Perfusion of the myocardium with protein C in the presence of thrombin (EC 3.4.21.5) elicits a potent anticoagulant activity, which is identified as activated protein C on the basis of synthetic substrate hydrolysis and anticoagulant properties. The rate of activated protein C formation during the transit through the myocardium is at least 20,000 times that of thrombincatalyzed activation of protein C in the perfusion solution. The capacity ofthe heart to activate protein C is maintained for at least 1 hr when thrombin is present in the perfusate, but decays (halflife 30 min) once thrombin is omitted. Addition of diisopropylphospho-thrombin increases this decay rate more than 10-fold. Coperfusing diisopropylphospho-thrombin with active thrombin lowers the amount of protein C activation in the myocardium. Cultured monolayers of human endothelium enhance the rate of thrombin-catalyzed protein C activation. As with myocardium, the activation rate is inhibited by including diisopropylphosphothrombin in the medium. It is proposed that the surface ofvascular endothelium provides a cofactor that enhances the rate of protein C activation by thrombin.

Frataxin deficiency is the primary cause of Friedreich ataxia (FRDA), an autosomal recessive cardiodegenerative and neurodegenerative disease. Frataxin is a nuclear-encoded mitochondrial protein that is widely conserved among eukaryotes. Genetic inactivation of the yeast frataxin homologue (Yfh1p) results in mitochondrial iron accumulation and hypersensitivity to oxidative stress. Increased iron deposition and evidence of oxidative damage have also been observed in cardiac tissue and cultured fibroblasts from patients with FRDA. These findings indicate that frataxin is essential for mitochondrial iron homeostasis and protection from iron-induced formation of free radicals. The functional mechanism of frataxin, however, is still unknown. We have expressed the mature form of Yfh1p (mYfh1p) in Escherichia coli and have analyzed its function in vitro. Isolated mYfh1p is a soluble monomer (13,783 Da) that contains no iron and shows no significant tendency to self-associate. Aerobic addition of ferrous iron to mYfh1p results in assembly of regular spherical multimers with a molecular mass of approximately 1. 1 MDa (megadaltons) and a diameter of 13+/-2 nm. Each multimer consists of approximately 60 subunits and can sequester >3,000 atoms of iron. Titration of mYfh1p with increasing iron concentrations supports a stepwise mechanism of multimer assembly. Sequential addition of an iron chelator and a reducing agent results in quantitative iron release with concomitant disassembly of the multimer, indicating that mYfh1p sequesters iron in an available form. In yeast mitochondria, native mYfh1p exists as monomer and a higher-order species with a molecular weight >600,000. After addition of (55)Fe to the medium, immunoprecipitates of this species contain >16 atoms of (55)Fe per molecule of mYfh1p. We propose that iron-dependent self-assembly of recombinant mYfh1p reflects a physiological role for frataxin in mitochondrial iron sequestration and bioavailability.

Rationale Analyses of circulating cell membrane-derived microvesicles (MV) have come under scrutiny as potential diagnostic and prognostic biomarkers of disease. However, methods to isolate, label and quantify MV have been neither systematized nor validated. Objective To determine how pre-analytical, analytical and post-analytical factors affect plasma MV counts, markers for cell of origin and expression of procoagulant surface phosphatidylserine. Methods and Results Peripheral venous blood samples were collected from healthy volunteers and patients with cardiovascular disease and/or diabetes. Effects of blood sample collection, anticoagulant and sample processing to platelet free plasma (PFP), and MV isolation, staining and storage (freeze-thaw) and cytometer design were evaluated with replicate samples from these populations. The key finding is that use of citrate or EDTA anticoagulants decreases or eliminates microvesicles from plasma by inducing adhesion of the microvesicles to platelets or other formed elements. Protease inhibitor anticoagulants, including heparin, preserve MV counts. A centrifugation protocol was developed in which recovery of isolated MV was high with resolution down to the equivalent light scatter of 0.2 micron latex beads. Each procedure was systematically evaluated for its impact on the MV counts and characteristics. Conclusion This study provides a systematic methodology for MV isolation, identification and quantification, essential for development of MV as diagnostic and prognostic biomarkers of disease.

Frataxin is a conserved mitochondrial protein required for iron homeostasis. We showed previously that in the presence of ferrous iron recombinant yeast frataxin (mYfh1p) assembles into a regular multimer of approximately 1.1 MDa storing approximately 3000 iron atoms. Here, we further demonstrate that mYfh1p and iron form a stable hydrophilic complex that can be detected by either protein or iron staining on nondenaturing polyacrylamide gels, and by either interference or absorbance measurements at sedimentation equilibrium. The molecular mass of this complex has been refined to 840 kDa corresponding to 48 protein subunits and 2400 iron atoms. Solution density measurements have determined a partial specific volume of 0.58 cm(3)/g, consistent with the amino acid composition of mYfh1p and the presence of 50 Fe-O equivalents per subunit. By dynamic light scattering, we show that the complex has a radius of approximately 11 nm and assembles within 2 min at 30 degrees C when ferrous iron, not ferric iron or other divalent cations, is added to mYfh1p monomer at pH between 6 and 8. Iron-rich granules with diameter of 2-4 nm are detected in the complex by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. These findings support the hypothesis that frataxin is an iron storage protein, which could explain the mitochondrial iron accumulation and oxidative damage associated with frataxin defects in yeast, mouse, and humans.