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H R Roberts

Publications and source records attributed to H R Roberts.

At least 127 records · Page 7Linked to original sources
PubMed1983

Identification of the molecular defect in factor IX Chapel Hill: substitution of histidine for arginine at position 145.

Hemophilia B Chapel Hill is a mild hereditary hemorrhagic disorder in which the factor IX antigen is present in normal amounts but factor IX biological activity is markedly reduced. Previous studies have demonstrated that purified factor IX Chapel Hill has 8% of the activity of normal human factor IX and that the activation of factor IX Chapel Hill is defective in that only one of the two peptide bonds hydrolyzed during activation of normal factor IX is cleaved. The tryptic peptides from normal human factor IX and factor IX Chapel Hill were subjected to analysis by high-performance liquid chromatography. Comparison of the elution profile of the peptides obtained from factor IX Chapel Hill and normal factor IX demonstrated that the tripeptide Leu-Thr-Arg, which is derived from the normal molecule (positions 143-145) immediately amino-terminal from the Arg-Ala peptide bond at 145-146 that is cleaved during the activation of factor IX with factor XIa, was absent in the digest obtained from factor factor IX Chapel Hill. The elongated "activation peptide" from factor factor IX Chapel Hill was obtained by further high-performance liquid chromatographic fractionation and subjected to primary structure analysis. The following sequence, corresponding to positions 143-147, was obtained: Leu-Thr-His-Ala-Glu. Thus, the primary molecular defect in factor factor IX Chapel Hill is the substitution of histidine for arginine at position 145. This substitution precludes cleavage by factor XIa at this peptide bond, and the activation peptide region remains associated with the light chain of factor IXa Chapel Hill.

Amino Acid Sequence↗
PubMed1983

Treatment of a high titer anti-factor-VIII antibody by continuous factor VIII administration: report of a case.

Daily administration of large doses of factor VIII concentrate in a hemophiliac with a high titer factor VIII inhibitor resulted in marked reduction in the titer and response of the inhibitor to factor VIII administration and made possible elbow and bilateral knee replacements under conventional factor VIII coverage. Studies performed during the course of treatment indicated that the reduction in the inhibitor was the result of specific tolerance to factor VIII.

Adult↗
PubMed1983

The management of musculoskeletal problems in hemophilia. Part I. Principles of medical management of hemophilia.

Musculoskeletal bleeding in general and arthropathy in particular are the central problems among many in the two major forms of hemophilia: classic hemophilia, caused by factor VIII deficiency, and Christmas disease, caused by factor IX deficiency. Currently available replacement therapy, if properly used in a multidisciplinary setting, should provide significant benefit to hemophilic patients despite its cost and occasional complications.

Anemia, Hemolytic↗
PubMed1982

Influence of metal ions on prothrombin self-association. Demonstration of dimer formation by intermolecular cross-linking with dithiobis(succinimidylpropionate).

Interaction of certain metal ions with prothrombin and prothrombin fragment 1 has been shown to result in conformational change(s). Self-association of prothrombin and prothrombin fragment 1 in the presence of divalent cations has been reported. The present study has made use of a covalent cross-linking reagent, dithiobis(succinimidylpropionate), to study the self-association of prothrombin in the presence of divalent cations. In the presence of certain divalent cations, prothrombin dimer is the product of such cross-linking. Optimal dimerization of bovine prothrombin requires preincubation with calcium ions prior to the addition of cross-linking reagent. Calcium ions are also required for dimerization after this preincubation period. A similar time dependence is not seen with human prothrombin. The dimerization of bovine prothrombin is also supported by strontium and gadolinium to an extent comparable to that of calcium and to a lesser extent by barium or manganese. Magnesium ions do not support dimerization. The results suggest that certain divalent cations either induce or stabilize a conformation of prothrombin which can self-associate to form dimers. The results further suggest that divalent cations are also necessary for the actual cross-linking process subsequent to this conformational change.

