A model for the initial phase of cell/surface interactions based on ligand binding phenomena

The interaction between mammalian cells and solid surfaces plays an important role in a number of biological phenomena. Of particular clinical importance is the migration of cells suspended in blood to the wall of a blood vessel in the event of tissue damage. While the resultant inflammation often represents a desirable response to an external challenge, responses of this type can also lead to adverse consequences. Although the cell migration phenomenon is well known, a plausible mechanism for controlling the critical 'rolling' stages of adhesion has yet to be proposed. In this report we suggest how a simple consideration of ligand/receptor binding interactions can be used to explain a switch between a situation where a cell population is almost entirely in free suspension, to one where a significant fraction is attached to the solid surface.

Decreased sialidase activity in mononuclear leucocytes of type 1 diabetic subjects: relationship to diabetic complications and glycaemic control

Leucocyte surface sialic acid content influences surface charge, deformability, and leucocyte-endothelial interaction. Abnormal leucocyte structure and function contributes both to microvascular damage and diabetic complications. The aim of this study was to investigate altered leucocyte SA metabolism in diabetic subjects and measure lysosomal sialidase which regulates leucocyte surface sialylation. We examined 26 Type 1 (insulin-dependent) diabetic subjects with retinopathy, 26 Type 1 diabetic subjects without complications, and 38 matched normal control subjects. Sialidase was assayed in freshly prepared sonicates of pure mononuclear leucocytes (MNLs), using the fluorometric substrate 4-methyl-umbelliferyl-N-acetylneuraminic acid. In the subjects with diabetes there was a significant negative correlation between MNL sialidase activity and both HbA1c (rs = 0.37, p = 0.007) and fructosamine (rs = -0.31, p = 0.026). MNL sialidase activity was significantly decreased in diabetic subjects with clinical evidence of complications compared to control subjects. HbA1c was significantly higher (p = 0.036) in diabetic patients with complications compared to those without. The observed decrease in MNL sialidase activity related to diabetic control may be important in the pathogenesis of vascular damage. Diabetes-associated changes in sialylation of functional cell surface glycoconjugates may have important clinical consequences.

Freeze-stable sialidase activity in human leucocytes: substrate specificity, inhibitor susceptibility, detergent requirements and subcellular localization

Human leucocytes contain a freeze-stable sialidase (neuraminidase; EC 3.2.1.18) activity in addition to the better-characterized lysosomal freeze-labile enzyme. In order to discriminate between the sialidase activities detected with the synthetic fluorimetric substrate 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (MU-Neu5Ac), different tritiated sialoglycoconjugate substrates were prepared. Using this sensitive radioactive assay system, leucocyte sialidase activity towards glycoproteins was shown to be labile to repeated freeze-thawing, but a Triton-stimulated activity towards gangliosides was entirely freeze-stable. Assay conditions were optimized for this freeze-stable ganglioside sialidase activity. Subcellular fractionation of mononuclear leucocytes (MNLs) on Percoll-density gradients showed that this ganglioside sialidase activity was entirely associated with the plasma membrane. Study of the detergent requirements showed that MNLs also demonstrated ganglioside sialidase activity when sodium cholate was present in place of Triton. Cholate-stimulated ganglioside sialidase activity was found to be entirely freeze-stable and localized at the plasma membrane. Studies on whole homogenates of MNLs demonstrated that the Triton-stimulated and cholate-stimulated activities showed similar acidic pH optima at < or = 3.9 and were both strongly inhibited by 2-deoxy-2,3-didehydro-N-acetylneuraminic acid and Cu2+, but not by free N-acetylneuraminic acid, N-(4-nitrophenyl)oxamic acid or heparan sulphate. These results suggest that human MNLs contain, in addition to the lysosomal freeze-labile sialidase, a single sialidase activity which is freeze-stable, ganglioside-specific, plasma membrane-associated and stimulated both by Triton and by cholate.

Kinetic analysis of papaya proteinase omega

Papaya proteinase omega (pp omega) has been purified from dried latex both by immunoaffinity and traditional methods. Kinetic analysis revealed that (1), the pp omega-catalysed hydrolysis of N-benzoyl-L-arginine p-nitroanilide (BApNA) has a lower specificity (kcat/Km) than the same reaction catalysed by papain; (2), the pp omega-catalysed hydrolysis of a tripeptide substrate having phenylalanine at the second position (S2-site) showed a more similar specificity to that catalysed by papain; (3), the significant difference between the two enzymes is that steady state kinetics with both L-BApNA and a tripeptide enables the identification in pp omega of other ionizations affecting binding. The active sites of papain and pp omega can therefore be distinguished by pH-dependence of kcat/Km.

