Phenoxyl radical complexes of gallium, scandium, iron and manganese

The hexadentate macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L CH 3H3 ), 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L(Bu) H3 ) and 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane (L OCH 3-H3 ) form very stable octahedral neutral complexes LM(III) with trivalent (or tetravalent) metal ions (Ga(III) , Sc(III) , Fe(III) , Mn(III) , Mn(IV) ). The following complexes have been synthesized: [L(Bu) M], where M = Ga (1), Sc (2), Fe (3); [L(Bu) Mn(IV) ]PF6 (4'); [L OCH 3M], where M = Ga (1 a), Sc (2 a), Fe (3 a); [L OCH 3Mn(IV) ]PF6 (4 a'); [L CH 3M], where M = Sc (2 b), Fe (3 b), Mn(III) (4 b); [L CH 3Mn(IV) ]2 (ClO4 )3 (H3 O)(H2 O)3 (4 b'). An electrochemical study has shown that complexes 1, 2, 3, 1 a, 2 a and 3 a each display three reversible, ligand-centred, one-electron oxidation steps. The salts [L OCH 3Fe(III) ]ClO4 and [L OCH 3Ga(III) ]ClO4 , have been isolated as stable crystalline materials. Electronic and EPR spectra prove that these oxidations produce species containing one, two or three coordinated phenoxyl radicals. The Mössbauer spectra of 3 a and [3 a](+) show conclusively that both compounds contain high-spin iron(III) central ions. Temperature-dependent magnetic susceptibility measurements reveal that 3 a has an S = 5/2 and [3a](+) an S = 2 ground state. The latter is attained through intramolecular antiferromagnetic exchange coupling between a high-spin iron(III) (S1 = 5/2) and a phenoxyl radical (S2 = 1/2) (H = - 2JS1 S2 ; J = - 80 cm(-1) ). The manganese complexes undergo metal- and ligand-centred redox processes, which were elucidated by spectroelectrochemistry; a phenoxyl radical Mn(IV) complex [Mn(IV) L OCH 3](2+) is accessible.

Reaction of Dithiothreitol and Para-nitroacetophenone with Different Radical Precursors of .OH Radical-induced Strand Break Formation of Single-stranded DNA in Anoxic Aqueous Solution

The yields of single-strand breakage (ssb) in single-stranded calf thymus DNA (ssDNA) have been determined after 60Co gamma-irradiation of aqueous anoxic solutions in the presence of different concentrations of dithiothreitol (DTT), ascorbate or trans-4,5-dihydroxy-1,2-dithiane, using low-angle laser light scattering. The influence of DTT on the kinetics of ssb formation has been determined by conductivity measurements in pulse radiolysis. The results suggest that strand breakage in ssDNA proceeds via two modes of about equal contribution and with half-lives of about 7 ms and 0.8s, respectively. Both modes reflect reactions of at least two DNA radicals, which react with DTT by hydrogen-atom transfer reactions with similar rate constants of about 5-9 x 10(5) dm3 mol-1 s-1. These hydrogen-atom transfer reactions inhibit strand break formation. The slow mode is shown to represent the decay of base-radicals to generate sugar radicals. The involvement of the oxidizing .OH adduct radical of guanine in the formation of strand breaks can be ruled out and there is no evidence for a contribution from the anion or radical anion of DTT to the inhibition of strand breaks via electron transfer reactions to DNA radicals.

Intramolecular Transformation Reaction of the Glutathione Thiyl Radical into a Non-sulphur-centred Radical: A Pulse-radiolysis and EPR Study

The thiyl radical derived from glutathione (GSH) is shown to decay rapidly in aqueous solution by intramolecular rearrangement reactions into the non-sulphur-centred radical 1. The reaction is induced by OH- with a rate constant of 5 x 10(9) dm3 mol-1 and is also observable at near-neutral conditions (at physiological pH values around 7.5 the rate of formation of 1 amounts to approximately 1 x 10(3) s-1). The activation enthalpy and entropy at pH 8.4 and 20 degrees C were found to be 26.7 kJ mol-1 and -77 J mol-1 K-1, respectively. Radical 1 was unequivocally identified by EPR as the alpha-amino radical at the glutamyl residue of GSH. It is relatively long-lived with typical bimolecular decay rate constants of the order of (2-20) x 10(6) dm3 mol-1 s-1. At higher GSH concentrations the formation of 1 is retarded but not inhibited. All radicals, sulphur- as well as non-sulphur-centred ones are connected via equilibria, partly under the action of 'repair' processes of GSH. These repair processes, however, are slow (k much less than 1.4 x 10(5) dm3 mol-1 s-1). The equilibria are established quite rapidly and were found to be far on the side of the non-sulphur-centred radical under all conditions employed. Radical 1 possesses reducing properties as evidenced by its fast reaction with 4-nitro-acetophenone (PNAP) to yield PNAP.- (k = 7 x 10(8) dm3 mol-1 s-1).

Experimental evidence for the noninnocence of o-aminothiophenolates: coordination chemistry of o-iminothionebenzosemiquinonate(1-) pi-radicals with Ni(II), Pd(II), Pt(II)

The ligand 2-mercapto-3,5-di-tert-butylaniline, H[L(AP)], an o-aminothiophenol, reacts with metal(II) salts of Ni and Pd in CH3CN or C2H5OH in the presence of NEt3 under strictly anaerobic conditions with formation of beige to yellow cis-[M(II)(L(AP))2] (M = Ni (1), Pd (2)) where (L(AP))1- represents the o-aminothiophenolate(1-) form. The crystal structure of cis-[Pd(II)(L(AP))2][HN(C2H5)3][CH3CO2] has been determined by X-ray crystallography. In the presence of air the same reaction produces dark blue solutions from which mixtures of the neutral complexes trans/cis-[M(II)(L(ISQ))2] (M = Ni (1a/1b), Pd (2a/2b), and Pt (3a/3b)) have been isolated as dark blue-black solid materials. By using HPLC the mixture of 3a/3b has been separated into pure samples of 3a and 3b, respectively; (L(ISQ))1- represents the o-iminothionebenzosemiquinonate(1-) pi-radical. The structures of 1a.dmf and 3a.CH2Cl2 have also been determined. All compounds are square-planar and diamagnetic. 1H NMR spectroscopy established the cis <==> trans equilibrium of 1a/1b, 2a/2b, and 3a/3b in CH2Cl2 solution where the isomerization rate is very fast for the Ni, intermediate for the Pd, and very slow for the Pt species. It is shown that the electronic structures of 1a/1b, 2a/2b, 3a, and 3b are best described as diradicals with a singlet ground state. The spectro- and electrochemistries of all complexes display the usual full electron transfer series where the monocation, the neutral species, the mono- and dianions have been spectroscopically characterized. X-band EPR spectra of the monocations [1a/1b]+ and [3a]+ support the assignment of an oxidation-state distribution as predominantly [M(II)(L(ISQ))(L(IBQ))]+ where (L(IBQ))0 represents the o-iminothionequinone level. In contrast, the EPR spectra of the monoanions [1a/1b]- and [3a]- indicate an [M(II)(L(ISQ))(L(AP)-H)]- distribution but with a significant contribution of the [M(I)(L(ISQ))(2)]- resonance hybrid; (L(AP)-H)2- represents the o-imidothiophenolato(2-) oxidation level. Analysis of the geometric features of 120 published structures of complexes containing ligands of the o-aminothiophenolate type show that high precision X-ray crystallography allows to discern the differing protonation and oxidation levels of these ligands. o-Aminothiophenolates are unequivocally shown to be noninnocent ligands; the (L(ISQ))1- radical form is quite prevalent in coordination compounds and the electronic structure of a number of published complexes must be reconsidered.