Amino Acid Sequence↗
PubMed1982

Cleavage and activation of human prothrombin by Echis carinatus venom.

The cleavage of human prothrombin by partially purified Echis carinatus venom (ECV) was investigated in the present report. Incubation of prothrombin with ECV resulted in the rapid cleavage of prothrombin to alpha-thrombin, with the release of fragment-1 and fragment-2. When dansyl arginine-N-(3-ethyl-1,5-pentanediyl) amide (DAPA), a very effective inhibitor of thrombin, was included in the ECV-prothrombin solution, meizothrombin was rapidly formed. Only small amounts of meizothrombin-1 could be detected. Prolonged incubation (23 h) in the presence of DAPA, however, resulted in nearly quantitative conversion of meizothrombin to meizothrombin-1 and fragment-1. Kinetic studies strongly suggested that the conversion of meizothrombin to meizothrombin-1 was due to ECV and not meizothrombin autolysis. In addition, EDTA, which inhibits ECV, blocked the cleavage of meizothrombin. Amino terminal sequence analysis indicated that ECV cleaves human prothrombin at two sites; Gly158-Ser159 and Arg322-Ile323. The former site differs from the site of autolytic cleavage of meizothrombin which occurs at Arg155-Ser156. In contrast to reports in the literature, the results of the present study indicate that the release of fragment-1 does not precede activation of human prothrombin by ECV.

Amino Acid Sequence↗
PubMed1982

Measurement of human factor IXa activity in an isolated factor X activation system.

To determine the functional properties of factor IX isolated from the plasma of CRM+ hemophilia B patients, an assay system using proteins isolated from human plasma had to be developed which would be amenable to kinetic studies under a variety of experimental conditions. The present study describes the activation of factor X by factor IXa, isolated from normal human plasma, in an assay system which allows manipulation of calcium, phospholipid and factor VIIIa concentrations. Initial rate measurements with factor VIIIa present in the system were made by incubating factor VIII with factor Xa immediately before factor IXa assay. With this approach, the initial rate of factor X activation was constant, with little evidence for a lag period. Within the framework of the assay system described in the present study, it should be possible to examine not only genetic variants of factor IX, but also variants of factor VIII, as well as providing a means of routine factor IXa and factor VIII(a) assays.

Blood Coagulation Tests↗
PubMed1982

The acceleration by polylysine of the activation of factor X by factor IXa.

The present study reports that polylysine can function as a cofactor in the conversion of factor X to factor Xa by factor IXa. In the presence of polylysine, factor X is converted to factor Xa by factor IXa as demonstrated by both clotting and amidolytic assays. The activation of factor X by factor IXa requires the prior activation of factor IX to IXa by factor XIa. Conversion of factor X to factor Xa by factor IXa is not observed in the absence of polylysine. The activation reaction proceeds optimally at pH 8.0 with an equal weight ratio of polylysine to factor X. The effect of polylysine is readily reversed by low concentrations of NaCl or elevated temperature suggesting that electrostatic interactions are of primary importance in the polylysine facilitation of the activation of factor X by factor IXa.

Animals↗
PubMed1982

Factor VIII inhibitors: a clinical overview.

There is much evidence to indicate that inhibitors to Factor VIII in patients with classical hemophilia are the result of an immunological response to exposure to material (VIII:C or VIII:CAg) that is absent or present in reduced amounts in these patients. The inhibitor is an antibody that is usually restricted in immunochemical composition and in many instances contains predominantly or exclusively gamma G3 or gamma G4 heavy chains. Exposure to Factor VIII in many inhibitor patients leads to typical anamnestic responses with marked increases in the level of the inhibitor. The tendency to develop inhibitors and the clinical characteristics of the inhibitor may be affected by genetic factors, basal levels of Factor VIII:C and/or VIII:CAg, and the nature and amount of the "immunizing" material. Currently accepted therapeutic modalities are aimed primarily at the management of acute bleeding episodes.

