Artificial dimers of native actin: Preparation and properties in biological functions

With the aid of tartryl-bis--aminocaprylazide artificial dimers were produced from F actin from rabbit striated muscle. These derivatives will not polymerize by themselves but are able to copolymerize fully with native G actin. By modification of a single side chain per dimer, this copolymerization was completely inhibited. The dimers are able to activate subfragment I ATPase of myosin and bind to DNase I with inactivation of the enzyme in the same manner as native G actin. Within the dimer, one ADP is immobilized and will exchange against ATP extremely slowly. The dimers do not bind to the mushroom toxin phalloidin.     

Characterization and chemical modification of the Na(+)-dependent bile-acid transport system in brush-border membrane vesicles from rabbit ileum.

The Na(+)-dependent uptake system for bile acids in the ileum from rabbit small intestine was characterized using brush-border membrane vesicles. The uptake of [3H]taurocholate into vesicles prepared from the terminal ileum showed an overshoot uptake in the presence of an inwardly-directed Na(+)-gradient ([Na+]out > [Na+]in), in contrast to vesicles prepared from the jejunum. The Na(+)-dependent [3H]taurocholate uptake was cis-inhibited by natural bile acid derivatives, whereas cholephilic organic compounds, such as phalloidin, bromosulphophthalein, bilirubin, indocyanine green or DIDS - all interfering with hepatic bile-acid uptake - did not show a significant inhibitory effect. Photoaffinity labeling of ileal membrane vesicles with 3,3-azo- and 7,7-azo-derivatives of taurocholate resulted in specific labeling of a membrane polypeptide with apparent molecular mass 90 kDa. Bile-acid derivatives inhibiting [3H]taurocholate uptake by ileal vesicles also inhibited labeling of the 90 kDa polypeptide, whereas compounds with no inhibitory effect on ileal bile-acid transport failed to show a significant effect on the labeling of the 90 kDa polypeptide. The involvement of functional amino-acid side-chains in Na(+)-dependent taurocholate uptake was investigated by chemical modification of ileal brush-border membrane vesicles with a variety of group-specific agents. It was found that (vicinal) thiol groups and amino groups are involved in active ileal bile-acid uptake, whereas carboxyl- and hydroxyl-containing amino acids, as well as tyrosine, histidine or arginine are not essential for Na(+)-dependent bile-acid transport activity. The irreversible inhibition of [3H]taurocholate transport by DTNB or NBD-chloride could be partially reversed by thiols like 2-mercaptoethanol or DTT. Furthermore, increasing concentrations of taurocholate during chemical modification with NBD-chloride were able to protect the ileal bile-acid transporter from inactivation. These findings suggest that a membrane polypeptide of apparent M(r) 90,000 is a component of the active Na(+)-dependent bile-acid reabsorption system in the terminal ileum from rabbit small intestine. Vicinal thiol groups and amino groups of the transport system are involved in Na(+)-dependent transport activity, whereas other functional amino acids are not essential for transport activity.     

Further characterization of membrane proteins involved in the transport of organic anions in hepatocytes Comparison of two different affinity labels: 4,4′-diisothiocyano-1,2-diphenylethane-2,2′-disulfonic acid and brominated taurodehydrocholic acid

4,4′-Diisothiocyano-1,2-diphenylethane-2,2′-disulfonic acid (H₂DIDS) known as an irreversible inhibitor of the anion transport in red blood cells (Cabantchik, Z.I. and Rothstein, A. (1972) J. Membrane Biol. 10, 311–330) blocks also the uptake of bile acids and of some foreign substrates in isolated hepatocytes (Petzinger, E. and Frimmer, M. (1980) Arch. Toxicol. 44, 127–135). [³H]H₂DIDS was used for labeling of membrane proteins probably involved in anion transport of rat liver cells. The membrane proteins modified in vitro by [³H]H₂DIDS were compared with those labeled by brominated taurodehydrocholic acid. The latter is one of a series of suitable taurocholate derivatives, all able to bind to defined membrane proteins of hepatocytes and also known to block the uptake of bile acids as well as of phallotoxins and of cholecystographic agents (Ziegler, K., Frimmer, M., Möller, W. and Fasold, H. (1982) Naunyn-Schmiedeberg's Arch. Pharmacol. 319, 254–261). The radiolabeled proteins were compared after SDS-electrophoresis with and without reducing agent present, solubilization by detergents, two-dimensional electrophoresis and after separation of integral and peripheral proteins. Our results suggest that the anion transport system of liver cells cannot distinguish between bile acids and the anionic stilbene derivative (DIDS). The labeling pattern for both kinds of affinity labels was very similar. Various combinations of separation techniques gave evidence that the radiolabeled membrane proteins are not subunits of a single native channel protein.     

