Topography of the chromatic pattern-onset VEP

The chromatic pattern-onset VEP has been used successfully as a sensitive and objective technique to determine congenital and acquired color vision deficiency. It also has been applied to characterize development, maturation and aging of the chromatic visual pathways. Here we determine the topographic components of the full-field VEP using the multifocal technique. Recordings were made with the VERIS system that extracts topographic VEPs using a pseudorandom stimulus sequence. Chromatic pattern stimuli were presented in an onset-offset temporal sequence, with colors modulated along different axes in the MBDKL color space. Additional experiments were conducted to verify the S-cone axis for each observer and that our chromatic stimuli were close to isoluminant at different field locations. Our data show reliable and robust chromatic onset VEP responses for multiple retinal areas that conform to pattern-onset full-field VEP waveform characteristics. For stimuli with chromatic contributions, pattern-onsets produced reliable and consistent waveforms whereas for stimuli with large luminance contributions pattern-reversal stimuli were superior. Our method for recording chromatic multifocal pattern-onset VEPs holds promise for clinical application to detect and monitor early retinal and optic nerve changes related to aging and disease.

Multiple ionization of atom clusters by intense soft X-rays from a free-electron laser

Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.

Transient experimental anemia in cholesterol-fed rabbits induces systemic overexpression of the reticulocyte-type 15-lipoxygenase and protects from aortic lipid deposition

Oxidative modification of low-density lipoprotein has been implicated in atherogenesis and the lipid peroxidizing enzyme 12/15-lipoxygenase (12/15-LOX) was suggested to be involved. For this study, we induced a strong and long-lasting systemic overexpression of the 15-LOX, in female New Zealand White rabbits by transient experimental anemia. After the hematopoietic parameters had returned to normal, these animals and age-matched controls were fed a lipid-rich Western-type diet for 10 weeks. Analyzing the lipid deposition in the aortic wall, we found that the 15-LOX overexpressing rabbits deposited significantly (P<0.01) less cholesteryl linoleate in the thoracic aorta than the corresponding controls. Similar results were obtained when free cholesterol and cholesteryl oleate were quantified. However, in the aortic arch where lipid deposition was much more severe a similar trend was observed, but the effects were not significant any more. Comparative determination (lipoxygenase overexpressing vs. control animals) of various plasma parameters as well as histological inspections of major organs did not reveal any indications for major organ malfunction. These data suggest that transient experimental anemia, which is accompanied by a long-lasting overexpression of the reticulocyte-type 15-LOX protects cholesterol-fed rabbits from lipid deposition in the aortic wall.

Phenotypes of 16 Stargardt macular dystrophy/fundus flavimaculatus patients with known ABCA4 mutations and evaluation of genotype-phenotype correlation

PURPOSE

To determine the phenotypic variability in patients with compound heterozygous or homozygous ABCA4 mutations, and to correlate the phenotypes with the functional properties of the altered protein.

METHODS

Sixteen patients from 13 families with signs of Stargardt macular dystrophy/fundus flavimaculatus and known mutations on both alleles of the ABCA4 gene (15 compound heterozygous, one homozygous) were characterized by clinical examination, fundus autofluorescence, psychophysics (color vision, kinetic and two-color dark- and light-adapted static threshold perimetry), and electrophysiology (Ganzfeld, multifocal ERG, EOG).

RESULTS

The homozygous 5917delG mutation resulted in the earliest disease manifestation (at 5 years) and a general cone-rod dysfunction, whereas the compound heterozygous mother (5917delG, G1961E) exhibited a very mild phenotype. Compound heterozygotes for the IVS40+5G-->A and the C1488Y or Y362X mutation showed also an early age of onset but only a central dysfunction. The effect of the 2588G-->C mutation, the G1961E mutation, and the complex mutation L541P-A1038V depended on the mutation in the second allele. Genotype-phenotype correlation appeared possible in most instances. Psychophysics revealed a simultaneous yet not necessarily congruent cone and rod dysfunction.

CONCLUSIONS

The type and combination of ABCA4 mutations in compound heterozygous patients determined were compatible with the severity of the phenotype as to age of onset and the functional consequences in the majority of patients. Unexplained phenotypic differences indicate the influence of other factors. ABCA4 mutations result in cone and rod dysfunction. Different disease durations limit the power of presently available genotype-phenotype correlations.

