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A hereditary (three family members) deficiency of antithrombin III (AT-III) in which AT-III antigen (AT-III ag) is normal in spite of low heparin cofactor and antithrombin activity is described. Plasma levels were: AT-III ag, 0.92-0.96 U/ml; AT-III heparin cofactor activity, 0.54-0.62 U/ml; progressive antithrombin activity index, 0.13-0.18; anti-Xa activity, 0.50-0.56 U/ml. Plasma crossed immunoelectrophoresis (CIE) patterns performed with and without added heparin were normal, but serum CIE revealed a decreased complex peak. Purification of the patient's plasma AT-III by heparin-sepharose affinity chromatography showed a normal protein recovery and elution profile, but the purified AT-III fraction showed only 50% of the normal progressive thrombin neutralization and anti-Xa activity. When thrombin-antithrombin (TAT) complexes were formed by incubating with excess thrombin, SDS-polyacrylamide gel electrophoresis (PAGE) analysis revealed that half the patient AT-III formed TAT complexes while the remainder migrated as free AT-Ill. All the control AT-III formed TAT complexes. The patient's nonreacting AT-EI (AT-rn "Denver"), isolated by affinity chromatography, showed CIE and SDS-PAGE migration patterns characteristic of normal AT-Ill but failed to bind thrombin or Xa. Calculations from turnover studies in one patient and normal subjects with autologous 131I-AT-III suggested that AT-III "Denver" is removed from the plasma slightly more rapidly than normal.These studies indicate that the patients' variant AT-III molecule was characterized by normal heparin interaction but defective binding and inhibition of thrombin and Xa. These characteristics allow isolation of the nonreactive variant molecule by heparin-sepharose affinity chromatography.

A 29-yr-old white female has suffered from recurrent venous thromboses over the last 12 yr. Plasma antithrombin III (AT-III) levels were 48% of normal by immunoelectrophoresis and 56% by chromogenic assay. Three of four siblings and the father had similar AT-III levels without associated venous thromboses. Heparin-Sepharose chromatography demonstrated normal behavior of the patient's AT-III. Her purified AT- III could not be distinguished from AT-III purified from a normal control either by SDS polyacrylamide gel electrophoresis or by crossed immunoelectrophoresis, and the heparin cofactor activity and the progressive antithrombin activity of both AT-III samples were identical. Turnover studies were made in the patient using her own purified AT-III labeled with 131I, (*I). The results did not differ significantly from studies made with autologous *I-AT-III in two normal control women. Her fractional breakdown rate of 0.54 total plasma AT- III per day compared with 0.45 and 0.52 in the controls. These studies indicate that the patient synthesizes a normal AT-III molecule at half normal rates.

The newborn infant has a physiologically low level of protein C which rises very slowly in postnatal life. The frequency and significance of severe neonatal protein C deficiency has not been reported. In this study, protein C levels were measured in 110 newborn infants at the time of birth using functional (amidolytic, Cact) and immunologic (Laurell rocket, Cag) assays. The protein C levels were compared with a marker of thrombin activation (D-dimer fragment of fibrin, +D-D) and infants were subsequently followed for signs and symptoms of thrombosis. Results are summarized below (protein C levels are expressed as U/ml).Thirteen infants had protein C levels compatible with the homozygous deficiency state. Extremely low levels of protein C (<0.20 U/ml) were not found in well term infants and were rarely noted in stable preterm infants. D-D were infrequently present and no thrombosis occurred. Near term infants born with fetal distress frequently showed +D-D but rarely demonstrated extremely low levels of protein C. None of these infants required indwelling arterial catheters and no thromboses occurred. Preterm infants with severe respiratory distress showed lower protein C levels at birth (p <0.01). Although 71% had +D-D, thromboses in these infants were all related to invasive catheterizations. In contrast, the study population of twins demonstrated a high frequency of severe protein C deficiency with negative D-D and frequent thromboses, three of which occurred in the absence of instrumentation. In summary, severe protein C deficiency and thrombin activation are common in sick preterm infants with the risk of thrombosis increased by intravascular catheterization. In contrast, twins with severe protein C deficiency may manifest a thrombotic risk which is independent of thrombin activation or catheterization.

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