Involvement of heparin chain length in the heparin-catalyzed inhibition of thrombin by antithrombin III

The mechanism of the heparin-promoted reaction of thrombin with antithrombin III was investigated by using covalent complexes of antithrombin III with either high-affinity heparin (Mr = 15,000) or heparin fragments having an average of 16 and 12 monosaccharide units (Mr = 4,300 and 3,200). The complexes inhibit thrombin in the manner of active site-directed, irreversible inhibitors: (Formula: see text) That is, the inhibition rate of the enzyme is saturable with respect to concentration of complexes. The values determined for Ki = (k-1 + k2)/k1 are 7 nM, 100 nM, and 6 microM when the Mr of the heparin moieties are 15,000, 4,300, 3,200, respectively, whereas k2 (2 S-1) is independent of the heparin chain length. The bimolecular rate constant k2/Ki for intact heparin is 3 X 10(8) M-1 S-1 and the corresponding second order rate constant k1 is 6.7 X 10(8) M-1 S-1, a value greater than that expected for a diffusion-controlled bimolecular reaction. The bimolecular rate constants for the complexes with heparin of Mr = 4,300 and 3,200 are, respectively, 2 X 10(7) M-1 S-1 and 3 X 10(5) M-1 S-1. Active site-blocked thrombin is an antagonist of covalent antithrombin III-heparin complexes: the effect is monophasic and half-maximum at 4 nM of antagonist against the complex with intact heparin, whereas the effect is weaker against complexes with heparin fragments and not monophasic. We conclude that virtually all of the activity of high affinity, high molecular weight heparin depends on binding both thrombin and antithrombin III to heparin, and that the exceptionally high activity of heparin results in part from the capacity of thrombin bound nonspecifically to heparin to diffuse in the dimension of the heparin chain towards bound antithrombin III. Increasing the chain length of heparin results in an increased reaction rate because of a higher probability of interaction between thrombin and heparin in solution.     

Monoclonal antibodies to the two most basic papaya proteinases

The proteinases fromCarica papaya include papain, isoenzymes of chymopapain and two proteinases A and B distinguished by their unusually high pI. The identity of one of the most basic proteinases has been questioned. The present report describes the preparation and characterisation of two monoclonal antibodies that react specifically with papaya proteinases A and B respectively and a third that identifies a common structural feature found in papain and proteinase A.     

Immunochemical analysis of respiratory-chain components of micrococcus luteus (lysodeikticus).

Membrane-bound antigens of the respiratory chain of Micrococcus luteus were analyzed by crossed immunoelectrophoresis after growth of the organism in the presence of 59Fe, the flavin adenine dinucleotide-flavin mononucleotide precursor D-[2-14C]riboflavin, or the heme precursor 5-amino-[4-(14)C]levulinic acid. Using zymograms and procedures of selective extraction in conjunction with autoradiography, it was possible to resolve and partially characterize a number of antigens. Succinate dehydrogenase (EC 1.3.99.1) was shown to possess covalently bound flavin and nonheme iron and was possibly present as a complex with cytochrome. Three other dehydrogenases, namely, NADH dehydrogenase, NAD(P)H dehydrogenase (EC 1.6.99.3), and malate dehydrogenase (EC 1.1.1.37), contained flavin in noncovalent linkage, the NAD(P)H dehydrogenase also possessing nonheme iron. Four other discrete antigens (or antigen complexes) containing both iron and heme centers also resolved, as were two minor immunogens possessing iron as the sole detectable prosthetic group.     

Stimulus-secretion coupling of glucose-induced insulin release. LI. Divalent cations and ATPase activity in pancreatic islets

Pancreatic islets can be viewed as a fuel-sensor organ. The amount of ATP used by the islet cells for the maintenance of adequate Ca2+ gradients across membranes is not known. An indirect approach to this issue consists in the measurement of Ca-ATPase activity. The kinetics of Ca-ATPase in islet homogenates yielded a Km for ATP close to 0.1 mM and two Km values for Ca2+ close to 0.13 and 4-6 microM, respectively. Within limits, the Ca-ATPase appeared as a distinct entity from Mg-ATPase. Several divalent cations, including Mg2+, inhibited the Ca-ATPase activity. Calmodulin also inhibited, significantly albeit modestly Ca-ATPase. The activity of the enzyme was increased at high pH or in the presence of bicarbonate. The reaction velocity at close-to-physiological concentrations of ATP, Ca2+ and H+ suggests that the consumption of ATP by the Ca-ATPase may account for a major fraction of the overall rate of ATP breakdown in intact islets.     

