Comparison of human alkaline phosphatase isoenzymes. Structural evidence for three protein classes

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Abstract
The structural relationships among human alkaline phosphatase isoenzymes from placenta, bone, kidney, liver and intestine were investigated by using 3 criteria. Immunochemical characterization by using monospecific antisera prepared against either the placental isoenzyme or the liver isoenzyme distinguishes 2 antigenic groups: bone, kidney and liver isoenzymes cross-react with anti-(liver isoenzyme) serum and the intestinal and placental isoenzymes cross-react with the anti-(placental isoenzyme) anti-serum. High-resolution 2-dimensional electrophoresis of the 32P-labeled denatured subunits of each enzyme distinguishes 3 groups of alkaline phosphatase: the liver, bone and kidney isoenzymes, each with a unique isoelectric point in the native form, can be converted into a single form by treatment with neuraminidase; the placental isoenzyme, whose position also shifts after removal of sialic acid; and the intestinal isoenzyme, which is distinct from all other phosphatases and is unaffected by neuraminidase digestion. Finally, the primary structure of each enzyme is compared by partial proteolytic-peptide mapping in dodecyl sulfate/polyacrylamide gels. The primary structural identity of liver and kidney isoenzymes and the non-identity of the placental and intestinal forms was confirmed. Direct experimental support for the existence of at least 3 alkaline phosphatase genes was provided.

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