Multifocal electroretinogram: age-related changes for different luminance levels

BACKGROUND

Age-related changes in the first-order multifocal electroretinogram (mfERG) responses were measured for two different luminance levels (200 and 700 cd.m(-2)). The relative contribution of optical and neural factors to senescent change in response was evaluated.

METHODS

Data were obtained from one eye of each of 71 normal phakic subjects, age 9-80 years. The mfERG responses were recorded with the 7" stimulus-refractor unit (EDI) and VERIS 4.3 using the following protocol: bipolar contact lens, 103 hexagons, consecutive stimulation with 200 and 700 cd.m(-2), pupils > or =6 mm, amplification of 10(5), filter cut-offs at 10 and 300 Hz.

RESULTS

Age-correlated decreases in amplitude and response density and increases in P1 implicit time were found for both luminance levels. The mean response density (nV.deg(-2)) was higher for the 700 cd.m(-2) stimulus, but the rate of change with age was not significantly different from that obtained with the 200 cd.m(-2) stimulus. Implicit time was not significantly different for the two light levels, nor was the rate of change with age. The decrease in response density and the increase in implicit time with age were significant across all retinal regions, dividing the 50 deg stimulus into six concentric rings. Age-related change in response density was greatest for the central retina and decreased with increasing retinal eccentricity.

CONCLUSION

Log mfERG response changes linearly as a function of age. Analyses of the effects of reduced ocular media transmission and increased stray light, along with ancillary data obtained from pseudophakes, imply that age-related changes in the mfERG are due to both optical and neural factors.

Structural basis for lipoxygenase specificity. Conversion of the human leukocyte 5-lipoxygenase to a 15-lipoxygenating enzyme species by site-directed mutagenesis

Mammalian lipoxygenases constitute a heterogeneous family of lipid-peroxidizing enzymes, and the various isoforms are categorized with respect to their positional specificity of arachidonic acid oxygenation into 5-, 8-, 12-, and 15-lipoxygenases. Structural modeling suggested that the substrate binding pocket of the human 5-lipoxygenase is 20% bigger than that of the reticulocyte-type 15-lipoxygenase; thus, reduction of the active-site volume was suggested to convert a 5-lipoxygenase to a 15-lipoxygenating enzyme species. To test this "space-based" hypothesis of the positional specificity, the volume of the 5-lipoxygenase substrate binding pocket was reduced by introducing space-filling amino acids at critical positions, which have previously been identified as sequence determinants for the positional specificity of other lipoxygenase isoforms. We found that single point mutants of the recombinant human 5-lipoxygenase exhibited a similar specificity as the wild-type enzyme but double, triple, and quadruple mutations led to a gradual alteration of the positional specificity from 5S- via 8S- toward 15S-lipoxygenation. The quadruple mutant F359W/A424I/N425M/A603I exhibited a major 15S-lipoxygenase activity (85-95%), with (8S,5Z,9E,11Z,14Z)-8-hydroperoxyeicosa-5,9 ,11, 14-tetraenoic acid being a minor side product. These data indicate the principle possibility of interconverting 5- and 15-lipoxygenases by site-directed mutagenesis and appear to support the space-based hypothesis of positional specificity.

Alterations in leukotriene synthase activity of the human 5-lipoxygenase by site-directed mutagenesis affecting its positional specificity

The positional specificity of arachidonic acid oxygenation is currently the decisive parameter for classification of lipoxygenases. Although the mechanistic basis of lipoxygenase specificity is not completely understood, sequence determinants for the positional specificity have been identified for various isoenzymes. In this study we altered the positional specificity of the human 5-lipoxygenase by multiple site-directed mutagenesis and assayed the leukotriene A(4) synthase activity of the mutant enzyme species with (5S,6E,8Z,11Z,14Z)-5-hydroperoxy-6,8,11,14-eicos atetraenoic acid (5S-HpETE) as substrate. The wild-type 5-lipoxygenase converts 5S-HpETE almost exclusively to leukotriene A(4) as indicated by the dominant formation of leukotriene A(4) hydrolysis products. Since leukotriene synthesis involves a hydrogen abstraction from C(10), it was anticipated that the 15-lipoxygenating quadruple mutant F359W + A424I + N425M + A603I might not exhibit a major leukotriene A(4) synthase activity. Surprisingly, we found that this quadruple mutant exhibited a similar leukotriene synthase activity as the wild-type enzyme in addition to its double oxygenation activity. The leukotriene synthase activity of the 8-lipoxygenating double mutant F359W + A424I was almost twice as high, and similar amounts of leukotriene A(4) hydrolysis products and double oxygenation derivatives were detected with this enzyme species. These data indicate that site-directed mutagenesis of the human 5-lipoxygenase that leads to alterations in the positional specificity favoring arachidonic acid 15-lipoxygenation does not suppress the leukotriene synthase activity of the enzyme. The residual 8-lipoxygease activity of the mutant enzyme and its augmented rate of 5-HpETE conversion may be discussed as major reasons for this unexpected result.