Molecular and electronic structure of octahedral o-aminophenolato and o-iminobenzosemiquinonato complexes of V(V), Cr(III), Fe(III), and Co(III). Experimental determination of oxidation levels of ligands and metal ions

The coordination chemistry of the ligands 2-anilino-4,6-di-tert-butylphenol, H[L(AP)], and N,N"'-bis[2-(4,6-di-tert-butylphenol]diethylenetriamine, H(2)[(L(AP))N(L(AP))], has been studied with the first-row transition metal ions V, Cr, Fe, and Co. The ligands are noninnocent in the sense that the aminophenolato parts, [L(AP)](-) and [L(AP)-H](2)(-), can be readily oxidized to their o-iminobenzosemiquinonato, [L(ISQ)](-), and o-iminobenzoquinone, [L(ISB)], forms. The following neutral octahedral complexes have been isolated as crystalline materials, and their crystal structures have been determined by X-ray crystallography at 100 K: [Cr(III)(L(ISQ))(3)] (1), [Fe(III)(L(ISQ))(3)] (2), [Co(III)(L(ISQ))(3)] (3), [V(V)(L(ISQ))(L(AP)-H)(2)] (4), [V(V)(L(AP)-H)(2)(L(AP))] (5), and [V(V)O[(L(AP))N(L(AP)-H)]] (6). From variable-temperature magnetic susceptibility measurements and X-band EPR spectroscopy it has been established that they possess the ground states: 1, S = 0; 2, S = 1; 3, S = (3)/(2); 4, S = (1)/(2); 5, S = 0; 6, S = 0. The o-iminobenzosemiquinonato radicals (S(rad) = (1)/(2)) couple strongly intramolecularly antiferromagnetically to singly occupied orbitals of the t(2g) subshell at the respective metal ion but ferromagnetically to each other in 3 containing a Co(III) ion with a filled t(2g)(6) subshell. It is demonstrated that the oxidation level of the ligands and metal ions can be unequivocally determined by high-quality X-ray crystallography in conjunction with EPR, UV-vis, and Mössbauer spectroscopies. The spectro- and electrochemistry of these complexes have also been studied in detail. Metal- and ligand-based redox chemistry has been observed. The molecular and electronic structures are compared with those of their o-semiquinonato analogues.

Phenylthiyl radical complexes of gallium(III), iron(III), and cobalt(III) and comparison with their phenoxyl analogues

Three hexadentate, asymmetric pendent arm macrocycles containing a 1,4,7-triazacyclononane-1,4-diacetate backbone and a third, N-bound phenolate or thiophenolate arm have been synthesized. In [L(1)](3)(-) the third arm is 3,5-di-tert-butyl-2-hydroxybenzyl, in [L(2)](3)(-) it is 2-mercaptobenzyl, and in [L(3)](3)(-) it is 3,5-di-tert-butyl-2-mercaptobenzyl. With trivalent metal ions these ligands form very stable neutral mononuclear complexes [M(III)L(1)] (M = Ga, Fe, Co), [M(III)L(2)] (M = Ga, Fe, Co), and [M(III)L(3)] (M = Ga, Co) where the gallium and cobalt complexes possess an S = 0 and the iron complexes an S = (5)/(2) ground state. Complexes [CoL(1)].CH(3)OH.1.5H(2)O, [CoL(3)].1.17H(2)O, [FeL(1)].H(2)O, and [FeL(2)] have been characterized by X-ray crystallography. Cyclic voltammetry shows that all three [M(III)L(1)] complexes undergo a reversible, ligand-based, one-electron oxidation generating the monocations [M(III)L(1)(*)](+) which contain a coordinated phenoxyl radical as was unambiguously established by their electronic absorption, EPR, and Mössbauer spectra. In contrast, [M(III)L(2)] complexes in CH(3)CN solution undergo an irreversible one-electron oxidation where the putative thiyl radical monocationic intermediates dimerize with S-S bond formation yielding dinuclear disulfide species [M(III)L(2)-L(2)M(III)](2+). [GaL(3)] behaves similarly despite the steric bulk of two tertiary butyl groups at the 3,5-positions of the thiophenolate, but [Co(III)L(3)] in CH(2)Cl(2) at -20 to -61 degrees C displays a reversible one-electron oxidation yielding a relatively stable monocation [Co(III)L(3)(*)](+). Its electronic spectrum displays intense transitions in the visible at 509 nm (epsilon = 2.6 x 10(3) M(-)(1) cm(-)(1)) and 670sh, 784 (1.03 x 10(3)) typical of a phenylthiyl radical. The EPR spectrum of this species at 90 K proves the thiyl radical to be coordinated to a diamagnetic cobalt(III) ion (g(iso) = 2.0226; A(iso)((59)Co) = 10.7 G).

