Human placental ecto-5'-nucleotidase: isoforms and chemical crosslinking products of the membrane-bound and isolated enzyme.

Human placental ecto-5'-nucleotidase is specifically detected on Western blots using poly- or monoclonal antibodies. In two-dimensional electrophoresis 5'-nucleotidase, purified as well as membrane bound, is resolved in up to 13 isoforms distinguished by a different content of neuraminic acid. These forms span a range of molecular masses of about 67-71 kDa and of isoelectric points of 5.8-7.0. Chemical cross-linking of the purified enzyme with either homoor heterobifunctional reagents yields a dimer of about 140 kDa exclusively. On the other hand treatment of plasma membranes with the same reagents results in a crosslinking product of 5'-nucleotidase of about 97 kDa. The partner of the enzyme subunit in this crosslink, a protein of about 30 kDa, is unknown. Using specific antibodies the cytoskeletal components actin and tropomyosin were excluded as possible candidates.