The role of fibrinogen A alpha chains in ADP-induced platelet aggregation in the presence of fibrinogen molecules containing gamma' chains

Human plasma fibrinogen (Fgn) is heterogenous with respect to the size of its gamma chains, which differ in that residues 408 to 411 of gammaA chains (93% of total) are replaced in gamma' chains by a unique 20 amino acid sequence (gamma408 to gamma427). In this study, we compared the contribution to adenosine diphosphate (ADP)-induced platelet aggregation of the A alpha chains in Fgn molecules containing predominantly (fraction 1-2) or exclusively (peak 1 Fgn) gammaA chains with that of molecules containing approximately 50% gamma' chains (peak 2 Fgn). Using washed human platelets, we confirmed that the number of peak 2 Fgn molecules binding to platelets in the presence of ADP was about half the number of peak 1 Fgn molecules (18,962 +/- 2,298 v 44,366 +/- 16,096 molecules per platelet), and that isolated S- carboxymethylated (SCM) gammaA chains supported ADP-induced platelet aggregation nearly as well as peak 1 Fgn. In contrast, SCM-gamma' chains alone supported aggregation poorly, whereas a mixture of SCM- gammaA and gamma' chains (1:1 ratio) gave intermediate results. Despite the findings with isolated SCM-gamma' chains, we found that peak 2 Fgn supported platelet aggregation nearly as well as peak 1 Fgn. However, peak 2 Fgn from which carboxy (COOH)-terminal A alpha chain segments had been removed by digestion with plasmin showed a markedly decreased platelet aggregation potential. Peak 1 Fgn core fraction from an 88% to 90% coagulable plasmin digest, or Fgn fraction 1-9, which has a high gammaA/gamma' chain ratio (93:7), but lacks COOH-terminal regions of A alpha chains, supported platelet aggregation to the same extent as did intact peak 2 Fgn. These findings indicate that Fgn molecules containing gamma' chains can approach the aggregation potential of Fgn molecules containing predominantly or exclusively gammaA chains only if intact A alpha chains are also present.     

Overexpression of the ferritin H subunit in cultured erythroid cells changes the intracellular iron distribution

To test the hypothesis that variations in H- and L-subunit composition in the ferritin shell affect intracellular iron metabolism, we established stable transfectants of mouse erythroleukemia cells overexpressing the H-ferritin subunit. Analyses were performed on individual clones of transfected cells induced to differentiate with hexamethylenbisacetamide (HMBA). The results showed that there was a reduction in the amount of hemoglobin produced, in inverse relationship with the level of H-subunit overexpression. Incorporation of [2- 14C]glycine into heme was reduced by 20% t0 30% in the clones overexpressing H-ferritin subunit compared with control clone. However, the reduction in hemoglobin production was not reversed by addition of heme precursors (delta-aminolevulinic acid or iron) or by hemin itself. A reduced accumulation of beta-globin mRNA was also observed, which could account for the impaired hemoglobin synthesis. Furthermore, synthesis of the endogenous L-ferritin subunit was greatly repressed. Gel retardation assays performed on cytoplasmic extracts of transfected cells using an iron-responsive element (IRE) as a probe revealed that in overexpressing cells, the iron-regulatory protein (IRP) had a conformation with a high RNA-binding affinity, thus leading to translational repression of the endogenous L-ferritin synthesis. These data suggest that an increased formation of H-rich isoferritins leads to a rapid chelation of the regulatory iron pool. While the mechanism underlying the reduction in beta-globin mRNA remains to be elucidated, this study provides direct evidence for the role of IRP-mediated regulation of ferritin expression in erythroid cell metabolism.     

