Abstract
Serum 5'-nucleotidase in rat and man is derived from the plasma membrane rather than the cytosol by the criteria of inhibition with [alpha beta-methylene]ADP and antisera. In individuals with cholestasis the serum enzyme is mainly present as a high-Mr form that in the presence of the zwitterionic detergent Sulphobetaine 14 has the electrophoretic characteristics of liver plasma-membrane ectoenzyme. A minor form of 5'-nucleotidase in cholestatic serum and all the enzyme in normal serum appears to be half the molecular size of the liver plasma-membrane ectoenzyme. 5'-Nucleotidase from both normal and cholestatic rat serum was found to contain a polypeptide chain of apparent Mr 70 000 by immunoblotting techniques. It is suggested that the major form of 5'-nucleotidase in cholestatic serum is an ectoenzyme dimer derived from liver plasma membrane. All of the enzyme in normal serum and some of the enzyme in cholestatic serum is present as an active monomer derived from the ectoenzyme dimer.