Electronic structure of bis(o-iminobenzosemiquinonato)metal complexes (Cu, Ni, Pd). The art of establishing physical oxidation states in transition-metal complexes containing radical ligands

The ligand 2-anilino-4,6-di-tert-butylphenol and its 2-(3,5-dichloroanilino)-4,6-di-tert-butylphenol analogue react in CH(3)CN or CH(3)OH solutions with divalent transition metal ions in the presence of air and triethylamine. Depending on the metal:ligand ratio (1:1, 1:2, or 1:3) and the presence (or absence) of the cyclic amine 1,4-dimethyl-1,4,7-triazacyclononane (dmtacn), the following complexes have been isolated as crystalline solids: [Co(III)(L(ISQ))(3)] (1); [Cu(II)(dmtacn)(L(ISQ))]PF(6) (2); [Cu(II)(L(ISQ))(2)] (3); [Ni(II)(L(ISQ))(2)] (4a); [Ni(II)((Cl)L(ISQ))(2)] (4b); [Pd(II)(L(ISQ))(2)] (5). (L(ISQ))(-) represents the monoanionic o-iminobenzosemiquinonate radical (S(rad) = (1)/(2)). Compounds 1-5 have been characterized by single-crystal X-ray crystallography at 100(2) K. For all complexes it is unambiguously established that the O,N-coordinated o-iminobenzosemiquinonato(1-) ligand is present. Complexes 3, 4b, and 5 are square planar molecules which possess an S(t) = (1)/(2), 0, and 0 ground state, respectively, as was established by (1)H NMR and EPR spectroscopies and variable-temperature magnetic susceptibility measurements. Complex 2 possesses an S(t) = 1 ground state which is attained via strong intramolecular ferromagnetic coupling (J = +195 cm(-1)) between the d(x)2-(y)2 magnetic orbital of the Cu(II) ion and the pi-orbital of the ligand radical. Complex 1 contains three mutually orthogonal (L(ISQ))(-*) ligands and has an S(t) = (3)/(2) ground state. It is shown that the electronic structure of 4a and 5 is adequately described as singlet diradical containing a divalent, diamagnetic d(8) configurated central metal ion and two strongly antiferromagnetically coupled (L(ISQ))(-) radical ligands. It is concluded that the same electronic structure prevails in the classic bis(o-diiminobenzosemiquinonato)- and bis(o-benzosemiquinonato)metal complexes of Ni(II), Pd(II), and Pt(II). The electrochemistry of all complexes has been investigated in detail. For 3, 4a, and 5 a series of reversible one-electron-transfer waves leads to the formation of the anions and cations [M(L)(2)](2-),(1-),(1+),(2+) which have been characterized spectroelectrochemically. All redox processes are shown to be ligand-based.

Free-radical-induced chain breakage and depolymerization of poly(methacrylic acid): equilibrium polymerization in aqueous solution at room temperature

Hydroxyl radicals, generated by ionizing radiation in N2O saturated aqueous solutions, abstract H atoms from poly(methacrylic acid) at the methyl and methylene groups, and radicals 1 and 2 are formed, respectively. The reactions of the poly(methacrylic acid) radicals were investigated by pulse radiolysis (using optical and conductometric detection), EPR, product analysis, and kinetic simulations. The conductometric detection allowed us to measure the rate of chain scission and monomer release. Under conditions in which the polymer is largely deprotonated, the primary radical 1 abstracts a hydrogen (k= 3.5 x 10(2)s(-1)) from the methylene group, and this yields the more stable secondary radical 2. This radical undergoes chain scission by beta-fragmentation (k= 1.8 s(-1)), and the terminal (end-of-chain) radical 3 is formed. The polymer radicals terminate only slowly (2k= 80 dm3mol(-1)s(-1)). This allows effective depolymerization (depropagation) to take place (k=0.1 s(-1)). The yield of monomer release is higher than the original radical yield by up to two orders of magnitude. Once monomer is formed, it reacts with 3 (propagation, k= 15 dm3mol(-1)s(-1)), and a situation close to an equilibrium radical polymerization is approached. From these data, the equilibrium monomer concentration is calculated at 6.7 x 10(-3) mol dm(-3) at room temperature. The standard entropy of propagation is estimated at -185 to -150 J mol(-1)K(-1). Because the monomer reaches concentrations in the millimolar range, the *OH radicals increasingly react with monomers (results in oligomerization) rather than with the polymer. This effect is reflected by, for example, a lowering of chain-scission yields upon prolonged irradiation. In acid solutions, the decay of the polymer radicals becomes much faster (estimated at about 10(7)dm3mol(-1)s(-1) at pH3.5), and monomer release is no longer observed.

Pulse radiolysis of the DNA-binding bisbenzimidazole derivatives Hoechst 33258 and 33342 in aqueous solutions

PURPOSE

The DNA-minor-groove-ligands bisbenzimidazoles Hoechst 33258 and 33342 have been reported to protect against radiation-induced DNA-strand breakage. In order to elucidate the mechanisms of protection by these DNA-binding compounds, pulse radiolysis studies on the reactions of the OH radical, the solvated electron and the H atom with Hoechst as well as OH-radical-induced nucleotide radical quenching by free Hoechst (model level) was investigated.

MATERIALS AND METHODS

The pulse radiolysis of Hoechst 33258 and 33342 was studied in N2O and N2O/O2-(4:1)-saturated aqueous solutions in the absence and presence of azide and bromide ions and nucleotides.

RESULTS

In a fully scavenged system (3 x 10(-2) mol x dm(-3) t-butanol, N2O/O2-saturated), a transient is formed which in the presence of phosphate buffer is no longer observed. This is assigned metastable quinonoid forms of Hoechst (lambdamax(Hoechst) = 340; lambdamax(transient) = 370 nm) which is generated in protonation/ deprotonation reactions by H+/OH- formed during the pulse. To prevent their formation 10(-3) mol x dm(-3) phosphate buffer was added in all other experiments. The transient spectra formed upon OH-radical attack (k=9 x 10(9) dm3 x mol(-1) x s(-1)) indicate that a major part of the primary OH-adduct radicals undergo rapid transformation (k approximately 5 x 10(5) x s(-1)), attributed to water elimination yielding an N-centered radical. This intermediate, also generated by N3. (k = 4 x 10(9) dm3 mol(-1) x s(-1)), subsequently complexes with a Hoechst molecule [k = 8 x 10(8) dm3 x mol(-1) x s(-1) epsilon(440 nm) = 1.4 x 10(4) dm3 mol(-1) x cm(-1)]. The N-centered radical does not react with O2 (k < 5 x 10(5) dm3 mol(-1) x s(-1)), but reacts readily with the superoxide radical (k= 1.0 x 10(9) dm3 x mol(-1) x s(-1)). Hoechst reacts with the peroxyl radicals derived from uridine (k approximately 5 x 10(6) dm3 x mol(-1) x s(-1)) or 5'-UMP (k approximately 1 x 10(7) dm3 mol(-1) x (s-1)), but not with the less oxidizing (e.g. methylperoxyl radical) yielding intermediates whose spectral properties are similar to those of the N-centered radical. However, they decay at a much lower rate (2k approximately 1 x 10(8) dm3 mol(-1) x s(-1)) than the N-centered radicals generated by N3. (2k= 1.1 x 10(9) dm3 x mol(-1) s(-1)), and it has been suggested that these peroxyl radicals form adducts rather than undergoing electron transfer. The H atom (k= 7 x 10(9) dm3 x mol(-1) x s(-1)) and the solvated electron (k= 2.3 x 10(10) dm3 x mol(-1) x s(-1)) yield, albeit noticeably different, H-adduct radicals which also strongly absorb in the 440 nm region. The reduction potential of Hoechst 33258 has been determined electrochemically at 0.84-0.90 V vs. NHE at pH 6.8.

