Homodimer and heterodimer forms of adult rat intestinal alkaline phosphatase

Three forms of alkaline phosphatase have been isolated from different sections of the small intestine: F3 180 kDa from the duodenum; F2 150 kDa principally jejunal; F1 120 kDa the only ileal form. Their catalytic properties have been compared as well as the electrophoretic properties the dimer and monomer of their phosphorylated intermediates. Pi was a competitive inhibitor of F1 and F3, whereas glycerophosphate was competitive inhibitor only of F3. Pi was a non competitive inhibitor of F2 and of a mixture F1 plus F3. Heating the phosphorylated enzyme preparations led to their dissociation into the phosphorylated monomers: F1 and F3 appear to be homodimers 65 kDa and 90 kDa peptides respectively whilst F2 seems to be a dimer formed from one of each monomer. F1 was phosphorylated faster but less intensively than F3. F2 was strongly phosphorylated over a long time-course and its 65 kDa monomer fraction was phosphorylated more strongly for longer than that from F1.