The distance between the high affinity sites of troponin-C measured by interlanthanide ion energy transfer

Trivalent lanthanide ions are known to be good substituents for Ca2+ at all four calcium-binding sites of rabbit skeletal troponin-C (TnC). In particular, the visible luminescence of terbium ions bound at the two high affinity sites can be excited via a tyrosine residue. We have carried out energy transfer measurements using Tb3+ as the donor and a number of lanthanide ions as acceptors in order to measure the distance between the two high affinity sites in TnC. Luminescence decay measurements showed that, in the absence of acceptors, TnC-bound Tb3+ decays with a single lifetime of 1.31 ms. In the presence of a good acceptor such as Nd3+, the decay was resolvable into two components of roughly equal amplitude. The first was the same in lifetime as that of TnC-bound Tb3+ alone; the second has a shorter lifetime, presumably due to interlanthanide ion energy transfer. From these lifetimes and published critical transfer distances (Horrocks, W. DeW., Jr., Rhee, M.-J., Snyder, A.P., and Sudnick, D.R. (1980) J. Am. Chem. Soc. 102, 3650-3652), we obtained a distance of 0.92 nm between the two high affinity sites. This distance is consistent with the fact that the two high affinity sites have been located in the COOH-terminal half of TnC and with the proposal that each half of TnC is homologous in structure to parvalbumin (Kretsinger, R. H., and Barry, C. D. (1975) Biochim. Biophys. Acta 405, 40-52).

Metal-binding properties of calmodulin

Metal-binding properties of calmodulin have been studied by using trivalent lanthanide ions as analogs of Ca2+. In agreement with a report published as this work was in progress [Kilhoffer, M.-C., Demaille, J. G., and Gerald, D. (1980) FEBS Lett. 116, 269-272] we found that sites I and II are the high-affinity sites, while sites III and IV are the low-affinity sites for Tb3+. Competition experiments suggest the same preference in binding also applies to Ca2+. With calmodulin selectively nitrated at either of the two tyrosine residues we found that, although both tyrosine groups can transfer energy to the bound Tb3+, the fluorescence of only Tyr-138 is sensitive to metal binding. Direct excitation of bound Eu3+ ions using a laser indicates that all four sites possess very similar microenvironments. These studies demonstrate that the binding properties of calmodulin are different from those of the homologous protein troponir C.

Serial measurement of circulating immune complexes in healthy subjects

Circulating immune complex (CIC) levels were serially studied in healthy subjects and in normal rats. In human 3 methods (Clq solubility, complement consumption, and granulocyte phagocytosis tests) were used for the measurement of CIC; in rats, CIC levels were determined by the modified complement consumption test. A marked fluctuation in the CIC level was observed both in healthy human subjects and in normal rats. No correlation between results of the 3 assays used for CIC detection was found indicating that not only the level but the composition of CIC changes continuously in health.

Immunogenicity of antigen complexed with antibody. I. Role of different isotypes

IgM-, IgG1-, IgG2a-, IgG2b- and IgE-type anti-ovalbumin antibodies were isolated from rat immune sera and the complexes formed by antibodies of a defined isotype and the antigen were compared for antibody avidity and interaction with homologous complement. IgG2a-containing complexes consumed total complement with the greatest efficiency. IgG2a-, IgG2b- and IgM-containing complexes displayed similar activities in activation of the alternative pathway while IgG1 was less efficient. Reduction and alkylation of IgG2a antibodies abolished the capacity of the complex to activate the alternative pathway, but not their total complement consumption. The complement-dependent solubilization was greatest when complexes contained IgG1- or IgM-type antibodies and lowest with IgG2a-containing complexes. Inbred Long Evans rats immunized by complexes containing IgG1 or IgG2b antibodies displayed a markedly lower delayed-type hypersensitivity (DTH) compared with those immunized by antigen alone. Immunization with IgG2a- or IgE-containing complexes resulted in a slightly decreased DTH, while IgM-containing complexes induced DTH like ovalbumin alone. IgG2a-containing complexes elicited an antibody response markedly higher than that found in animals immunized by antigen alone. The same effect was found when complexes contained reduced-alkylated IgG2a. Immunization with complexes containing the other isotypes involved in the study induced an antibody response similar to the antigen in phosphate-buffered saline.