Effect of nitric oxide on expression of transferrin receptor and ferritin and on cellular iron metabolism in K562 human erythroleukemia cells

Nitric oxide (NO) is known to increase the affinity of the intracellular iron-regulatory protein (IRP) for iron-response elements (IREs) in transferrin receptor and ferritin mRNAs, suggesting that it may act as a regulator of cellular iron metabolism. In this study, exogenous NO produced by adding the NO-generator S-nitroso-N-acetyl penicillamine gave a dose-dependent upregulation of transferrin receptor expression by K562 erythroleukemia cells and increased levels of transferrin receptor mRNA. NO did not affect the affinity of transferrin binding by the transferrin receptor. NO alone did not alter intracellular ferritin levels, but it did abrogate the inhibitory effect of the iron chelator desferrioxamine and potentiated the stimulatory effect of additional iron. NO also caused some increase in ferritin mRNA levels, which might mask any IRP-/IRE-mediated inhibitory effect of NO on ferritin translation. Although NO did not affect net iron uptake, it increased release of iron from K562 cells pulsed previously with 59Fe, and subcellular fractionation showed that it also increased the proportion of intracellular iron bound to ferritin. These findings provide direct evidence that NO can affect cellular iron metabolism and suggest that NO produced in vivo by activated bone marrow macrophages might affect erythropoiesis.     

Electron microscopy of fibrin Paris I

Fibrinogen Paris I, a congenital fibrinogen abnormality, is characterized by delayed fibrin aggregation and poor clot retraction owing to the replacement of normal gamma-chains by mutant gamma-chains, which are termed gamma-Paris I. Available evidence indicates that the structural abnormality involves the amino acid sequence near the COOH- terminus of the mutant chain and probably includes the region containing the normal gamma-chain crosslinking site. Electron microscopy was carried out on Paris I fibrin. In place of the normally interwoven network of branching cross-striated fibers, negatively or positively contrasted Paris I fibrin was characterized by nonfibrous clumps of material connected by distince fibrous strands tending to be thinner and more irregular in width than normal fibrin. Most Paris I fibrin fibers tended to the aperiodic, although cross-striations were observed occasionally in negatively contrasted specimens and rarely in positively contrasted specimens. In addition, Paris I fibrin frequently showed relatively short, abruptly terminating fibers. The gross ultrastructural differences between normal and Paris I fibrin suggest that for fibrin assembly to take place normally, a region(s) in the fibrin molecule near to or possibly overlapping the COOH-terminal gamma- chain crosslinking site must be preserved or at least not sterically hindered.     

Partitioning of hepatitis C virus during Cohn-Oncley fractionation of plasma

Because of concern about the safety of immune globulins with respect to transmission of hepatitis C, the partitioning of hepatitis C virus (HCV) during alcohol fractionation of a plasma pool prepared exclusively from anti-HCV-reactive donations was examined. Quantitation of HCV RNA was accomplished by nested polymerase chain reaction (PCR) at limiting dilutions. One PCR unit was arbitrarily defined as the minimum amount of HCV RNA from which an amplified product could be detected. The starting plasma pool contained 1.4 x 10(5) PCR units per mL. Most of the HCV RNA was found in cryoprecipitate and in Cohn fractions I and III, but it was also detected in fraction II, which is used for immunoglobulin G preparations. A 3.4-percent solution of IgG prepared from this fraction II contained 30 PCR units per mL. The fractionation process leading to immune globulin resulted in overall reduction in HCV RNA by a factor of 4.7 x 10(4). Although the presence of HCV RNA in the final product does not necessarily imply the presence of infectious virus, this work suggests that the safety of immune globulins with respect to HCV transmission is not due solely to the partitioning of HCV away from the immunoglobulin fraction.     

Low risk of viral infection after administration of vapor-heated factor VIII concentrate. International Investigator Group

A multicenter prospective study was carried out to evaluate whether a vapor-heated factor VIII concentrate transmitted blood-borne viral infections over a surveillance period of 15 months. Thirty-five patients with hemophilia and von Willebrand disease who had never received any blood components were treated. Twenty-eight were analyzed and found not to have non-A, non-B hepatitis. Sera from 20 of these 28 patients were also tested for the antibody to the hepatitis C virus. None had sero-converted during the follow-up period. None of the patients analyzed developed markers of the hepatitis B virus (n = 17) or the human immunodeficiency virus (n = 31). This vapor-heated factor VIII concentrate carries a low risk of transmitting hepatitis and human immunodeficiency virus infection.