CONCLUSION

Hoechst reacts fast only with strongly oxidizing radicals by electron transfer (e.g. with the adenine-and guanine-derived heteroatom-centered radicals), but also more slowly with nucleo-base-derived peroxyl radicals, here albeit via addition. This may have important implications with regard to its protection owing to DNA-radical quenching under oxic vs. anoxic conditions.

Molecular and electronic structures of bis(pyridine-2,6-diimine)metal complexes [ML2](PF6)n (n = 0, 1, 2, 3; M = Mn, Fe, Co, Ni, Cu, Zn)

A series of mononuclear, octahedral first-row transition metal ion complexes mer-[M(II)L0(2)](PF6)2 containing the tridentate neutral ligand 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (L0) and a Mn(II), Fe(II), Co(II), Ni(II), Cu(II), or Zn(II) ion have been synthesized and characterized by X-ray crystallography. Cyclic voltammetry and controlled potential coulometry show that each dication (except those of Cu(II) and Zn(II)) can be reversibly one-electron-oxidized, yielding the respective trications [M(III)L0(2)]3+, and in addition, they can be reversibly reduced to the corresponding monocations [ML2]+ and the neutral species [ML2]0 by two successive one-electron processes. [MnL2]PF6 and [CoL2]PF6 have been isolated and characterized by X-ray crystallography; their electronic structures are described as [Mn(III)L1(2)]PF6 and [Co(I)L0(2)]PF6 where (L1)1- represents the one-electron-reduced radical form of L0. The electronic structures of the tri-, di-, and monocations and of the neutral species have been elucidated in detail by a combination of spectroscopies: UV-vis, NMR, X-band EPR, Mossbauer, temperature-dependent magnetochemistry. It is shown that pyridine-2,6-diimine ligands are noninnocent ligands that can be coordinated to transition metal ions as neutral L0 or, alternatively, as monoanionic radical (L1)1-. All trications are of the type [M(III)L0(2)]3+, and the dications are [M(II)L0(2)]2+. The monocations are described as [Mn(III)L1(2)]+ (S = 0), [Fe(II)L0L1]+ (S = 1/2), [Co(I)L0(2)]+ (S = 1), [Ni(I)L0(2)]+ (S = 1/2), [Cu(I)L0(2)]+ (S = 0), [Zn(II)L1L0]+ (S = 1/2) where the Mn(II) and Fe(II) ions are low-spin-configurated. The neutral species are described as [Mn(II)L1(2)]0, [Fe(II)L1(2)]0, [Co(I)L0L1]0, [Ni(I)L0L1]0, and [Zn(II)L1(2)]0; their electronic ground states have not been determined.

Synthesis and characterization of tris(bipyridyl)ruthenium(II)-modified mono-, di-, and trinuclear manganese complexes as electron-transfer models for photosystem II

With the aim of modeling the arrangement of redox-active and photoactive components along the electron-transfer pathway of photosystem II, tetra- to nonanuclear transition metal complexes have been synthesized, comprising one, two, or three manganese ions, oxidizable phenolates, and tris(2,2'-bipyridyl)ruthenium(II)-type units as photosensitizers. These model complexes are considered to be mononuclear ([LnMn](PF6)m), dinuclear ([L1aMnIV2(mu-O)2](PF6)6), or trinuclear ([LnMnIIMnIIMnIILn](PF6)12) with respect to the number of manganese centers present. Electronic coupling between the manganese ions is strongly antiferromagnetic in the case of the di(mu-oxo)-dimanganese compound [L1aMnIV2(mu-O)2](PF6)6, where the "ligand" [H2L1a]4+ consists of two tris(bipyridyl)ruthenium(II)-type units covalentely bound to a bismacrocyclic Me2dtne backbone to which the manganese ions are coordinated via an additional phenolate oxygen (Me2dtne = 1,2-bis(4-methyl-1,4,7-triazacyclononyl)ethane). Weak antiferromagnetic coupling is observed in compounds [LnMnIIMnIIMnIILn](PF6)12, where the three metals are in a linear arrangement (face-sharing octahedral). They are bridged by three phenolate oxygens of each of the deprotonated "ligands" [H3Ln]6+, respectively. Each ligand [H3Ln]6+ (n = 1, 2) consists of a tacn ring with three pendent arm phenols which are each bound to a tris(bipyridyl)ruthenium(II)-type unit (tacn = 1,4,7-triazacyclononane). In these compounds several electron-transfer steps were detected by electrochemical methods which are assigned to different redox processes located at individual electrochemically active components (Mn, Ru, bipyridyl, phenolate). For example, in the "mononuclear" compounds [LnMn](PF6)m (n = 1 or 2) Mn(II), Mn(III), and Mn(IV) are accessible and three Ru(II) centers are reversibly oxidized to Ru(III), and in addition, the coordinated phenolate can be oxidized to a highly reactive, coordinated phenoxyl radical. In several cases very slow heterogeneous electron-transfer rates were observed for redox processes involving the manganese centers.

[Spectrographic studies on the radioresistance of Miltex and miltefosine]

AIM

With simultaneous application of Miltex and radiation therapy in the combined treatment of topical relapses and skin metastases in breast carcinoma patients the question arises, how radioresistant is the new cytotoxic agent. Because of the long penetration times of the active agent miltefosine the answer is important with particular regard to the time of the external application of Miltex.

MATERIAL AND METHOD

After the application of a single dose of 10 Gy we studied the stability of the commercial preparation and its active agent miltefosine by means of absorption spectroscopy.

RESULTS

Immediately following the irradiation no alterations in absorption spectra of Miltex and miltefosine were found. However, 2 and 8 h post radiation the absorption curves of Miltex and miltefosine solutions were distinctly changed. The radiation induced changes of Miltex dilutions were smaller than those of the miltefosine solutions. For the commercial preparation the amount of the radiation-induced destruction is 0.10.

CONCLUSIONS

Consequently Miltex has shown a sufficient radioresistance or its decrease in the effectiveness is small. With daily single doses of 2 Gy in the radiotherapy of the topical relapses and skin metastases the destruction degree should be reduced to 0.02 assuming linear changes. Because of the distinct changes in the spectra and relative slow penetration of miltefosine in various cell lines [10, 11, 14] we will propose an application of the commercial cytotoxic agent 5 h before the radiation fractions. The smaller effect on Miltex is discussed in relation to the solution mediators of the active agent.