Binding of different ligands to IgM-Fc receptors of rat leucocytes

The primary receptor ligand interaction between rat leucocyte membrane receptors for IgM and their ligands were examined. We found that the incubation time for optimal IgM binding is different on the two types of IgM-Fc receptor-bearing spleen cells and peritoneal macrophages. The investigation of cytophilic activity of polyclonal or monoclonal IgM proteins and their fragments from various species indicated the fine specificity of receptors. Finally, data are presented which suggest a multiple point co-operative binding between IgM-coated erythrocytes and IgM-Fc receptor-bearing cells.

Complement-mediated fragmentation of soluble and insoluble immune complexes containing porcine anti-DNP antibodies

Complement-mediated release of soluble immune complexes and immune precipitates containing DNP-PSA and precipitating or non-precipitating porcine anti-DNP antibodies was studied. A decrease in the average size of soluble immune complexes indicating their fragmentation was observed during incubation in excess human serum, the extent of the complex release was found to be in direct proportion to the time of incubation. The effect was complement-dependent. In the second part of the study, complement-dependent solubilization of the immune precipitates of the precipitating antibody preparation was compared to the solubilization of the precipitates of the non-precipitating antibody formed in the presence of PEG. Although, both types of precipitates activated complement in about the same extent, complexes of non-precipitating antibody were solubilized much slower than those of the precipitating one. As avidity of both antibody preparations to the antigen was high, the observed differences in the rate of the complex solubilization probably reflected differences in the structure of the two types of complexes.

The influence of tissue transglutaminase on the function of Fc receptors

In contrast to FcRII the soluble Fc receptor (FcRI) of human peripheral mononuclear blood cells (PMBC) is shed from PMBC following a 4-37 degrees C temperature shift and inhibits rosette formation of nonshed PMBC with antibody-coated erythrocytes (EA). Purified FcR, could be polymerized by tissue transglutaminase as was revealed by SDS-polyacrylamide gel electrophoresis. Comparing the Sephadex G-150 elution profile of the EA rosette inhibitory capacity of FcRI vs FcRI incubated in the presence of transglutaminase, the latter was found in a higher mol. wt region and could inhibit rosette formation by both FcRI and FcRII. Furthermore, the shedding of FcRI could be prevented by the addition of transglutaminase or Ca2+-ionophore A23187 (which leads to the activation of PMBC transglutaminase) to the cell suspension. The function of FcRII was not affected by either the addition of transglutaminase or Ca2+-ionophore to the cells. The results point to the involvement of transglutaminase in the determination of the functional state of the Fc receptor on the cell surface.

Proteolytic fragments of troponin C. Interactions with the other troponin subunits and biological activity

Fragments of rabbit skeletal muscle Ca2+-binding subunit of troponin (TnC), obtained by cleavage with trypsin, thrombin, and CNBr, were tested for their ability to form binary and ternary complexes with ATPase inhibitory subunit (TnI) and tropomyosin-binding subunit (TnT) and their ability to replace TnC in reversing TnI inhibition of actomyosin ATPase activity. Three regions of TnC were found to be involved in interaction with TnI. Regions near Ca2+-binding sites II and III require Ca2+ for the interaction, while a third region near Ca2+-binding site IV binds TnI whether or not Ca2+ is present. The TnT binding site has been localized in the NH2-terminal half of TnC. Several of the TnC fragments form soluble ternary complexes with TnI and TnT. Fragments that contain amino acid residues 89-100 and at least one pair of Ca2+-binding sites are able to reverse the TnI inhibition of actomyosin ATPase activity, which exhibits the same [Ca2+]1/2 regardless of which of the Ca2+-binding sites are present in the fragment.

Effect of soluble immune complexes of Fc and C3 receptor-dependent phagocytosis by human monocytes

The capacity was studied of bovine serum albumin (BSA) rabbit anti-BSA and ovalbumin (OA) rabbit anti-OA immune complexes of different composition to inhibit the Fc receptor-dependent adherence and phagocytosis of sensitized sheep red blood cells by human monocytes. Parallel experiments were performed on the ability of the immune but complement-reacted complexes to inhibit the C3 receptor-dependent phagocytosis of C3-coated yeast particles. The extent of inhibition of both the Fc and C3 receptor-dependent phagocytosis was proportional to the antibody avidity of the complex used. Immune complexes made at equivalence and at moderate antibody excess markedly inhibited both types of phagocytosis, whereas those made at moderate antigen excess had only a weak inhibitory effect. These findings can be explained by the correlation between the Fc receptor-binding and complement-activating capacities of immune complexes of different composition. An alternative explanation, however, is also discussed.

