Chemical synthesis of radiolabeled bleomycin A2 and its binding to DNA

A method for the preparation of biologically active [3H]- and [13C]bleomycin A2 is described. Demethyl Cu(II):bleomycin A2, isolated after pyrolysis of Cu(II):bleomycin A2, was methylated with either [3H]-or [13C]methyl iodide, which resulted in Cu(II):bleomycin A2 labeled in the dimethylsulfonium moiety. Copper was removed by treatment with dithizone in chloroform, and structures were verified by thin-layer chromatography and 1H and 13C nuclear magnetic resonance spectroscopy. Copper-free [3H]-and [13C]bleomycin A2 are active in the degradation of DNA in vitro. Gel exclusion chromatograhy and equilibrium dialysis were used to determine the apparent equilibrium constants for binding of [3H]bleomycin A2 and Cu(II):[3H]bleomycin A2 to calf thymus DNA, noncovalently associated polydeoxyguanylate:polydeoxycytidylate, and noncovalently associated polydeoxyadenylate:polydeoxythymidylate. In 2.5 mM sodium phosphate buffer, pH 7.0, binding data obtained by gel filtration with calf thymus DNA reveal an apparent equilibrium constant for [3H]bleomycin A2 of 5.7 X 10(5)/mol and for Cu(II):[3H]bleomycin A2 of 3.9 X 10(5)/mol. One molecule of [3H]bleomycin A2 binds for every 3.7 base pairs in DNA, and one molecule of Cu(II):[3H]bleomycin A2 binds for every 2.8 base pairs in DNA. Analysis of binding data with calf thymus DNA, noncovalently associated polydeoxyguanylate:polydeoxycytidylate, and noncovalently associated polydeoxyadenylate:polydeoxythymidylate obtained by equilibrium dialysis reveals, in each instance, 2 types of binding sites for both the copper and metal-free form of the antibiotic. For those sites in calf thymus DNA with tighter binding affinity, the apparent equilibrium constant for [3H]bleomycin A2 was 6.8 X 10(5)/mol and for the Cu(II):[3H]bleomycin A2 complex, 4.4 X 10(5)/mol. As seen with calf thymus DNA, the affinity of [3H]bleomycin A2 is slightly greater than that of Cu(II):[3H]bleomycin A2 for the synthetic DNAs, although more of the copper form of the drug binds to these polymers.

Scope and mechanism of carbohydrase action. Stereocomplementary hydrolytic and glucosyl-transferring actions of glucoamylase and glucodextranase with alpha- and beta-D-glucosyl fluoride

Rhizopus niveus glucoamylase and Arthrobacter globiformis glucodextranase, which catalyze the hydrolysis of starch and dextrans, respectively, to form D-glucose of inverted (beta) configuration, were found to convert both alpha- and beta-D-glucosyl fluoride to beta-D-glucose and hydrogen fluoride. Each enzyme directly hydrolyzes alpha-D-glucosyl fluoride but utilizes th beta-anomer in reactions that require 2 molecules of substrate and yield glucosyl transfer products which are then rapidly hydrolyzed to form beta-D-glucose. Various D-glucopyranosyl compounds serve as acceptors for such reactions. Mixtures of beta-D-glucosyl fluoride and methyl-alpha-D-glucopyranoside[14C], incubated with either enzyme, yielded both methyl-alpha-D-glucopyranosyl-(1 leads to 4)-alpha-D-[14C]glucopyranoside and methyl-alpha-D-glucopyranosyl-(1 leads to 6)-alpha-D-[14C]glucopyranoside. Glucoamylase produced more of the alpha-maltoside; glucodextranase produced more of the alpha-isomaltoside. Thus, both "exo-alpha-glucan hydrolases" emerge as glucosylases that catalyze stereospecifically complementary hydrolytic and transglucosylative reactions with glucosyl donors of opposite configuration. These reactions not only provide a new view of the catalytic capabilities of these supposedly strict hydrolases; they also furnish a basis for defining a detailed mechanism for catalysis. Present results, together with those of several recent studies from this laboratory (especially similar findings obtained with beta-amylase acting on alpha- and beta-maltosyl fluoride (Hehre, E. J., Brewer, C. F., and Genghof, D. S. (1979) J. Biol. Chem. 254, 5942-5950), provide strong new evidence for the functional flexibility of the catalytic groups of carbohydrases.