Impact of notch filter use on waveforms of First- and Second-Order-Kernel responses from multifocal ERGs

The filter settings of the amplifier section of ERG recording systems have large impact on the waveforms of the recorded responses. In this study, the effects of a 50 Hz notch filter were assessed for both First-Order-Kernel- (FOK) and Second-Order-Kernel-responses (SOK 1st slice) from multifocal ERGs recorded with a VERIS III system. Amplitude and phase responses of the amplifier section were recorded for typical filter settings with the notch filter active and inactive. Multifocal ERGs (MERGs) from a group of 11 normal subjects were recorded for both amplifier settings, the waveforms resulted from those recordings were compared in the time and frequency domain. To verify the results, the recordings without the notch filter were digitally filtered with a simulated notch filter and compared to the responses recorded with an active analog notch filter. The line filter has the biggest attenuation at 50 Hz with an additional phase jump of 180 degrees. The FOK responses of MERGs are assembled by frequencies below 65 Hz with main spectral components between 19 and 47 Hz. The 1st slice of the SOK consists of frequencies up to 100 Hz with main components between 19 and 84 Hz. Thus, if FOK recordings are to be analyzed, the notch filter of the amplifier can be used in order to cope with noise problems caused by the line frequency of 50 Hz. However, one must be aware that the attenuation of higher frequency components will lead to changes of the waveforms. For SOK analysis, main spectral components are attenuated and/or shifted in phase, which leads to completely different waveforms and severe distortion of the recording results.

Bacterial and nonbacterial expression of wild-type and mutant human phospholipid hydroperoxide glutathione peroxidase and purification of the mutant enzyme in the milligram scale

15-Lipoxygenases and phospholipid hydroperoxide glutathione peroxidases are counterparts in the metabolism of hydroperoxy lipids and a balanced regulation of both enzymes is essential for normal cell function. Glutathione peroxidases contain selenocysteine as catalytically active amino acid and this selenocysteine is encoded by a TGA stop codon. Detailed protein chemical investigations on phospholipid hydroperoxide glutathione peroxidases and crystal trials have been hampered in the past by limited protein supply. There is no efficient natural source for large-scale enzyme preparation and overexpression of the functional protein in recombinant systems has not been reported so far. To avoid problems with recognition of the selenocysteine stop codon we mutated the selenocysteine to a cysteine and expressed the Sec46Cys mutant in milligram amounts in the baculovirus/insect cell system and as His-tag fusion protein in Escherichia coli. The recombinant enzyme species were purified by conventional fast protein liquid chromatography (nonfusion protein) or by affinity chromatography on a nickel matrix (His-tag protein). Surprisingly, we found that both protein variants were functional although their specific activities were reduced when compared with the wild-type enzyme. Basic protein chemical and enzymatic properties of the purified enzyme species were determined and monoclonal antibodies which recognize the native phospholipid hydroperoxide glutathione peroxidases were raised using our enzyme preparations as antigen. The described strategies for overexpression of mutant phospholipid hydroperoxide glutathione peroxidase species and their purification from recombinant sources provide sufficient amounts of enzyme for future protein chemical investigations and detailed crystal trials.

Early-onset severe rod-cone dystrophy in young children with RPE65 mutations

PURPOSE

To describe the ocular phenotype of patients with RPE65 mutations in infancy and young childhood.

METHODS

Four children from three families with severe early-onset visual impairment related to electrophysiologically detectable retinal dystrophy were screened for mutations in the RPE65 gene. Visual function from infancy to the age of 10 years was assessed with age-adapted methods. Clinical examinations and electroretinograms (ERGs) were also performed on the six parents.