The use of azidoarylimidoesters in RNA-protein cross-linking studies with Escherichia coli ribosomes

A series of related hetero-bifunctional RNA-protein cross-linking reagents has been prepared, carrying an imidoester or N-hydroxysuccinimide ester function at one end of the molecule, and a phenylazido function at the other. These compounds have been applied to RNA-protein cross-linking studies with ribosomal subunits, and one of them, p-azido-phenylacetic imidoester, has proved to be a particularly useful reagent for this purpose. The reagent first reacts specifically with protein amino groups, and subsequent photolysis of the azide group leads to cross-linking to the RNA in yields of up to 8% of the total protein. The whole reaction takes place under very mild conditions in aqueous solution.The individual proteins concerned in the cross-links have been identified by two-dimensional gel electrophoresis, and the existence of a covalent cross-link was confirmed by the isolation by two different methods of protein-oligonucleotide complexes carrying a ³²P label. Although most of the ribosomal proteins could be cross-linked to their corresponding ribosomal RNA within the individual subunits, RNA-protein cross-links at the ribosomal subunit interface were only detectable in vanishingly small amounts.The advantages of this type of genuine hetero-bifunctional reagent in RNA-protein cross-linking studies are discussed.     

Chemical modification of membrane proteins in relation to inhibition of anion exchange in human red blood cells

Mono-, di-, and trisulfonic acids, including 4,4′-diacetamido stilbene-2,2′-disulfonic acid (DAS) and 2-(4′-amino phenyl)-6-methylbenzene thiazol-3′,7-disulfonic acid (APMB) produce a reversible inhibition of sulfate equilibrium exchange in human red cells. A study of the sidedness of the action of a number of these sulfonic acids in red cell ghosts revealed that some, like DAS, inhibit only at the outer membrane surface while others, like APMB, inhibit at either surface. This finding suggests that at least two different types of membrane sites are involved in the control of anion permeability. The nature of the anion permeability controlling sites in the outer cell surface was investigated by studying the effects of DAS on the inhibition by dinitrofluoro-benzene (DNFB) of anion equilibrium exchange and on the binding of DNFB to the proteins of the red blood cell membrane. After exposure to DNFB in the presence of DAS for a certain period of time, there was a reduction of both the inhibitory effect of DNFB on sulfate exchange and the binding of DNFB to the protein in band 3 of SDS polyacrylamide gel electropherograms (nomenclature of Steck, J. Cell. Biol., 62: 1, 1974). Since binding to other membrane proteins was not affected, this observation supports the assumption that the protein in band 3 plays some role in anion transport. In accordance with the absence of an inhibitory effect at the inner membrane surface, internal DAS does not affect DNFB binding to the protein in band 3. DAS protected the anion exchange system not only against inhibition by DNFB but also by m-isothiocyanato benzene sulfonic acid. In contrast to DAS, the equally inhibitory phlorizin does not reduce the rate of dinitrophenylation of the protein in band 3. This suggests that either not all inhibitors of anion exchange exert their action by a combination with sites on the protein in band 3 or that in spite of the described evidence this protein is not involved in the control of anion movements. The effect of the irreversibly binding inhibitor 4-acetamido-4′-isothiocyanato-stilbene-2,2′-disulfonic acid (SITS) on DNFB binding to the protein in band 3 was studied in an attempt to differentiate DNFB binding related to inhibition of anion permeability from DNFB binding which is not involved. At least three distinguishable populations of DNFB binding sites were found: (1) binding sites common for DNFB and SITS which are probably related to inhibition, (2) other common sites which are not related to inhibition and (3) different sites whose dinitrophenylation is not affected by SITS. The number of sites in population (1) was estimated to be 0.8–1.2 ± 10⁶/cell. A study of the concentration dependence of the inhibition of anion equilibrium exchange with 4,4′-isothiocyanato-2,2′-stilbene disulfonic acid (DIDS) and APMB further suggests that among the sites in population (1) a major fraction is susceptible to modification by APMB and DIDS while the rest is only susceptible to DIDS. It remains undecided whether these differences of susceptibility reflect differences of accessibility or reactivity.     

Influence of fibrinogen on fibrin polymerization. Ultracentrifugation studies

During the transformation of fibrinogen to fibrin, excess fibrinogen suppresses further polymerization of fibrin, thereby enabling the nascent fibrin to be transported in a soluble form in blood. The question of possible complex formation between fibrin and fibrinogen was addressed by analyzing fibrin/fibrinogen (1:20, mol/mol) mixtures in the presence of calcium ions in stable linear sucrose density gradients by ultracentrifugation at 37 degrees C. During the period of ultracentrifugation in independent experiments, 40% of desAA-fibrin and 30% of desAABB-fibrin, respectively, precipitated without the participation of fibrinogen. The desAABB-fibrin, recovered in the gradient fractions, appeared as high-molecular-weight polymers (22 S), whereas the recovered desAA-fibrin exhibited only a slight increase in molecular weight (9 S) compared to fibrinogen (8 S). In contrast to this finding, both types of fibrin were totally recovered in gradient fractions provided that fibrinogen was present in the gradient at a uniform concentration of 2 mg/ml. In addition, the presence of fibrinogen but not human serum albumin reduced the size of desAABB-fibrin polymers (17 S). However, stable fibrin-fibrinogen complexes could not be demonstrated, since cosedimentation of differently labelled desAABB-fibrin and fibrinogen was not detectable. These studies suggest a specific but weak interaction of the solubilizing fibrinogen with the soluble fibrin polymers as demonstrated by a rapid exchange of both macromolecules.     