Conformation as the determinant of saccharide binding in concanavalin A: Ca2+-concanavalin A complexes

The existence of two conformational states of concanavalin A (Con A) with different metal ion binding properties has been recently demonstrated (Brown, R. D., Brewer, C. F., & Koenig, S. H. (1977) Biochemistry 16, 3883). Introduction of Mn2+ to the S1 site and Ca2+ to the S2 site of apo-Con A was shown to induce a conformational change in the protein, ascribed to a cis-trans isomerization of a peptide bond in the secondary structure, which results in extremely tight binding of the metal ions. This induced conformation is referred to as "locked" and the initial conformation as "unlocked". The locked ternary complex is identical with the native protein. In the present paper, we report evidence for the formation of a relatively stable, locked, ternary Ca2+-Con A complex that possesses properties similar to those of native Ca2+-Mn2+Con A. The experimental technique involves measurement of the magnetic field and time dependence of the nuclear magnetic relaxation rate (1/T1) of solvent water protons in solutions of Ca2+-Con A, after the addition of Mn2+ ion which slowly bind to the protein. The kinetic data can be fit by a model for Ca2+ interactions with Con A which indicates that Ca2+, in the absence of Mn2+, can bind at both the S1 and S2 sites of the protein and, furthermore, can induce the protein to undergo the unlocked to locked conformational transition. In terms of this model, the time-dependent binding of the Mn2+ ions is due to replacement of Ca2+ ions at the S1 sites in the locked protein. The off-rate of Ca2+ from the S2 site of the locked ternary Ca2+-Con A complex is much greater than that from the locked Ca2+-Mn2+-Con A complex. From the effects of added alpha-methyl D-mannopyranoside on the rate of replacement of Ca2+ by Mn2+ at the S1 site of the locked ternary Ca2+-Con A complex, it is concluded that the latter complex binds saccharides as strongly as the locked Ca2+-Mn2+-Con A complex. In addition, analysis of the data indicates that apo-Con A in the locked conformation binds alpha -methyl D-mannopyranoside with approximately 7% of the affinity of the fully metallized locked form of the protein. This strong saccharide-binding activity of locked apo-Con A, compared with that of the unlocked apo-Con A, was further demonstrated by equilibration of unlocked apo-Con A with alpha-methyl D-mannopyranoside, which resulted in the formation of the locked apo-Con A-saccharide complex. These results demonstrate that it is the locked conformation of Con A that is primarily responsible for saccharide-binding activity, and that the function of the bound metals is primarily to maintain the protein in the locked conformation.

Structure-activity study of the inhibition of microtubule assembly in vitro by podophyllotoxin and its congeners

This study investigates the inhibition of microtubule assembly in vitro by podophyllotoxin and its derivatives, which include in part the antitumor compounds 4'-demethylepipodophyllotoxin ethylidene beta-D-glucoside (VP-16-213) and 4'-demethylepipodophyllotoxin thenylidene beta-D-glucoside (VM-26); the cyclic ethers, cyclic sulfides, and cyclic sulfones of podophyllotoxin and deoxypodophyllotoxin; epipodophyllotoxin; picropodophyllotoxin; and several 4'-demethyl compounds. The inhibitory activity of these derivatives is sensitive to the configuration and size of substituents at position 4 in ring C and to steric features of substituents at position 12 in ring D. Decreasing activity correlates with the increasing size of the substituent at position 12, as indexed by their van der Waals radii. These results suggest that rings C and D of these drugs are involved in their interaction with the podophyllotoxin-binding site in tubulin.

Scope and mechanism of carbohydrase action: stereospecific hydration of D-glucal catalyzed by alpha- and beta-glucosidase

A unique demonstration is presented of the capacity of glycosidases to create anomeric configuration de novo. Purifed Candida tropicalis alpha-glucosidase and sweet almond beta-glucosidase have been found to attack the same substrate, D-glucal, and to convert this unusual glycosyl substrate (which lacks alpha or beta anomeric configuration) to 2-deoxy-alpha-(or beta-) D-glucose, respectively. The stereospecificity of the hydration reaction catalyzed by each enzyme in D2O was revealed by the use of high-resolution (270 MHz) 1H magnetic resonance spectroscopy. The alpha-glucosidase caused a specific axial protonation (deuteration) of D-glucal at C-2, and formation of 2-deoxy-alpha-D-[2(a)-2H]glucose. The beta-glucosidase catalyzed an oppositely directed axial protonation at C-2 and formation of 2-deoxy-beta-D-[2(e)-2H]glucose. These results are not accounted for by the generally accepted mechanisms of carbohydrase action derived from studies with glycosidically linked substrates alone. D-Glucal apparently binds to the enzymes with essentially the same overall orientation as the D-glucosyl moiety of glycosidically linked substrates (with the double bond of D-glucal lying essentially in the plane of the similarly bound D-glucosyl group). Thus, the alpha-glucosidase evidently protonates D-glucal from above the double bond and alpha-D-glucosidic substrates from below the glycosidic oxygen; beta-glucosidase apparently protonates D-glucal from below the double bond and beta-D-glucosides from above the glycosidic oxygen. A detailed mechanism is proposed for the hydration of D-glucal by each enzyme, involving an incipient glycosyl carbonium ion and assuming the presence at the active site of two carboxyl groups arranged to account for catalysis of glycosylations from glycosidically linked substrates. That D-glucal serves as a glycosyl substrate for these enzymes strongly supports the concept that glycosidases and glycosyltransferases are catalysts of glycosylation (i.e., glycosylases), since this concept does not make the usual assumption that carbohydrases are restricted to acting on substrates having a glycosidic bond and either alph- or beta-anomeric configuration.