Comparison of AMP and NADH binding to glycogen phosphorylase b

The binding sites for the allosteric activator, AMP, to glycogen phosphorylase b are described in detail utilizing the more precise knowledge of the native structure obtained from crystallographic restrained least-squares refinement than has hitherto been available. Localized conformational changes are seen at the allosteric effector site that include shifts of between 1 and 2 A for residues Tyr75 and Arg309 and very small shifts for the region of residues 42 to 44 from the symmetry-related subunit. Kinetic studies demonstrate that NADH inhibits the AMP activation of glycogen phosphorylase b. Crystallographic binding studies at 3.5 A resolution show that NADH binds to the same sites on the enzyme as AMP, i.e. the allosteric effector site N, which is close to the subunit-subunit interface, and the nucleoside inhibitor site I, which is some 12 A from the catalytic site. The conformations of NADH at the two sites are different but both conformations are "folded" so that the nicotinamide ring is close (approx. 6 A) to the adenine ring. These conformations are compared with those suggested from solution studies and with the extended conformations observed in the single crystal structure of NAD+ and for NAD bound to dehydrogenases. Possible mechanisms for NADH inhibition of phosphorylase activation are discussed.     

Human class I histocompatibility antigens (HLA-A,B,C). A small proportion only is phosphorylated

Human Class I HLA antigens (HLA-A,B,C) were isolated by immune precipitation from cells labelled with 32P, [35S]methionine or 125I (by lactoperoxidase-catalysed cell-surface iodination) and were analysed using both one- and two-dimensional electrophoretic systems. In several B-lymphoblastoid cell lines and in human peripheral blood lymphocytes the electrophoretic mobility of the 32P-labelled HLA-A,B,C heavy chains consistently differed from that of molecules labelled by other means. Thus the 32P-labelled heavy chains appeared to be larger and to possess a more acidic pI than did heavy chains labelled with [35S]methionine or 125I, or identified by Coomassie Blue staining. Phosphatase treatment of immunoprecipitates, under conditions where 32P-labelled antigens were shown to be dephosphorylated, did not affect the mobilities of the [35S]methionine-labelled heavy chains. On glycosidase treatment, the positions of the 32P-labelled heavy chains were affected by neuraminidase but not by endo-beta-N-acetylglucosaminidase H. These results imply that phosphorylated HLA-A,B,C antigens comprise only a small proportion (relative to the total cellular HLA-A,B,C antigens) of the biosynthetically mature molecules. The possible significance of such heterogeneity is discussed.     

Interactions of lens proteins. Self-association and mixed-association studies of bovine alpha-crystallin and gamma-crystallin

Concentrated solutions of calf alpha-crystallin (up to 45 g/l) and gamma-crystallin (up to 67 g/l) were subjected to frontal exclusion chromatography at pH 7.3, ionic strength 0.17 and 20 degrees C. The experimental concentration dependence of the weight-average partition coefficient was compared with theoretical expressions, which include considerations of thermodynamic non-ideality effects, for the concentration dependence of a single solute and of a solute undergoing reversible self-association. Two types of association pattern were examined, discrete dimerization and indefinite self-association. The partition chromatography results are consistent with an indefinite self-association of gamma-crystallin, governed by an isodesmic association constant of 6.7 X 10(-3) l/g. alpha-Crystallin appears to self-associate either very weakly, with a maximal association constant of 0.9 X 10(-3) l/g, or not at all; the distinction depends on the assessment of the non-ideality coefficients. The consequences of excluded volume effects on these self-association equilibria at high total protein concentration are discussed. Mixtures of alpha-crystallin and gamma-crystallin were analyzed by frontal exclusion chromatography (up to 14 g/l) and sedimentation velocity (up to 115 g/l): no interaction was observed.     

Leg chaetotaxy and the classification of the Uropodina (Acari: Mesostigmata)

Details of the segmental chaetotaxy of the legs of 47 species of Uropodina are given. On the basis of the ontogenetic development of the chaetotaxy, the Uropodina may be divided into two groups which coincide with the concepts of the Lower (Polyaspidoidea) and Higher (Uropodoidea) Uropodina of certain authors. Chaetotactic criteria do not support the classification of the Polyaspidini and Trachyuropodini sensu Hirschmann and Z-Nicol within the Oplitinae Hirschmann & Z-Nicol, the Diarthrophallini within the Uroactiniinae Hirschmann & Z-Nicol or the genus Trachytes within the Uropodini.
A critical appraisal is given of the classification of the Uropodidae (based on "Gangmerk-male") by Hirschmann and Z-Nicol.