High-molecular-mass alkaline phosphatase in serum and bile: physical properties and relationship with other high-molecular-mass enzymes.

High-molecular-mass alkaline phosphatase has similar properties in serum and bile. It is of sufficiently large size to be excluded by Sepharose 6B and carries a high negative charge at alkaline pH, largely accounted for by the presence of sialic acid residues. It is slightly more heat labile than the lower-molecular-mass liver isoenzyme. Other enzymes originating from the hepatocyte plasma membrane are present as similar high-molecular-mass forms in normal bile, and in both bile and serum from patients with liver disease. The activities of these relatively large enzymic forms in serum were correlated with one another and we show they are converted similarly to lower-molecular-mass forms by solubilization with detergents. However, they carry slightly different net charges, thereby ruling out the possibility that they all consisted of a single homogeneous multi-enzyme complex. These findings are discussed in the light of two current hypotheses that the high-molecular-mass enzymes represent (a) membrane fragments released into the circulation in liver disease or (b) low-molecular-mass enzymes that aggregate, after release, with the lipid and protein in the circulation.