Thromboplastin (tissue factor) in plasma membranes of human monocytes.

The synthesis of thromboplastin, a potent trigger of blood coagulation, can be induced in human peripheral blood monocytes. Indirect evidence suggests that newly synthesized thromboplastin becomes in part available on the cell surface. We have attempted to study the localization and availability of thromboplastin more directly by isolating plasma membranes from isolated human peripheral blood monocytes. The specific activities of the plasma membrane markers increased 16-22-fold in these preparations with a recovery of about 15%. The contamination by mitochondria, lysosomes, nuclei and endoplasmic reticulum was low as estimated by marker enzymes and electron microscopy. In both unstimulated and stimulated monocytes thromboplastin was largely recovered in this plasma membrane fraction, providing direct evidence for its membrane localization. Phospholipase C (E.C. 3.1.4.3) is a potent inactivator of thromboplastin through its hydrolysis of the phospholipids necessary for thromboplastin activity [Otnaess, Prydz, Bjørklid & Berre (1972) Eur. J. Biochem. 27, 238-243]. About 70% of the total membrane thromboplastin activity was inactivated when whole cells were treated with phospholipase C and the membranes subsequently isolated. Following stimulation to induce thromboplastin synthesis, the plasma membranes showed a shift in their relative content of phosphatidylcholine and phosphatidylethanolamine consistent with a transmethylation process.

Inhibitory effect of 3-deazaadenosine on the thromboplastin response of stimulated human monocytes.

Immune complexes (IC), 12-O-tetradecanoylphorbol-13-acetate (TPA), endotoxin (LPS) and phytohaemagglutinin (PHA) induce thromboplastin activity in human peripheral blood monocytes. In the presence of transmethylation inhibitors 3-deazaadenosine (DZA) and homocysteine a dose-dependent inhibition of the thromboplastin response reaching about 60 per cent was observed, when IC, LPS or PHA was used as the stimulant. TPA-induced thromboplastin synthesis was more resistant (maximum 20 per cent inhibition).