Interleukin-2 and blood brain barrier in cats: pharmacokinetics and tolerance following intrathecal and intravenous administration.

Single bolus doses of glycosylated human interleukin-2 (n IL-2) in the range of 2.8 x 10(3) to 2.0 x 10(6) IU/kg were administered to anesthesized cats via the cephalic vein (n = 10) or using suboccipital puncture (n = 8). CSF (cerebrospinal fluid) and blood samples were collected by repeated puncture. The n IL-2 concentration in four cats was determined on the basis of its biologic activity using 3H-thymidine incorporation into human ConA-blasts and by radioimmunoassay. In additional experiments radioactivity was determined in cerebrospinal fluid and serum after intravenous and intrathecal (i.th.) application of 5.8 x 10(3) - 3.2 x 10(3) IU/kg of 14C-acetyl-n IL-2 in regular time intervals. CSF and serum concentration time-profiles show a biexponential decline in the plasma elimination phase with half-lives of 4 min (alpha-phase) and 90 min (beta-phase) after intravenous and 20-120 min (alpha-phase) and 2-16 hours (beta-phase) after intrathecal application. There is a trend towards longer terminal elimination half-lives with increasing doses. Interleukin-2 is able to penetrate the blood brain barrier from the circulation into the cerebrospinal fluid and vice versa. Due to a slow rate of penetration and rapid elimination from blood only traces of n IL-2 (2-8 IU/ml) are detected in CSF after i.v. injection of 2 x 10(6) IU/kg, whereas concentrations between 400 and 1600 IU/ml are maintained in CSF for several hours following i.th. administration of 2-10 x 10(5) IU/kg.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bile acid binding proteins in hepatocellular membranes of newborn and adult rats. Identification of transport proteins with azidobenzamidotauro[14C]cholate ([14C]ABATC)

Neonatal hepatocytes are less active in uptake of bile acids than are mature hepatocytes. This phenomenon has been further investigated by transport studies with azidobenzamidotaurocholate (ABATC). Taurocholate, cholate and the photolabile ABATC were taken up by liver cells of adult rats by a sodium-dependent and by an additional sodium-independent mechanism. In the dark, ABATC inhibited the uptake of taurocholate and cholate. Taurocholate decreased the transport of ABATC in a competitive manner, both in the presence and absence of sodium. In neonatal hepatocytes the Vmax for taurocholate and for ABATC was similar but was lower than in mature liver cells. In contrast, the Km was similar for neonatal and mature hepatocytes. For identification of binding proteins in both kinds of cells ABATC was photolysed after preincubation with isolated hepatocytes. Under our experimental conditions (single ultraviolet flash) about 80% of the azido groups was converted to nitrene. The covalently binding nitrene derivative inhibited bile salt transport irreversibly. Photolabeling of intact hepatocytes or of isolated plasma membranes with ABATC resulted in radioindication of membrane proteins with 67, 60, 54, 50 and 43 kDa in mature plasma membranes but of proteins with masses of 67, 54, 43 and 37 kDa in neonatal basolateral membranes. The 50 kDa protein in largely lacking in membranes of 9-day-old rats. The process of photolabeling itself was sodium-independent when isolated cells were treated with ABATC. In contrast, the degree of labeling of intact hepatocytes was markedly reduced in the absence of sodium and chloride. 100-fold molar excess of taurocholate, benzamidotaurocholate (BATC), phalloidin or cyclosomatostatin protected isolated plasma membranes against coupling of ABATC. Photolabeling of hepatoma cells known to be deficient in bile salt transport did not result in radiomodification of membrane proteins.     

RNA-protein cross-linking in Escherichia coli 30S ribosomal subunits; determination of sites on 16S RNA that are cross-linked to proteins S3, S4, S5, S7, S8, S9, S11, S13, S19 and S21 by treatment with methyl p-azidophenyl acetimidate.

RNA-protein cross-links were introduced into E. coli 30S ribosomal subunits by treatment with methyl p-azidophenyl acetimidate. After partial nuclease digestion of the RNA moiety, a number of cross-linked RNA-protein complexes were isolated by a new three-step procedure. Protein and RNA analysis of the individual complexes gave the following results: Proteins S3, S4, S5 and S8 are cross-linked to the 5'-terminal tetranucleotide of 16S RNA. S5 is also cross-linked to the 16S RNA within an oligonucleotide encompassing positions 559-561. Proteins S11, S9, S19 and S7 are cross-linked to 16S RNA within oligonucleotides encompassing positions 702-705, 1130-1131, 1223-1231 and 1238-1240, respectively. Protein S13 is cross-linked to an oligonucleotide encompassing positions 1337-1338, and is also involved in an anomalous cross-link within positions 189-191. Protein S21 is cross-linked to the 3'-terminal dodecanucleotide of the 16S RNA.