BINDING OF CALCIUM-IONS AND THEIR EFFECT ON CLOTTING OF FIBRINOGEN MILANO-III, A VARIANT WITH TRUNCATED A-ALPHA-CHAINS

Calcium ions are known to be required for normal polymerisation of fib rin monomers. Normal human fibrinogen has three high-affinity calcium binding sites. Two of these are located in the D-domains whereas the t hird binding site was tentatively assigned either to the E-domain or t o the C-terminal part of the An-chain Furthermore, binding of calcium to the low-affinity binding sites (n greater than or equal to 10) faci litates fibrin monomer polymerisation. In several abnormally clotting fibrinogen variants, the polymerisation defect was partially normalise d following addition of calcium ions. In this study,we show normal bin ding of calcium to fibrinogen Milano III, a homozygous fibrinogen vari ant with truncated An-chains (An 452 Gly-Pro-Asp-->Trp-Ser-Stop). Thes e results confirm that the C-terminal parts of the An-chains beyond re sidue 451 Ile are not involved in calcium binding. The thrombin time w as severely prolonged and the final clot turbidity was strongly reduce d in fibrinogen Milano III. Moreover, calcium ions did not significant ly improve the abnormal clotting behavior of this dysfibrinogen. The p olymerisation defect in fibrinogen Milano III appears to be due to tru ncated An-chains as well as to the disulphide-linked albumin.