Calcium and the assays of human plasma clotting Factor XIII

1. A continuous fluorimetric assay for blood-clotting Factor XIII based on the incorporation of dansylcadaverine into casein was investigated. 2. Hammarsten casein was fractionated to yield the β-casein, which was dephosphorylated and acetylated to give a substrate which itself did not bind calcium and which was not cross-linked by the activated Factor. 3. The modified β-casein was used as substrate for a continuous fluorimetric assay and as substrate for the incorporation of radioactive glycine ethylester. 4. A linked fluorimetric assay for the zymogen is described. 5. The Km for calcium was redetermined and at 0.2mm was in the physiological range and much lower than the values reported by others using substrates which interact with calcium. 6. The Km for casein is about 14μm.