The reaction of triplet 2-methyl-1,4-naphthoquinone (menadione) with DNA and polynucleotides

The photoreaction of 2-methyl-1,4-naphthoquinone (MQ, menadione) with DNA and polynucleotides in argon-saturated aqueous solution (pH 7) was studied. Results from laser flash photolysis experiments indicate that triplet quinone reacts with DNA and polyA but not detectably with polyU by one-electron oxidation of the bases of the nucleic acid with formation of the radical anion of the quinone. Irradiation of argon-saturated solutions containing MQ and DNA or polynucleotides (polyU, polyA, polyG or polyC) with 334 nm light leads to an increase in molecular weight for single-stranded DNA, polyA and to a much less extent for polyU. This finding indicates crosslink formation with quantum yields in the range of 10(-5)-10(-3).

Single-strand breaks in double-stranded DNA irradiated in anoxic solution: contribution of tert-butanol radicals

Yields of single-strand breaks induced by 60Co gamma or pulse irradiation in double-stranded calf thymus DNA have been measured in N2O-saturated aqueous solution as a function of the concentration of tert-butanol. The yields were found to be dependent on dose rate. The experimental data were analyzed using a theoretical model based on non-homogeneous scavenging kinetics. It is concluded from this analysis that after 60Co gamma irradiation in the absence of oxygen, aside from breaks caused by hydroxyl radicals, additional breaks occur which are initiated by hydrogen atoms and secondary radicals of tert-butanol. The efficiency of hydrogen atoms in causing single-strand breaks in double-stranded calf thymus DNA was determined to be 2.3%, while the rate constant for the reaction of tert-butanol radicals with DNA and their efficiency in causing single-strand breaks was determined to be 4.1 x 10(3) dm3 mol-1 s-1 and 2%, respectively.

Yields of single-strand breaks in double-stranded calf thymus DNA irradiated in aqueous solution in the presence of oxygen and scavengers

Yields of radiation-induced single-strand breaks in double-stranded calf thymus DNA have been measured as a function of OH. scavenger concentration in N2O/O2-saturated aqueous solution. The experimental data are well represented by a theoretical model based on non-homogeneous reaction kinetics, without the need to adjust any parameter. The good agreement between experimental and theoretical data is taken as evidence that, in the presence of oxygen, the main effect of added scavengers with respect to the formation of single-strand breaks in double-stranded DNA is OH. radical scavenging.

Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis. Part I. Transformation reactions of two isomeric OH-adducts

The absorption spectra of polyadenylic acid (polyA) radicals in N2O saturated aqueous solution have been measured as a function of time (up to 15 s) following an 0.4 microsecond electron pulse. The spectra and their changes were analysed by comparison with those from monomeric adenine derivatives (nucleosides and nucleotides) which had been studied by Steenken. The reaction of OH. radicals with the adenine moiety in polyA results in the formation of two hydroxyl adducts at the positions C-4 [polyA4OH.] and C-8 [polyA8OH.]. Each OH-adduct undergoes a unimolecular transformation reaction before any bimolecular or other unimolecular decay occurs. These reactions are characterized by different rate constants and pH dependencies. The polyA4OH. adduct undergoes a dehydration reaction to yield a neutral N6 centered radical (rate constant kdeh = 1.4 x 10(4)s-1 at pH 7.3). This reaction is strongly inhibited by H+. In comparison with the analogous reactions in adenosine phosphates, the kinetic pK value for its inhibition is two pH units higher. This shift is the result of the counter ion condensation or double-strand formation. The polyA8OH. adduct undergoes an imidazole ring opening reaction to yield an enol type of formamidopyrimidine radical with the resulting base damage (kr.o. = 3.5 x 10(4)s-1 at pH 7.3). This reaction in contrast is strongly catalysed by H+ and OH-, similar as for adenosine but different compared to the nucleotides.

Non-homogeneous kinetics in the competition of single-stranded calf-thymus DNA and low-molecular weight scavengers for OH radicals: a comparison of experimental data and theoretical models

The yield of single-strand breaks induced by 60Co gamma-radiation in single-stranded calf-thymus DNA has been measured in aqueous, N2O/O2 saturated DNA solutions as a function of the concentration of added OH scavengers. Single-strand break yields have also been determined, relative to the yield of OH radicals, using pulse radiolysis. Essentially the same dependence on scavenging capacity was obtained under both irradiation conditions. At the highest scavenging capacity used (approximately 10(9) s-1), about eight times more strand breaks are formed than would be predicted from homogeneous competition kinetics. The experimental data are compared with values obtained from theoretical models which are based on non-homogeneous kinetics, and include the effects of spurs and of the direct action of radiation. The comparison shows that the observed dependence of the single-strand break yield on the scavenging capacity can be quantitatively accounted for without the need of empirical adjustment of parameters using a DNA model where the macromolecules are approximated by structureless cylinders of radius Rc = 0.80 nm which on their surface react with OH radicals at a diffusion-controlled rate. The factors determining the scavenger dependence of the single-strand break yield are discussed. The increase on going to higher scavenging capacities in the apparent rate constant of the reaction of OH with DNA is rationalized in terms of the mean diffusion length of OH radicals in solution. Estimates are given for the fractions of strand breaks due to randomized OH radicals, OH radicals from spurs, and the direct radiation effect.

Single- and double-strand break formation in double-stranded DNA upon nanosecond laser-induced photoionization

Double-stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (phi c-), single-strand breaks (phi ssb) and double-strand breaks (phi dsb) were determined for various laser pulse intensities (IL). phi c- and phi ssb increase linearly with increasing IL; however, phi ssb has a tendency to reach saturation at high IL (greater than 5 X 10(6) Wcm-2). The ratio phi ssb/phi c-, representing the number of ssb per radical cation, is about 0.08 at IL less than or equal to 5 X 10(6) Wcm-2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, phi ssb and phi dsb become larger by factors of approximately 5 and 10-15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O-scavenging of the photoelectrons. While phi ssb is essentially independent of the dose (Etot), phi dsb depends linearly on Etot in both argon- and N2O-saturated solutions. The linear dependence of phi dsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon- and N2O-saturated solutions h was found to be of the order of 40-70 phosphoric acid diester bonds. On addition of electron scavengers such as 2-chloroethanol (or N2O plus t-butanol), phi dsb is similar to that in neat, argon-saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser-pulse-induced dsb of DNA.