Transport of D-fructose and its analogues by Trypanosoma brucei

Kinetic parameters for entry of D-fructose into Trypanosoma brucei brucei have been determined. The net uptake of D-fructose was found to be rapid and occurred at a rate which was comparable with that observed for uptake of D-glucose. The Km and Vmax were 3.91 +/- 1.58 mM and 69.1 +/- 7.2 nmol min-1 (mg protein)-1. D-Fructose was metabolized to pyruvate under aerobic conditions and to pyruvate and glycerol under anaerobic conditions in a manner similar to D-glucose. Comparisons of the kinetic parameters for D-fructose transport and metabolism indicated that uptake was rate limiting. Inhibition constants (Ki) for inhibition of 6-deoxy-D-glucose by D-fructose and D-fructose transport by 6-deoxy-D-glucose were consistent with the Km values for these two substrates. These interactions indicate that D-fructose and 6-deoxy-D-glucose share a single common transporter. 1,5-Anhydro-D-glucitol and 1,5 anhydro-D-mannitol (the fused pyranose ring analogues of D-glucose and D-mannose) have been found to interact well with the transporter, while L-sorbose (a D-fructose analogue with a pyranose ring) had only low affinity. However, 2,5-anhydro-D-mannitol (a fused furanose ring analogue of D-fructose) inhibited both 6-deoxy-D-glucose and D-fructose transport with a Ki of approx. 0.8 mM. The high affinity for 2,5-anhydro-D-mannitol (2-deoxy-D-fructofuranose) indicates that D-fructose is transported in the furanose ring form.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and characterization of soluble dextran-adenosine phosphate complexes: Kinetic effects of coenzyme loading

Soluble dextran-ATP complexes have been synthesized using a bifunctional oxirane as the coupling agent. The degree of coupling is time-dependent, allowing materials of varying coenzyme loadings to be produced very simply. Characterization studies have shown that at the maximum coenzyme loading obtained (34 molecules per complex) all coenzyme moieties were coenzymically active with hexokinase. The extent of coenzyme loading was shown to have a considerable influence on the values of Km and Vmax of the complex as a substrate for hexokinase. Enzyme activity was also found with acetate kinase and myokinase, and coenzyme recycling (ATP, ADP) was demonstrated in an ultrafiltration reactor.

Specificity and kinetics of hexose transport in Trypanosoma brucei

Transport of 6-deoxy-D-glucose was studied in Trypanosoma brucei in order to characterise the kinetics of hexose transport in this organism using a nonphosphorylated sugar. Kinetic parameters for efflux and entry, measured using zero-trans and equilibrium exchange protocols, indicate that the transporter is probably kinetically symmetrical. Comparison of the kinetic constants of D-glucose metabolism with those for 6-deoxy-D-glucose transport shows that transport across the plasma membrane is likely to be the rate-limiting step of glucose utilisation. The transport rate is nevertheless very fast and 6-deoxy-D-glucose, at concentrations below Km, enters the cells with a half filling time of less than 2 s at 20 degrees C. Thus the high metabolic capacity of these organisms is matched by a high transport rate. The structural requirements for the trypanosome hexose transporter were explored by measuring inhibition constants (Ki) for a range of D-glucose analogues including fluoro and deoxy sugars as well as epimeric hexoses. The relative affinities shown by these analogues indicated H-bonds from the carrier to the C-3, C-4 and C-5 hydroxyl oxygens and from the C-1 and C-3 hydroxyl hydrogens to the binding site. Hydrophobic interactions are likely at the C-2 and C-6 regions of the glucose molecule. Spatial constraints appear to occur around C-4 indicating that the transport site at this position is not freely open to the external solution as is the case with the mammalian hexose transporter. However, the trypanosome transporter appears to accept D-fructose but the common mammalian (erythrocyte type) hexose transporter does not.

Two distinct succinate thiokinases in both bloodstream and procyclic forms of Trypanosoma brucei

Two succinate thiokinase activities specific for either adenine or guanine nucleotides have been found in Trypanosoma brucei. Key glycolytic and citric acid cycle enzymes were measured to show repression of glycolysis and derepression of the citric acid cycle in the procyclic form, relative to the bloodstream form. A marked rise in adenine-linked succinate thiokinase activity accompanied a rise in activity of citric acid cycle enzymes. However, guanine-linked succinate thiokinase was found to increase only slightly in activity. These results implicate the adenine-linked enzyme as an essential component of the citric acid cycle, whereas the guanine-linked enzyme appears to be under separate control. This communication also reports for the first time the occurrence of citrate synthase activity in the bloodstream (long slender) form of T. brucei.

Sugar transport in Trypanosoma brucei: a suitable kinetic probe

A transport assay has been developed for use in the investigation of 1-deoxy-D-glucose uptake in trypanosomes. 1-Deoxy-D-glucose has high affinity for the trypanosome sugar transport system (net influx Km = 4.03 +/- 0.42 mM; V = 0.052 +/- 0.005 mM X s-1. D-Glucose oxidation is competitively inhibited by 1-deoxy-D-glucose. However, we show that 1-deoxy-D-glucose is not a substrate for metabolism and that the competition occurs because of interaction at the transport system. D-Glucose competitively inhibits 1-deoxy-D-glucose influx.