Carbon-13 nuclear magnetic resonance study of chondroitin 4-sulfate in the proteoglycan of bovine nasal cartilage

The Fourier transform 13C nuclear magnetic resonance spectra of bovine nasal cartilage proteoglycan subunit and complex and whole bovine nasal cartilage were obtained and compared with that of chondroitin 4-sulfate. The spectrum of chondroitin 4-sulfate in solution revealed multiple resolvable resonances with linewidths that are consistent with considerable segmental motion in the polysaccharide chain. The spectra of proteoglycan subunit and complex in solution and that of whole cartilage were very similar to that of free chondroitin 4-sulfate chains. This indicates that the linkage of multiple chondroitin sulfate chains to proteoglycan core protein and the association of proteoglycan with collagen and other constituents of cartilage matrix does not significantly alter the structure and motions of these chains.

13C NMR studies of the interaction of concanavalin A with saccharides

The binding of alpha- and beta-methyl-D-glucopyranoside and beta-(o-iodo-pheryl)-D-glucopyranoside to concanavalin A has been studied by carbon-13 nuclear magnetic resonance techniques. The kinetics and binding orientations of these saccharides relative to the transition metal ion site in the protein have been determined.

Binding of 13 C-enriched -methyl-D-glucopyranoside to concanavalin A as studied by carbon magnetic resonance

Binding of alpha-methyl-D-glucopyranoside, uniformly enriched with 14% (13)C, to zinc and manganese derivatives of concanavalin A at pH 5.6 was studied by pulsed Fourier transform carbon magnetic resonance techniques. Spin-lattice relaxation (T(1)) of the [(13)C]carbons of the sugar was measured in the absence and presence of the two transition metal derivatives of the protein. In the presence of the manganese derivative of concanavalin A, selective relaxation of the sugar carbons was observed. The values for T(1) reflect different distances between the carbons of the bound sugar and the manganese ion. Calculation of the distance between the manganese ion and each carbon of the sugar permit the 3-dimensional orientation of the bound sugar to be determined relative to the transition metal site in the protein. The results indicate that alpha-methyl-D-glucopyranoside binds to the protein in the Cl chair conformation with its 3- and 4- carbons closest to the manganese ion at a mean distance of 10 A.

Solvent proton magnetic relaxation dispersion in solutions of concanavalin A

Concanavalin A, a protein isolated from jack beans, exhibits several important biological properties, all of which are related to its ability to bind and precipitate specific polysaccharides. Concanavalin A is a dimer at pH 5.6, and has one transition-metal and one calcium-ion binding site per monomer unit of molecular weight 27,000. Both metal-ion sites must be occupied for the protein to be active. It is of interest to determine the role of the transition metal ion in Concanavalin A and its relationship to the sugar binding activity of the protein. We report the magnetic field and temperature dependences of the spin-lattice magnetic relaxation rates of solvent protons in aqueous solutions of zinc and manganese derivatives of Concanavalin A, and the influence of monosaccharide binding on these rates. The results of a leastsquares fit of the data to the theory, with five adjustable parameters, indicate that there is one rapidly exchanging water molecule ligand on the Mn(2+) ion, with a residence lifetime of 2.5 musec at 25 degrees , and with its protons 0.27 nm (2.7 A) from the Mn(2+) ion. We find that at low magnetic fields (proton Larmor frequencies below about 10 MHz), the correlation time for the dipolar interaction between the Mn(2+) electronic spin moment and the protons on the water ligand is the spin-lattice relaxation time tau(S) of the Mn(2+) moment, but that at higher magnetic fields the correlation time for the dipolar interaction is determined by the Brownian rotational tumbling of the protein, because of the substantial variation of tau(S) with magnetic field. Monosaccharide binding to manganese Concanavalin A has little effect on the relaxation rates of solvent protons, a result that indicates that the sugars do not bind directly to the transition metal in the protein.