Structural evidence that human liver and placental alkaline phosphatase isoenzymes are coded by different genes.

Human liver alkaline phosphatase [ortho-phosphoric monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1] was purified, and some of its physical and chemical properties were examined and compared to those of human placental alkaline phosphatase. The results indicated a different peptide structure for each, based upon HB2-terminal residue sequence, two-dimensional tryptic peptide maps, and different amino acid compositions. These data are interpreted to indicate that the enzymes are synthesized by different structural genes. Other molecular properties differentiating the two enzymes were a higher apparent molecular weight for the liver enzyme from sodium dodecyl sulfate gel electrophoresis, a higher S20,w value, different carbohydrate content, and a different isoelectric point. The immunochemical specificity of each enzyme was not affected by removal of sialic acid groups. Both enzymes are similar in that they are dimers of equal molecular weight subunits, and are probably homodimers.