An intracellular pool of the procoagulant thromboplastin in human monocytes

Human monocytes cell extracts have been analysed by density gradient centrifugation in Percoll gradients. Two peaks of activity of the plasma membrane marker enzyme 5′ nucleotidase were detected. The main peak was at a density of 1.040 and a secondary one at a lower density. Thromboplastin activity was recovered associated with the main peak of 5′ nucleotidase activity in control cells. In NH₄Cl treated cells, thromboplastin activity is found at both densities. In agreement with this, labelling of the cell surface with ¹²⁵I shows a reduction in availability of proteins for labelling between control and NH₄Cl treatment. The availability of thromboplastin in monocytes for attack by exogenous trypsin or for initiation of coagulation is also reduced by cultivation with amines. We suggest that the low density peak of 5′nucleotidase and thromboplastin represents an intracellular compartment.