Dual mechanism of laminin modulation of ecto‐5′‐nucleotidase activity

The myoblast cell surface activity of ecto‐5′‐nucleotidase was stimulated by a laminin substrate, whereas fibronectin and gelatin did not increase the AMPase activity of ecto‐5′‐nucleotidase. This increase was related to a higher expression of ecto‐5′‐nucleotidase on the surface of cells seeded on a laminin substrate, but without the mobilization of an intracellular pool of enzyme. Furthermore, laminin and its fragments E′ 1 and E 8 modified the AMPase activity of the ecto‐5′‐nucleotidase purified from chicken striated muscle and reconstituted in liposomes. Over the range of concentrations used, intact laminin and its fragment E 8 , consisting of the distal half of the long arm, stimulated the AMPase activity of ecto‐5′‐nucleotidase. By contrast, the large fragment derived from the short arms, designated E′ 1 , inhibited the AMPase activity. Furthermore, the monoclonal anti‐ecto‐5′‐nucleotidase antibody, CG37, abolished the stimulatory effect of fragment E 8 on the AMPase activity of ecto‐5′‐nucleotidase but did not reverse the inhibitory effect of fragment E′ 1 . In conclusion, laminin stimulates the AMPase activity of ecto‐5′‐nucleotidase by two mechanisms: inducing the expression of ecto‐5′‐nucleotidase to the cell surface and direct modulation of the enzymatic activity.