Interference of beta 2-microglobulin specific autoantibodies with EA-binding of human peripheral lymphocytes; inhibition of B-cell and enhancement of T-lymphocyte Fc-receptors

The effect of anti-beta 2 m-specific autoantibodies was investigated on the FcRs of human PBMCs. Anti-beta 2 m autoantibodies inhibited the FcRs of the lymphocyte subpopulation detectable by rosetting with EA(hu). On the contrary, when EA(ox) indicator system was used, in the majority of the cases an enhancement of EA rosette formation was detected. Using separated lymphocyte subpopulations we found that the binding of anti-beta 2 m autoantibodies increased the number of FcR+ non-B-cells and inhibited that of B-lymphocytes.

Classes and subclasses of rat antibodies: reaction with the antigen and interaction of the complex with the complement system

Antibodies against the dinitroaminophenyl (DNAP) haptenic group were raised in outbred CFY rats using HSA or LPS as carrier. Antibodies isolated by immunoadsorbent techniques were resolved into fractions representing distinct isotypes, and the resulting fractions were tested for avidity. Subclass IgG2a was found to contain antibodies of an avidity index lower than those of other IgG subclasses or IgM. IgG2a was the only isotype detected when DNAP-LPS was used for immunization. Complexes containing defined isotypes were compared for their capacity to activate homologous complement. IgGl type antibody-containing complexes displayed a low complement activating capacity compared with those containing IgG2b, IgG2c or IgG2a. The latter subclass when complexed with antigen can thus induce complement-dependent processes in spite of a low avidity. Insoluble complexes of IgG antibodies were rapidly solubilized in rat serum (CRA phenomenon), except those containing predominantly IgG2c.

Binding of lanthanide ions to troponin C

Tb3+ and Eu3+ bound to troponin C were detected by (1) changes in the fluorescence of the tyrosine chromophore of the protein or (2) the luminescence of the ions themselves excited by energy transfer from the protein or by direct excitation using a pulsed laser light source [Horrocks, W. DeW., Jr., & Sudnick, D. R. (1979) Science (Washington, D.C.) 206, 1194-1196]. Titrations carried out in the absence and presence of Ca2+ suggest two classes of binding sites (two sites in each class) for the lanthanides, corresponding to the high- and the low-affinity sites for Ca2+. Computer analysis, assuming competition between Ca2+ and the lanthanide, using the binding constants of Ca2+ for TnC [Potter, J. D., & Gergely, J. (1975( J. Biol. Chem. 250, 4628-4633] yields 5.2 X 10(8) M-1 and 9.7 X 10(6) M-1 for Tb3+ and 4.7 X 10(9) M-1 and 5.3 X 10(7) M-1 for Eu3+, for the high- and the low-affinity sites, respectively. From lifetimes of laser-induced luminescence in H2O and in D2O, the number of water molecules coordinated to Eu3+ was two at the high and three at the low affinity sites.

Alpha-1-antitrypsin-induced inhibition of complement-dependent phagocytosis

In a previous investigation, inhibition of complement-dependent rosette formation by alpha 1-antitrypsin (alpha 1-AT) was observed, and it was demonstrated that alpha 1-AT interacts through its carbohydrate portion with C3 and its fragments. In the present study, the effect of alpha 1-AT on the complement-receptor-mediated phagocytosis by human peripheral blood monocytes was examined. Purified alpha 1-AT inhibited in a dose-dependent manner phagocytosis of C3-carrying yeast particles. Inhibition was selective, concerned only C3-receptor-mediated phagocytosis, neither Fc-receptor-mediated phagocytosis nor uptake of untreated yeast particles was blocked by alpha 1-AT. It was demonstrated that alpha 1-AT towards C3 and fragments of C3 was not mediated by its antiprotease effect, but by its carbohydrate moiety. This finding suggests that alpha 1-AT may have an impact on various immune functions involving complement receptors.