The aerobic/anaerobic transition of glucose metabolism in Trypanosoma brucei

The ratio of glycerol to pyruvate produced by T. brucei incubated with glucose at various oxygen tensions has been used as an index of the aerobic and anaerobic pathways of glucose metabolism. A minimal model is presented which fits the observed data. The value of the notional K of the aerobic/anaerobic transition from the model is close to that of the Km of trypanosomal glycerophosphate oxidase. The anaerobic pathway appears to be almost completely inoperative at oxygen tensions in the range of those found in venous and arterial blood.

Dihydrolipoamide dehydrogenase from halophilic archaebacteria

Dihydrolipoamide dehydrogenase has been discovered in the halophilic archaebacteria for the first time. The enzyme from both classical and alkaliphilic halobacteria has been investigated. (1) The enzyme specifically catalysed the stoichiometric oxidation of dihydrolipoamide by NAD+. Enzymic activity was optimal at 2 M-NaCl and was remarkably resistant to thermal denaturation. (2) The relative molecular masses (Mr) of the native enzyme from the various species of halobacteria were determined to be within the range 112000-120000. (3) The enzyme exhibited a hyperbolic dependence of catalytic activity on both dihydrolipoamide and NAD+ concentrations. From these steady-state kinetic measurements the dissociation constant (Ks) of dihydrolipoamide was determined to be 57 (+/- 5) microM. (4) The enzyme was only susceptible to inactivation by iodoacetic acid in the presence of its reducing ligands, dihydrolipoamide or NADH. The rate of inactivation followed a hyperbolic dependence on the concentration of dihydrolipoamide, from which the Ks of this substrate was calculated to be 55 (+/- 7) microM. Together with the steady-state kinetic data, the pattern of inactivations is consistent with the involvement in catalysis of a reversibly reducible disulphide bond, as has been found in dihydrolipoamide dehydrogenase from non-archaebacterial species. In eubacterial and eukaryotic organisms, dihydrolipoamide dehydrogenase functions in the 2-oxo acid dehydrogenase complexes. These multienzyme systems have not been detected in the archaebacteria, and, in the context of this apparent absence, the possible function and evolutionary significance of archaebacterial dihydrolipoamide dehydrogenase are discussed.

Estimation of Michaelis constant and maximum velocity from the direct linear plot

When estimates of Michaelis-Menten parameters are obtained from kinetic observations taken in pairs, as in the direct linear plot, bias can arise in the final estimates if any pairs lead to negative values of the maximum velocity V. This bias can be removed by treating such negative values as if they were large and positive, and by treating the corresponding values of Km in the same way.

Reaction of the aminic form of aspartate transaminase with difluoro-oxaloacetate

Addition of difluoro-oxaloacetate to the aminic form of aspartate transaminase causes a rapid shift of absorbance maximum of the enzyme from 332 nm to 328 nm, followed by a much slower shift to 360 nm corresponding to complete conversion of the aminic form of the enzyme into the aldimine form or a species with similar spectral parameters in rapid equilibrium with it. Kinetic analysis of both the initial fast reaction and the overall slow reaction by using repeated spectral scanning and stopped-flow techniques allows formulation of a basic reaction mechanism involving at least two intermediate enzyme complexes. Computer simulation of the progress curves of the initial fast reaction based on the suggested reaction mechanism gives kinetic parameters that are consistent with all the data obtained by other methods. A molecular reaction scheme involving a ketimine Schiff-base intermediate is proposed.

[19F]fluorine nuclear-magnetic-resonance study of the interaction of difluoro-oxaloacetate with aspartate transaminase

Difluoro-oxaloacetate interacts with the aldimine form of aspartate transaminase to give a complex, the dissociation constant of which has been determined spectrophotometrically and by 19F n.m.r. (nuclear magnetic resonance). The 19F n.m.r. line-width-pH and chemical-shift-pH profiles of difluoro-oxaloacetate in the presence of the aldimine form of the enzyme both show inflexion points in the pH5 and pH8 regions, which may arise from variations in the binding of difluoro-oxaloacetate as specific groups on the enzyme are successively protonated. Difluoro-oxaloacetate also interacts with apoenzyme to form a complex, the dissociation constant of which was determined by 19F n.m.r. The 19F n.m.r. line-width-pH and chemical-shift-pH profiles of difluoro-oxaloacetate in the presence of apoenzyme show a single inflexion point in the region of pH8. The absence, in this case, of an inflexion in the pH5 region indicates that the latter, present in the corresponding profiles for the aldimine form of the enzyme, results from ionization of an enzyme group associated with the pyridoxal phosphate cofactor.

Interaction of difluoro-oxaloacetate with aspartate transaminase

Diffluoro-oxaloacetate behaves as a competitive inhibitor of 2-oxoglutarate and as an uncompetitive inhibitor with respect to aspartate in steady-state kinetic experiments with cytoplasmic aspartate transaminase. In the presence of high concentrations of aspartate transaminase, difluoro-oxaloacetate is slowly transaminated to difluoro-aspartate, suggesting its use as a kinetic probe to study the reactions of the aminic form of the enzyme.