Determination of the constants of the Alper formula for single-strand breaks from kinetic measurements on DNA in aqueous solution and comparison with data from cells

On the basis of rate constants measured in aqueous solution for (i) DNA single-strand break (ssb) formation induced by OH radicals, (ii) prevention of ssb formation by reaction of DNA radicals with glutathione, and (iii) addition of O2 to DNA radicals, oxygen enhancement ratios (OER) and K values of the Alper equation have been calculated. The values obtained were compared with OER and K values determined for ssb formation in lambda DNA irradiated in Escherichia coli as a function of the oxygen concentration. Without adjustment of any parameter the two sets of data are similar when the corrected Alper formula is used. The results support the oxygen fixation-thiol repair model of Howard-Flanders and Alper, and indicate that under selected conditions DNA in aqueous solution may serve as a model system for DNA in cells.

The influence of glutathione on single-strand breakage in single-stranded DNA irradiated in aqueous solution in the absence and presence of oxygen

The yields of strand break formation (Gssb) in single-stranded DNA (ssDNA) initiated by radiation-generated OH radicals have been determined using the method of low-angle laser light scattering (LALLS). The irradiations were carried out in aqueous, N2O-saturated solutions in the absence and presence of oxygen and at different concentrations of glutathione (GSH). GSH exhibits a protective effect, which is shown to be mainly due to OH radical scavenging. To quantify this, the rate constants for the reactions of OH with GSH and DNA have been redetermined under our experimental conditions. The values obtained were 9.0 x 10(9) and 4.5 x 10(8) dm3 mol-1 s-1, respectively. From the Gssb values obtained under anoxic conditions it is concluded that GSH protects against strand breakage (in addition to OH scavenging) by reacting with DNA radicals in competition to strand break formation. The rate constant of the repair reaction is 8.1 x 10(4) dm3 mol-1 s-1 at room temperature. For irradiations carried out in the presence of oxygen the rate of strand break formation is determined by the decay of DNA peroxyl radicals. Under these conditions we observed no protective effect of GSH apart from OH radical scavenging. The results are compatible with those that are expected from the oxygen-fixation hypothesis.

The kinetics of hydroxyl-radical-induced strand breakage of hyaluronic acid. A pulse radiolysis study using conductometry and laser-light-scattering

Hydroxyl radicals were generated radiolytically in N2O- and N2O/O2(4:1)-saturated aqueous solutions of hyaluronic acid. The hydroxyl radicals react rapidly with hyaluronic acid mainly by abstracting carbon-bound H atoms. As a consequence of subsequent free-radical reactions, chain breakage occurs the kinetics of which has been followed using the pulse radiolysis technique. In the absence of oxygen, strand breakage was followed by the change in conductivity induced by the release of cationic counterions condensed at the surface of hyaluronic acid which is a polyanion consisting of subunits of glucuronic acid alternating with N-acetyl-glucosamine. It appears that strand breakage is not due to one single first-order process, however, the contributions of the different components cannot be adequately resolved. At pH 7 the overall half-life is 1.4 ms, in both acid and basic solutions the rate of free-radical induced strand breakage is accelerated (at pH 4.8, t1/2 = 0.6 ms; at pH 10, t1/2 = 0.18 ms). In the absence of oxygen there is no effect of dose rate on the kinetics of strand breakage. In the presence of oxygen in addition to conductometric detection, strand breakage was also followed by changes in low-angle laser light-scattering. These two techniques are complementary in that in this system the conductometry requires high doses per pulse while the light-scattering technique is best operated in the low-dose range. In the presence of oxygen a pronounced dose-rate effect is observed, e.g. at pH 9.7 after a dose of 9.4 Gy the overall half-time is approx. 0.5 s, while after a dose of 6.6 Gy the half-time is approx. 0.23 s. Both the yield and the rate of strand breakage increase with increasing pH, e.g. at pH 7 G(strand breaks) = 0.7 x 10(-7) mol J-1 and at pH 10.4, 4.8 x 10(-7) mol J-7. The radiolytic yields of CO2, H2O2, organic hydroperoxides, O2.- and oxygen consumption have been determined in gamma-irradiated N2O/O2(4:1)-saturated solutions of both hyaluronic acid and beta-cyclodextrin.

[Treatment of newly diagnosed type II diabetic patients with special reference to prescribing glibenclamide in low doses]

In the period of 15 months 90% of all newly registered type II diabetics of a district area (70 males, 122 females) were prospectively thoroughly examined above all in hospital, standardizedly stabilized and have been followed up for 1 year with regard to their therapy behaviour. From the features obtained (age, body weight, heredity, symptomatology, blood glucose and insulin concentrations basally and after 75 g oGTT) subgroupings were worked out relevant for the therapy. 64% of the newly registered type II diabetics could be stabilized purely dietetically (males 70%, females 60.7%). Female type II diabetics revealed the worse course of therapy. Younger age, overweight, genetic load and higher insulin concentrations are markers of the importance of the insulin resistance and a domain of the basis therapy. Older age, normal weight in connection with higher blood glucose and lower insulin concentrations as an expression of the insulin deficit of higher degree render in most cases possible a treatment with SH-preparations. Small doses of glibenclamide proved to be very favourable both in not purely dietetically manageable younger patients and in older patients. 68% of all SH-cases did with a dosage of 1-3 mg glibenclamide a day, 28% with 1 mg a day. The pathophysiologic and pharmacologic mechanisms are discussed.

Hydroxyl radical-induced strand break formation in single-stranded polynucleotides and single-stranded DNA in aqueous solution as measured by light scattering and by conductivity

Combining conductivity measurements and molecular weight determination by means of low-angle laser light scattering, we have found for the polyribonucleotides (polyuridylic acid (poly(U], polyadenylic acid (poly(A], polycytidylic acid (poly(C] and polyguanylic acid (poly(G] and for single-stranded DNA (ssDNA) that, on average, 8.5 counterions per single-strand break (ssb) are liberated under salt-free conditions. This relationship allows us to estimate, from conductivity measurements alone, G-values of single-strand break formation (G(ssb] for the polydeoxyribonucleotides (polydeoxyriboadenylic acid (poly(dA], polydeoxyribocytidylic acid (poly(dC], polydeoxyribothymidylic acid (poly(dT], polydeoxyribouridylic acid (poly(dU] and polydeoxyriboguanylic acid (poly(dG]. The following G(ssb) values (units of mumol J-1) have been obtained for anoxic conditions: poly(dA), 0.23; poly(dC), 0.14; poly(dT), 0.06; poly(dU), 0.046 and poly(dG), 0.009. Time-resolved conductivity measurements in pulse radiolysis enable us to measure the rate of strand break formation. The rate has been found to be similar for poly(dA) and ssDNA over a range of pH values. Poly(dC) and poly(dU) exhibit conductivity increase components with half-lives similar to those of poly(dA) and ssDNA at corresponding pH values. The implications of these results are discussed.