RESULTS

In all three families, patients were compound heterozygous for mutations of the RPE65 gene (ins144T/IVS1+5G-->A, R91W/Y368H, 1114delA+T457N/IVS1+5G-->A). Visual acuity was measurable in all patients at the age of 6 to 10 years, despite severe visual impairment noted during infancy and congenital nystagmus in three of the four patients. Photophobia was not a feature. Funduscopic changes were discrete, the most prominent finding being increased granularity in the macula and the periphery. Peripheral vision was well preserved, measured by Goldmann perimetry. Rod ERGs were not recordable, whereas cone ERGs were detectable in early childhood. All features taken together suggest a specific form of Leber congenital amaurosis (LCA) distinguishable on clinical grounds. ERGs were normal in five of the six parents. One father had an ERG compatible with congenital stationary night blindness unrelated to his heterozygous state for the RPE65 mutation.

CONCLUSIONS

RPE65 mutations on both alleles may be associated with early-onset severe rod-cone dystrophy. Visual functions of the four patients were better than is usually seen in LCA, in particular in cases associated with retGC1 mutations. RPE65 mutations should be suspected in infants who appear to be blind in dim surroundings but react to objects in bright illumination and have nonrecordable rod ERGs and residual cone ERGs.

Shape and specificity in mammalian 15-lipoxygenase active site. The functional interplay of sequence determinants for the reaction specificity

Previous mutagenesis studies along with molecular modeling using the x-ray coordinates of the rabbit 15-lipoxygenase have led to the suggestion that the size of the substrate binding pocket may play an essential role in determining the oxygenation specificity of 5-, 12-, and 15-lipoxygenases. Based on the x-ray crystal structure of rabbit 15-lipoxygenase, Ile(593) appeared to be important in defining size and shape of the substrate-binding site in 15-lipoxygenases. We found that substitution of Ile(593) with alanine shifted the positional specificity of this enzyme toward 12-lipoxygenation. To compare the importance of position 593 with previously defined determinants for the oxygenation specificity, we introduced small (alanine-scan) or large amino acids (phenylalanine-scan) at critical positions surrounding the putative fatty acid-binding site, so that the volume of the pocket was either increased or decreased. Enlargement or alteration in packing density within the substrate binding pocket in the rabbit 15-lipoxygenase increased the share of 12-lipoxygenase products, whereas a smaller active site favored 15-lipoxygenation. Simultaneous substitution of both large and small residues in the context of either a 15- or 12-lipoxygenase indicated that there is a functional interplay of the sequence determinants for lipoxygenation specificity. If the 15-lipoxygenase active site is enlarged excessively, however, no lipoxygenation was observed anymore. Together these results indicate the importance of the overall size and shape of the arachidonic acid binding pocket in defining the specificity of lipoxygenase reaction.

Molecular analysis of 29 pyruvate kinase-deficient patients from central Europe with hereditary hemolytic anemia

We investigated the DNA of 29 unrelated pyruvate kinase (PK) deficiency (PKD) patients from Central Europe with hereditary nonspherocytic hemolytic anemia for mutations in the PK-L/R gene. Among 58 potentially affected alleles, 53 mutations were identified, of which 17 were different from each other. Of these 17 mutations, 13 were single-nucleotide (nt) substitutions resulting in amino acid exchanges, G787A (Gly263-Arg), G994A (Gly332-Ser), G1006T (Ala336-Ser), G1010A (Arg337-Gln), A1081G (Asn361-Asp), G1127T (Ser376-Ile), G1174A (Ala392-Thr), G1281T (Glu427-Asp), C1454T (Ser485-Phe), C1456T (Arg486-Trp), G1493A (Arg498-His), G1529A (Arg510-Gin), and C1594T (Arg532-Trp); 1 in-frame triplet deletion, 1060delAAG (delLys354); 1 in-frame triplet insertion, 1203insAGC (insSer after Cys401); 1 splicesite mutation, 101-1G-A; and 1 frameshift deletion, 628delGT. Six mutations, 628delGT, G787A, G1010A, G1127T, G1281T, and C1454T, are described for the first time. To test the hypothesis of a single origin of the most common PK mutation in the European population, G1529A, we investigated all patients at four polymorphic sites in the PK-L/R gene: C/A at nt 1705, C/T at nt 1992, the (ATT)n microsatellite in intron J, and a polymorphism (T)10/(T)19 in intron I. Nine patients homozygous for mutation G1529A were consistent in all four markers. In the group of patients homozygous for mutation G1529A, the hematologic parameters and clinical manifestations have been studied in detail. Although having an identical mutation in the PK-L/R gene, the patients are affected differently. Their appearance ranges from a very mild compensated hemolysis to a severe anemia. Possible molecular explanations are discussed.

