Change in alkaline phosphatase isoenzyme pattern in urine as possible marker for renal disease

The human kidney contains two types of alkaline phosphatase (AP) isoenzymes: a hepatic type of AP and an intestinal-like AP. Intestinal-like AP, measured by immunotitration techniques, is a minor component (1 to 4%) of the total AP activity. It is found only in the particle-free fraction (cytoplasm) and is located, with immunofluorescent techniques, in some of the proximal convoluted tubules. Urinary AP activity is found after high-speed centrifugation in the supernatant (x 100,000g), as well as in the sediment, and may be extracted from the sediment after solubilization with n-butanol. Both types of these renal isoenzymes contribute to urinary AP activity. Biochemical characterization (effect of inhibitors, thermostability, denaturing with urea, and so on) revealed that urinary intestinal-like AP and renal intestinal-like AP are identical. Both, however, have been distinguished as multiple forms of AP from the small intestine. Most of the urinary AP activity of healthy persons (22 volunteers) was found in the sediment and consisted of liver-type AP. Urinary AP of patients with diseases, after application of potentially nephrotoxic drugs or during rejection episodes of renal allografts, contains little sediment activity, but it contains increased amounts of urinary intestinal-like AP.