High-molecular-mass alkaline phosphatase in serum and bile: nature and relationship with lipoprotein-X.

Using immunoelectrophoresis and other techniques, we have demonstrated an association between lipoprotein-X and (a) alkaline phosphatase and (b) other enzymes originating from the hepatocyte membrane, namely gamma-glutamyltransferase and leucine aminopeptidase. The high-molecular-mass forms of these enzymes, in both serum and bile, were precipitated by lipoprotein-X antiserum but not by antisera to other plasma proteins. The activity of high-molecular-mass alkaline phosphatase in serum was positively correlated with lipoprotein-X and with lipoprotein-X-associated alkaline phosphatase, both assessed semi-quantitatively. On the other hand, many sera possessed high activities of high-molecular-mass alkaline phosphatase but no detectable lipoprotein-X. Incubation of serum with conjugated bile salts and with synthetic detergents, at concentrations which did not dissociate the high-molecular-mass enzymes, caused parallel alterations in the electrophoretic mobility of serum lipoprotein-X and its associated enzyme activity. Incubation of normal dialyzed hepatic bile with normal, lipoprotein-X-negative serum produced an alteration in electrophoretic mobility of biliary lipoprotein and its associated enzyme activity from anodal to cathodal in agar gel. Digestion with papain had a variable effect on the different enzymes in the complex, without affecting the lipoprotein moiety. Leucine aminopeptidase was removed most readily from the complex to give the low-molecular-mass form present in normal serum; gamma-glutamyltransferase dissociated somewhat less readily, and alkaline phosphatase was completely resistant to dissociation from the complex. These results are discussed in the light of current knowledge, and a hypothesis is proposed for the nature of the high-molecular-mass enzymes in serum and bile.