Agents capable of accelerating the conversion of prothrombin into thrombin in the presence of Ca+ may be said to have thromboplastic activity. Early studies of thromboplastic activity were car- ried out with materials derived from tissues. Wooldridge (1, 2), in 1886, produced intravascu- lar clotting in animals by the intravenous admini- stration of aqueous extracts of red blood cells, white blood cells, and tissues. He was of the opinion that the active factor in these extracts was a protein-lecithin compound. In 1912, Ho- well (3) also reported that a lipoprotein was re- sponsible for the thromboplastic activity of aque- ous tissues extracts. The lipid portion was a phosphatide which could be separated from the protein with fat solvents, and had the properties of "cephalin." He assumed that the thrombo- plastic activity of certain lecithins tested was due to contamination with cephalin. McLean (4, 5) extended these observations and showed that the potency of the cephalin preparation varied directly with the degree of unsaturation of the constituent fatty acids. Gratia and Levene (6) confirmed the work of Howell, using their own cephalin preparations. Wadsworth, Maltaner and Mal- taner (7-9) prepared cephalin free of lecithin and showed that the lecithin had no thromboplastic ac- tivity. Cohen and Chargaff (10) and Chargaff (11) verified that the tissue thromboplastic factor was a lipoprotein and that the lipid portion was associated with the phosphatides.
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Marcus et al. (1958) studied this question.