Cross-linking of reversibly immobilized enzymes

Chemistry on solid supports: defining events and titers by use of cleavable, assayable linking molecules

The cleavable diamines cystamine, 5, and 1,6-diamino-3,4-dihydroxyhexane, 1, were bonded to solid supports and, with a simple, newly developed dinitrofluorobenzene-based assay, were used to define (a) titers of ligands and (b) chemistry distal to the support. Compound 1, which is cleavable with periodate, becomes a linking molecule which is stable to almost all conditions encountered in biochemistry and enjoys considerable hydrophilic character. Compound 5, which is cleavable with dithiothreitol, can be usefully applied to those systems which do not require reducing agents. These nucleophilic linking moieties were converted to cleavable electrophilic linkers by succinylation and p-nitrophenyl ester activation. The first preparation of a polysaccharide-linked support is described. The method also allows the chemical definition of ligands containing amino groups which are prepared by deblocking of protecting groups while on the support. The methodology should promote greater understanding of affinity chromatography materials and processes.

Immobilized subunits of alkaline phosphatase

The techniques developed in this investigation for the analysis of the structure and properties of immobilized alkaline phosphatase subunits can be applied to the characterization of other immobilized enzymes. The methods for determining the quaternary structure of immobilized subunits and of detecting contaminating oligomeric structures are important for the clear interpretation of the properties of subunit preparations. Complementation of immobilized subunits with labeled soluble subunits can be directly monitored by measuring isotope ratios; moreover, titrations of this type can be used to form specific hybrid oligomers which could not be prepared as a homogeneous population in solution. The use of immobilized protein derivatives makes it possible to obtain stable subunit preparations, and thus study the properties of single subunits. Even the rates of "inactivation" of subunits without catalytic activity can be quantitated after titration with soluble subunits. The availability of these methods for studying immobilized enzymes, combined with the advantages of using these derivatives, should increase the general potential of immobilization as a technique for studying oligomeric proteins.

Reversible covalent immobilization of ligands and proteins on polyacrylamide gels

Ligands and proteins were covalently but reversibly immobilized on polyacrylamide gels using novel acrylic monomers whose syntheses are reported here. These reagents have an acrylyl group at one end for copolymerization into gels, an N-succinimidyl ester at the other allowing rapid immobilization of molecules having an available primary amino group, and a cleavable disulfide bond in the middle. Two immobilization methods were developed using these reagents. In the first method, a ligand with a primary amino group was treated with the immobilization reagent in anhydrous ethanol and the resulting amide derivative was purified and copolymerized with acrylamide and bisacrylamide resulting in the desired reversible immobilization. In the second method, the immobilization reagents (at densities up to 50 mumol/ml) were directly copolymerized with acrylamide and bisacrylamide to form activated gels of the desired shape and porosity. Proteins or other ligands in aqueous buffers were then added to the activated gels resulting in their covalent immobilization. Ligands or proteins immobilized using the methods reported here remained stably bound even when gels were subjected to boiling in detergents or high-ionic-strength buffers. Immobilized ligands were readily released (greater than 97%) from gels by treatment with quantitative amounts of aqueous dithiothreitol (DTT) under mild conditions. Immobilized proteins were also released (up to 87%) from the gels by DTT treatment. Small ligands (e.g., aminohexyl glycosides), active enzymes, and glycoproteins were immobilized, and then recovered, using these reagents.

Use of concanavalin A as a topographical probe for protein-protein interaction. Application to lactose synthase

Galactosyltransferase (EC 2.4.1.38) has been shown to bind to Con A-Sepharose. Concentrations of methyl-alpha-mannoside greater than 0.7 M were required to release the enzyme from the immobilized lectin. Molecular weight determination by gel filtration revealed that galactosyltransferase formed a 1:1 complex with concanavalin A. Preincubation of the enzyme with excess concanavalin A did not affect its catalytic activity either in the presence or absence of alpha-lactalbumin. The galactosyltransferase-concanavalin A complex was retained on an alpha-lactalbumin-Sepharose column in the presence of N-acetylglucosamine and manganese chloride and was eluted from the column in their absence. Galactosyltransferase immobilized onto a Con A-Sepharose was still active either in the presence or absence of alpha-lactalbumin. Lactose synthase activity was also observed when the galactosyltransferase-concanavalin A complex was assayed with alpha-lactalbumin immobilized on Sepharose. These data indicate that the carbohydrate moiety of galactosyltransferase is involved in neither the catalytic process nor the binding of alpha-lactalbumin and must be linked to the enzyme at a location where it does not present any steric hindrance on the binding of concanavalin A, either free or immobilized on Sepharose.

Isolation of protein subunits by coupling to an insoluble matrix: analysis of interactions and application to G-actin

A criterion has been devised for the assessment of intermolecular contacts between protein subunits coupled to a gel matrix. After reversible coupling by way of disulfide groups to Sepharose 4B, the system is exposed to a bifunctional reagent. The protein is then released by reduction, and the proportion of dimers and higher oligomers formed by cross-linking is determined by gel electrophoresis, followed by densitometry of the stained gels. Experiments were performed with G-actin coupled to dithio-2-dipyridyl Sepharose 4B. At low concentration of coupled protein, no species other than the monomer were obtained; under these conditions any intermolecularly cross-linked species would represent pre-existing associated states coupled to the matrix. At higher protein concentrations dimers and higher species progressively appear. Their proportion is much higher than predicted on the basis of a random statistical distribution of coupled molecules throughout the accessible internal volume of the matrix. It follows that protein-protein contacts can occur either because of high flexibility of the polysaccharide chains of the matrix, or because the protein is partly (but not wholly) present as a shell on the surface of the beads. With Affi-gel 10, which has N-hydroxysuccinimide ester coupling groups, similar experiments were performed, taking advantage of the degradation of the matrix under relatively mild acid conditions. In this case the degree of cross-linking was much lower than in the Sepharose 4B system at the same protein concentration. However, this medium proved unsatisfactory for the measurement of interactions of the bound actin with other muscle proteins present in the mobile phase. The results with Sepharose 4B support the validity of previous studies on interaction of monomeric actin with other muscle proteins.