and similar immunologic and inhibitory (L-p-bromotetramisole; L-phenylalanine) characteristics compared to adult intestinal and fetal intestinal AP. Its electrophoretic mobility in agarose gel was intermediate between that of adult intestinal and fetal intestinal AP; after neuraminidase treatment it became indistinguishable from the adult intestinal isoenzyme. The intestinal-type AP found in the urine was not sensitive to neuraminidase and had a molecular weight significantly lower than the urinary tissue-unspecific AP isoenzyme. In conclusion, intestinal AP in the kidney is a specific marker for the brush border of the S3 segment of the proximal tubule, and this finding opens new perspectives in the cell biology of this particular part of the nephron.