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 K_m 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 K_m for casein is about 14μm.