Gamma-radiolysis of poly(A) in aqueous solution: efficiency of strand break formation by primary water radicals

gamma-Radiation-induced single-strand break formation (ssb) in polyadenylic acid (poly(A] has been determined in Ar and N2O-saturated aqueous solution in the presence of different concentrations of t-butanol. Strand breaks were monitored by a low-angle laser light-scattering technique. The efficiencies for strand breakage caused by solvated electrons, hydrogen atoms and OH radicals have been found to be 0.25, 0.20 and 7.8 per cent, respectively. The efficiency of OH radicals depends only slightly on pH (pH 5.0, 7.5 and 9.0) and is independent of the presence of salt (0.01 mol dm-3 NaC1O4) and of the irradiation temperature (20 degrees C and 70 degrees C). The efficiency of OH for ssb formation obtained in this work with poly(A) is much smaller than that of poly(dA). This is explained by the different molecular conformations of the sugar moiety of poly(A) (3'-endo) and poly(dA) (2'-endo). With increasing t-butanol concentration more strand breaks are formed than expected from simple homogeneous competition kinetics of poly(A) and t-butanol for OH radicals. This result is considered to be due to nonhomogeneous reaction kinetics in the above-mentioned competition. The rate constants for the reaction of OH and H with poly(A) have been determined.

Somatostatin-gastrin interactions in the rat stomach

Low concentrations of somatostatin and gastrin within or slightly above the range of physiologically circulating levels were perfused in the isolated, vascularly perfused rat stomach preparation. Somatostatin at 10 and 50 pg/ml significantly inhibited acetylcholine-stimulated gastrin secretion by 26% and 45%, respectively, whereas perfusion of 50 and 500 pg/ml exogenous gastrin did not modify gastric somatostatin secretion. Perfusion of somatostatin-antiserum significantly increased gastrin release by 235%. It is concluded that (1) somatostatin is a powerful inhibitor of the gastrin cell under in vitro conditions; the data are in accordance with a concept that endogenous somatostatin could act as a true hormone; (2) the secretory activity of the somatostatin cell is not significantly affected by circulating gastrin.

Single- and double-strand break formation in DNA irradiated in aqueous solution: dependence on dose and OH radical scavenger concentration

The yields of single- and double-strand breaks (SSB and DSB) in calf thymus DNA, after 60Co gamma irradiation in dilute aqueous solution, have been determined via molecular weight measurements using a low-angle laser light scattering technique. The irradiations were administered to N2O-containing solutions of DNA in the absence and presence of oxygen and with different concentrations of the OH radical scavengers phenol, tertiary butanol, and methanol. OH radicals were found to produce SSB linearly with dose with a G value of 55 nmol J-1 and 54 nmol J-1 in deoxygenated and oxygenated solutions, respectively. DSB were formed according to a linear-quadratic dose relationship and the G value of linearly formed DSB were GDSB alpha(r.t.) = 3.5 nmol J-1 in deoxygenated and 3.2 nmol J-1 in oxygenated solution. The ratio of GSSB/GDSB alpha(r.t.) = gamma of 19 +/- 6 was independent of the scavenger concentration in the case of tertiary butanol and methanol-containing solutions. GDSB alpha(r.t.) is interpreted to result from a radical site transferred from a sugar moiety of the cleaved strand to the complementary intact strand. This process of radical transfer and subsequent cleavage of the second strand occurs with a probability of about 6 +/- 2% in the presence of oxygen at all scavenger concentrations studied. These data on scavenging capacity on GDSB alpha(r.t.) suggest that the double-strand breakage produced via radical transfer remains higher than that resulting from direct effect, up to scavenging capacities of about 10(9) s-1.

Calcitonin gene-related peptide stimulates rat gastric somatostatin release in vitro

The influence of rat calcitonin gene-related peptide (rCGRP) on the secretion of gastric somatostatin and gastrin was studied in vitro using the isolated, vascularly perfused rat stomach preparation. rCGRP stimulated somatostatin secretion dose-dependently reaching 3-fold stimulation at 1 microM. The kinetics of somatostatin response were characterized by a sharp increase in the initial phase of rCGRP perfusion followed by sustained elevated levels. Gastrin secretion was moderately suppressed at 1 nM to 100 nM CGRP. Somatostatin responses to half-maximal stimulation with 100 nM CGRP were not affected by concomitant perfusion of atropine, propranolol, and tetrodotoxin. It is concluded that increases in somatostatin release in response to CGRP are probably due to a direct effect on the gastric somatostatin-producing D-cell and may be important for the potent acid-inhibitory activity of CGRP.

Hydroxyl radical-induced strand break formation of poly(U) in anoxic solution. Effect of dithiothreitol and tetranitromethane

The role of dithiothreitol (DTT) and tetranitromethane (TNM) on the yields of radiation-induced strand break formation in polyuridylic acid (poly(U] was studied in anoxic aqueous solutions at neutral pH by low-angle laser light-scattering. From G (single-strand breaks) as a function of DTT concentration it follows that two different processes lead to OH radical-induced single-strand break (ssb) formation. Only one of the two processes, which accounts for 80 per cent of the ssb formation, is inhibited by DTT, the other one, 20 per cent, is not inhibited. The 'repair' process is attributed to H-donation to the C-6-yl radical of the uracil moiety. The C-6-yl radical is produced by OH addition to the C-5 position of the uracil moiety. It follows that the sugar radicals, in contrast to earlier suggestions, do not seem to be repaired by DTT at the low concentrations used. The strand break formation not inhibited by DTT is induced by radicals other than the uracil-6-yl radical, e.g. the uracil-5-yl or the OH radicals reacting with the sugar moiety. The strong reduction of G(ssb) from 2.3 to 0.2 on addition of TNM is also discussed.

Hydroxyl radical-induced strand break formation of poly(U) in the presence of oxygen: comparison of the rates as determined by conductivity, e.s.r. and rapid-mix experiments with a thiol

The rate of OH radical-induced strand break formation of single-stranded poly(U) in N2O/O2-saturated aqueous solution was studied by measuring the time-dependence of the electrical conductivity following pulse radiolysis. The first half-life of the total conductivity increase depends slightly on pH and the molecular weight and on the dose per pulse. The activation parameters for strand break formation were found to be EA = 52 kJ mol-1 and A = 5 X 10(8) s-1. Similar first half-lives were observed when the decay of peroxyl radicals of poly(U) was measured by e.s.r. under various conditions. This indicates that poly(U)-peroxyl radicals are involved in the rate-determining step of strand break formation. After pulse radiolysis, strand break formation can be inhibited by the addition of dithiothreitol (DTT) in a rapid-mix apparatus. It is postulated that peroxyl radicals of poly(U) react with DTT by formation of hydroperoxides, thereby preventing strand breakage.