Adolescent↗
PubMed1982

Chronic hepatitis in patients with hemophilia A: histologic studies in patients with intermittently abnormal liver function tests.

Recent studies in multiply transfused patients with hemophilia A and persistent liver function abnormalities have shown a high incidence of chronic active hepatitis. The purpose of the present study was to determine the severity of liver disease in multiply transfused patients with intermittent liver enzyme abnormalities. Fifteen patients with elevated enzymes on two or three out of four determinations at 6-mo intervals were studied. None had signs or symptoms of chronic liver disease. Thirteen had serologic evidence of prior exposure to the hepatitis B virus. Liver biopsy performed on these patients after replacement therapy with factor VIII showed chronic persistent hepatitis or other mild forms of liver disease in 14 of the 15 patients. Patients with chronic persistent hepatitis had significantly higher mean liver enzymes at time of biopsy than patients with milder forms of hepatic inflammation, but there was no relationship between liver histology and hepatitis B serology or the amount of factor VIII used in the 6 mo preceding biopsy. These findings support the continued use of factor VIII concentrates in patients with hemophilia.

Adolescent↗
PubMed1981

Characterization of the defect in activation of factor IX Chapel Hill by human factor XIa.

Factor IXChapel Hill (Factor IXCH), an abnormal Factor IX molecule isolated from the plasma of a patient with mild hemophilia B, has previously been shown to exhibit delayed activation by Factor XIa and calcium. In this study, we have found that Factor IXCH is cleaved upon incubation with human Factor XIa and calcium; however, cleavage of this protein is not observed by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis under nonreducing conditions. Under reducing conditions, the rate of disappearance of the zymogen parallels both the appearance of the heavy chain and the generation of clotting activity. In addition, a protein band that migrates with an apparent molecular weight of 45,000 also increases in parallel with clotting activity. Factor IXCH and normal Factor IX (Factor IXN), after incubation with Factor XIa and calcium, were subjected to amino terminal sequence analysis. Activated Factor IXN is cleaved at an arginine-alanine (Arg-Ala) bond and an arginine-valine (Arg-Val) bond as demonstrated by formation of the three amino terminal sequences corresponding to the amino terminal of the light chain, heavy chain, and activation peptide. However, activated Factor IXCH has only two amino terminal sequences, corresponding to the original amino terminal sequence and the heavy chain (formed by cleavage at the Arg-Val bond). It is concluded that the major defect in Factor IXCH is the inability of Factor XIa to cleave the Arg-Ala bond at a significant rate. The rate of formation of clotting activity of Factor IXCH is approximately 60% of the rate of formation of clotting activity of Factor IXN. The specific clotting activity of activated Factor IXCH is between 20 and 33% of activated Factor IXN.

Amino Acid Sequence↗
PubMed1980

A survey of the effectiveness of prothrombin complex concentrates in controlling hemorrhage in patients with hemophilia and anti-Factor VIII antibodies.

The treatment of patients with hemophilia A and anti-Factor VIII antibodies is difficult. Between July 1977 and June 1978, a survey was carried out by an ad hoc working party of the subcommittee on Factor IX concentrates of the International Committee on Thrombosis and Hemostasis to assess the effectiveness of Prothrombin Complex Concentrates in controlling hemorrhage in these patients. The results are presented in this paper and, although subjective, support the view that these concentrates are not as effective in patients with inhibitors as Factor VIII concentrates are in patients without inhibitors.

Antibodies↗
PubMed1980

Haemostatic failure of prothrombin complex concentrates during elective dental procedure.

One haemophilia A patient with an inhibitor was exposed to a graded surgical procedure and treated with Prothrombin Complex Concentrates (PCC). Hemostasis was not obtained until the inhibitor was neutralized and the patient's factor VIII level was normal. This patient is presented to emphasize that PCC are not nearly as effective as once hoped in producing hemostasis in inhibitor patients. Consequently, major decisions should not be made presuming efficacy of these products in inhibitor patients.

Adult↗