Immunoassay in urine of a specific marker for proximal tubular S3 segment

Urinary intestinal alkaline phosphatase (EC 3.1.3.1; IAP) is a marker of the S3 segment of the human kidney proximal tubule. An accurate enzyme-antigen immunoassay (EAIA) with a high-affinity specific monoclonal antibody (IAP250) developed for this marker has a detection limit below the lowest IAP activity found in urine samples of normal subjects. The intra- and interassay CVs were less than 5%. Mean analytical recovery of pure IAP added to urine was 102% (SD 6%), and the EAIA results correlated well with immunoreactivity (measured by a sandwich ELISA), suggesting that the EAIA detected all of the IAP in urine. In healthy individuals (ages 20-80 years) the IAP concentrations, expressed as urinary creatinine ratios, ranged from 0.1 to 2.0 U/g (5-95 percentiles) without major differences related to sex and age. Workers exposed to mercury, which affects the S3 segment, showed an increased IAP elimination; abusers of analgesics, which affect more distal parts of the nephron, did not. As opposed to currently measured markers, the EAIA offers easy, accurate, and precise measurement of early alterations in the S3 segment.