Proliferating alveolar macrophages in BAL and lung function changes in interstitial lung disease

In interstitial lung disease, the number of alveolar macrophages (AMs) can be increased. This may be caused by recruitment of precursor cells from peripheral blood and/or local proliferation in the lung. We therefore analysed proliferation, by studying both the expression of the nuclear proliferation antigen, Ki67, and the deoxyribonucleic acid (DNA) content, using the Feulgen reaction followed by cytometry. The patients had interstitial lung disease, i.e. sarcoidosis (n = 20), extrinsic allergic alveolitis (n = 20), idiopathic lung fibrosis or lung involvement in collagen-vascular disease (n = 19). In all patient groups there was a significant increase in proliferating AMs compared to healthy controls (4.2 versus 1.4% Feulgen, 2.1 versus 0.5% Ki67), with a significant correlation between these two parameters. A positive correlation was also found in bronchoalveolar lavage (BAL) between numbers of lymphocytes and proliferating cells in sarcoidosis and in fibrosis. In fibrosis, numbers of eosinophils and proliferating cells were also positively correlated. Our main finding was, however, a positive correlation between numbers of proliferating cells (Feulgen) and lung function parameters, especially vital capacity and oxygen tension (PO2) at rest, in patients with sarcoidosis and lung fibrosis. By contrast, in extrinsic allergic alveolitis, no correlation could be observed between proliferating cells and cell population or lung function. Our results suggest that local proliferation of macrophages is an important element in interstitial lung disease.

Proliferating alveolar macrophages in BAL and lung function changes in interstitial lung disease

In interstitial lung disease, the number of alveolar macrophages (AMs) can be increased. This may be caused by recruitment of precursor cells from peripheral blood and/or local proliferation in the lung. We therefore analysed proliferation, by studying both the expression of the nuclear proliferation antigen, Ki67, and the deoxyribonucleic acid (DNA) content, using the Feulgen reaction followed by cytometry. The patients had interstitial lung disease, i.e. sarcoidosis (n = 20), extrinsic allergic alveolitis (n = 20), idiopathic lung fibrosis or lung involvement in collagen-vascular disease (n = 19). In all patient groups there was a significant increase in proliferating AMs compared to healthy controls (4.2 versus 1.4% Feulgen, 2.1 versus 0.5% Ki67), with a significant correlation between these two parameters. A positive correlation was also found in bronchoalveolar lavage (BAL) between numbers of lymphocytes and proliferating cells in sarcoidosis and in fibrosis. In fibrosis, numbers of eosinophils and proliferating cells were also positively correlated. Our main finding was, however, a positive correlation between numbers of proliferating cells (Feulgen) and lung function parameters, especially vital capacity and oxygen tension (PO2) at rest, in patients with sarcoidosis and lung fibrosis. By contrast, in extrinsic allergic alveolitis, no correlation could be observed between proliferating cells and cell population or lung function. Our results suggest that local proliferation of macrophages is an important element in interstitial lung disease.

Phosphofructokinase from Plasmodium berghei: a kinetic model of allosteric regulation

As in mammalian cells, phosphofructokinase (PFK) is of major regulatory importance in the glucose metabolism of Plasmodium berghei. The malarial enzyme shows allosteric properties similar to PFK from various sources; it is activated by fructose-6-phosphate and inhibited by ATP, but differs with respect to allosteric regulation. Enzyme activity is only marginally increased by AMP, a potent activator of many phosphofructokinases. Phosphoenolpyruvate, which is reported to inhibit PFK activity, efficiency activates the malarial enzyme. No activation by ADP was observed. Instead, ADP inhibits the enzyme non-allosterically and competitively to the substrate MgATP. Phosphate stimulates the catalytic activity of malarial PFK independently of the activation by F6P and PEP.

Phosphofructokinase from Plasmodium berghei. Influence of Mg2+, ATP and Mg2(+)-complexed ATP

The control enzyme phosphofructokinase is of regulatory significance in the metabolism of glucose by the malarial parasite Plasmodium berghei. (1) The enzyme was partially purified from erythrocytic stages of P. berghei by precipitation with poly(ethylene glycol) and chromatography on 2',5'-bisphosphoadenosine-Sepharose 4B. (2) Similarly to various other phosphofructokinases, the enzyme from P. berghei shows an allosteric behaviour. It is activated by fructose 6-phosphate and inhibited by ATP. (3) The effects of Mg2(+)-complexed ATP, free ATP and Mg2+ were studied by keeping constant the concentration of one of these and varying the concentrations of the other two. (4) The enzyme is shown to be allosterically inhibited by free ATP and by higher concentrations of Mg2+. Compared with phosphofructokinase of erythrocytes, inhibition by ATP is weaker by two orders of magnitude. Mg2(+)-complexed ATP has no effect on allosteric regulation. (5) The proposed kinetic model provides an adequate description of the data.

