Human platelets 5'-nucleotidase: a cell membrane ectoenzyme with a possible regulatory role in the aggregation reaction.

The existence of a 5'-nucleotidase has been demonstrated in human blood platelets. The enzyme has a low Km (20 microM) for adenosine monophosphate. It is prevalently located on the external surface of the plasma membrane as demonstrated by the similar degradation of exogenous AMP by intact and lysed platelets. Also results of inactivation studies by means of non penetrating chemical reagents point to this conclusion. Activity is inhibited by glucosyl moieties specific lectins (e.g. Concanavalin A) with varying sensitivity in intact platelets, isolated membranes and in the solubilized form. Also micromolar concentrations of ADP inhibit the activity, possibly with a competitive pattern since this effect is much more evident at low substrate concentrations. On the basis of these results, it is suggested that this enzyme is involved in in loco adenosine production in the platelets, a process which can assume a physiological important role.