Glucose-6-phosphate dehydrogenase from Plasmodium berghei: kinetic and electrophoretic characterization

Evidence is given for the existence of a parasite-specific glucose-6-phosphate dehydrogenase (G6PD) in Plasmodium berghei by characterization of its kinetic and electrophoretic properties. From infected rat erythrocytes the parasites were isolated, washed, and lysed. G6PD was purified by affinity chromatography with 2'5'-ADP-Sepharose 4B, although the separation of the malaria-specific enzyme from that of the host cell was not complete. Malarial G6PD significantly differed from the red cell enzyme with respect to its electrophoretic properties. In cellulose acetate electrophoresis, a band with catodic mobility was observed in addition to the anodically mobile host cell enzyme at pH 7.0. The subunits of the parasite-specific G6PD have a molecular weight of 55 kDa in contrast to 59 kDa of red cell G6PD subunits. The enzyme from P. berghei shows no cross-reactivity with polyclonal antibodies against G6PD from rat erythrocytes. Thus, a close evolutionary relationship between both proteins and the presence of proteolytic modifications could be excluded. The Km value for G6P of malarial G6PD is increased by one order of magnitude compared with the host cell enzyme.

A kinetic model of phosphofructokinase from Plasmodium berghei. Influence of ATP and fructose-6-phosphate

Phosphofructokinase (PFK) from the malarial parasite Plasmodium berghei shows the following kinetic features: the more the pH is decreased, the more the enzyme is inhibited by ATP; in contrast to PFK from erythrocytes, this inhibition is less potent by two orders of magnitude; as in the red cell, fructose-6-phosphate (F6P) is a positive effector. Kinetic modelling of PFK from P. berghei has been performed by taking the pH-dependence of activity into regard, implicitly by the estimation of pH-dependent kinetic parameters for the inhibition by ATP and the activation by F6P and explicitly by the assumption of protonation-steps involved in allosteric regulation. By means of a novel procedure of model discrimination [D. Buckwitz and H.-G. Holzhütter: A new method to discriminate between enzyme-kinetic models. In: Application of Computational Methods in Medicine (Györi, I., ed.), Akademai, Budapest, in press] we have selected among several kinetic models the best rate equation which provides an adequate quantitative description of the kinetic behaviour of the enzyme in the relevant ranges of substrate concentrations and pH (5.8-7.6). It thus becomes clear how the highly increased glycolytic flux in malaria-infected cells could be affected through PFK.

Rheology of fibrin clots. III. Shear creep and creep recovery of fine ligated and coarse unligated closts

Creep and creep recovery of human fibrin clots in small shearing deformations have been investigated over a time scale from 24 to 10(4) s. Coarse, unligated clots and fine clots ligated by fibrinoligase in the presence of calcium ions were studied to suppliement previous data on coarse ligated and fine unligated clots. Stress was found to be proportional to strain up to at least a maximum shear strain (in torsion geometry) of 6.2%. The initial modulus (25 s after imposition of stress) is proportional to approximately the 1.5 power of concentration for fine ligated and coarse unligated clots. For fine unligated closts there is comparatively little creep subsequent to the initial deformation; ligation (in this case involving mostly the gamma chains) reduces the creep to nearly zero. For coarse unligated clots, there is substantially more creep under constant stress, and creep recovery is not complete. Ligation (in this case involving both camma and alpha chains) alrgely supresses the creep and causes the recovery to be complete. If the structure if fully formed before creep begins, tests of creep recovery by the Boltzmann superposition principle show adherence to linear visoelastic behavior for all four clot types. Otherwise, the Boltzmann test fails and the recovery is much less than calculated. For fine ligated clots, the observed recovery agrees well with that calculated on the basis of a dual structure model in which an additional independent structure is built up in the deformed state, so that the state of ease after removal of stress is a balance between two structures deformed in opposite senses. It is postulated that the coherence and elastic modulus of the fine ligated clot are largely due to steric blocking of long protofibrils with a high flexural stiffness. In the coarse clot, it is proposed that the structure involves extensive branching of thick bundles of protofibrils, which become permanently secured by the ligation of the alpha chains of the fibrin.