Model studies for the direct effect of high-energy irradiation on DNA. Mechanism of strand break formation induced by laser photoionization of poly U in aqueous solution

Laser flash photolysis of polyuridylic acid (poly U) in anoxic aqueous solutions leads to biphotonic photoionization of the uracil moiety followed by the formation of single strand breaks (ssb). The rate constant for ssb formation (1.0 s-1, obtained from the slow component of conductivity increase at 23 degrees C and pH 6.8) increases with decreasing pH to 235 s-1 at pH 3.5. The activation energy (pre-exponential factor) was measured to be 66 kJ mol-1 (5 X 10(11) s-1) at pH 6.8. Addition of dithiothreitol (DTT) or glutathione (GSH) prevents ssb formation by reacting with a poly U intermediate (rate constant = 1.2 X 10(6) and 0.16 X 10(6) dm3 mol-1 s-1, respectively). Since with OH radicals as initiators very similar data have been obtained for the kinetics of ssb formation and for the reaction with DTT, we conclude that photoionization of the uracil moiety in poly U leads eventually to the same chemical pathway for ssb formation as that induced by OH radicals. Furthermore, we propose that protection by DTT and GSH occurs via H donation to the C-4' radicals of the sugar moiety of DNA and to the C-4' and the C-2' radicals of poly U.

Rate and rate-determining step of hydrogen-atom-induced strand breakage in poly(U) in aqueous solution under anoxic conditions

The rate constant for strand breakage in poly(U) after reaction with hydrogen atoms in deoxygenated aqueous solution has been determined to be k = 1.5 s-1 at pH = 4-5 and 24 degrees C. Dithiothreitol has been found to prevent strand break formation by reacting with H-adduct radicals of poly(U) with a rate constant of 5 X 10(6) M-1 s-1. It is concluded that the rate-determining step in H atom-induced strand breakage in poly(U) at pH less than or equal to 6 is the decay of uracil moiety H-adduct radicals via H-abstraction from the ribose moiety.

Yields of radiation-induced main chain scission of poly U in aqueous solution: strand break formation via base radicals

The G values for single-strand breaks G(ssb) in polyuridylic acid (poly U) have been measured by low-angle laser light scattering in aqueous solutions under various conditions (e.g. in the presence of N2O, Ar and t-butanol). In N2O-saturated solutions at room temperature and pH 5.6, the G(ssb) is 2.3. The efficiency of ssb formation was found to be 41 per cent for OH radicals, 19 per cent for H atoms and congruent to zero for e-aq. On the basis of 20 per cent and less than 5 per cent attack on the sugar moiety by OH radicals and H atoms, respectively, the large G(ssb) values obtained cannot be explained solely as resulting from radicals produced by reaction of OH radicals and H atoms on the sugar moiety. It is therefore proposed that base radicals produced by the reaction of OH radicals or H atoms with the uracil moiety can also lead to chain break formation in poly U via radical transfer to the sugar moiety.

Identification of a major pathway of strand break formation in poly U induced by OH radicals in presence of oxygen

A reaction mechanism is presented for strand break formation in poly U induced by OH radicals in N2O/O2- saturated aqueous solution based on experimental results obtained with different methods.

Rate of OH radical induced strand break formation in single stranded DNA under anoxic conditions. An investigation in aqueous solutions using conductivity methods

Single stranded DNA (ssDNA) in aqueous N2O-saturated solution was pulse-irradiated with electrons or irradiated under steady-state conditions with 60Co-gamma rays. The conductivity increase after irradiation was measured as a function of concentration, pH, temperature, metal cation content and additives. The conductivity increase could be shown to be due to the release of associated counterions (Na+ or H+) as a result of the formation of chain breaks. At 28 mg 1(-1) DNA approximately 8 sodium ions are liberated per 100 eV absorbed energy (G (Na+) = 8.3). On the basis of G value for single-strand breaks (ssb) of G (ssb) = 0.55, it is calculated that 8.3/0.55 = 15 sodium ions per strand break are set free. The release of Na+ monitored by 23Na-NMR at pH 7 as a function of dose corresponds to that of the conductivity increase. The rate of the conductivity increase does not depend on dose/pulse (range 2-20 J kg-1) and shows at least two components. The rate constant of the fast and dominant component is constant above pH 8.5 (k = 38 s-1, 20 degrees C) and increases linearly with proton concentration below pH 7. Values of 13 kcal/mol for the activation energy and 5 X 10(11) s-1 for the frequency factor were obtained at pH 7.3. Addition of p-benzoquinone (pBQ) increases the rate constant of the fast component proportionally to the concentration of pBQ. From these results the rate-determining step of the fast conductivity increase is concluded to be the splitting of DNA radicals. The pH dependence and the magnitude of the activation energy agree with results from low molecular weight model compounds for the C-4' mechanism; this mechanism involves a heterolytic splitting of the phosphoric acid ester bond starting from the 4' radical of DNA.

Adrenergic control of rat gastric somatostatin and gastrin release

The effect of adrenergic agonists and antagonists on the secretion of gastric somatostatin-like immunoreactivity (SLI) and gastrin was investigated in an isolated, vascularly perfused rat stomach preparation. Two- to six-fold increases in SLI secretion induced by isoproterenol, epinephrine, and norepinephrine were completely abolished by propranolol but were not influenced by phentolamine. Propranolol did not alter glucagon- and DB-cAMP-induced stimulation of SLI release. Experiments in which the beta 2-agonist salbutamol and the beta 1- and beta 2-blockers practolol and H35/25 were used showed that both subtypes of beta receptors are involved. Gastrin secretion revealed only minor changes in dose-response studies with a wide range of isoproterenol concentrations (2 X 10(-8) to 1.5 X 10(-4) M). The results obtained in this study suggest that in rats 1) the SLI response to adrenergic agonism is predominantly mediated by beta receptors; 2) both beta 1- and beta 2-adrenergic receptors are involved; 3) under in vitro conditions, adrenergic agonism is a weak stimulus for gastrin secretion.

Release of K+ and H+ from poly U in aqueous solution upon gamma and electron irradiation. Rate of strand break formation in poly U

Conductivity changes were found which followed the reaction of radiolytically generated OH radicals with the potassium salt of polyuridylic acid (poly U) in aqueous solution. After 60Co-gamma-irradiation the observed increase of conductivity at pH = 6.8 was shown to consist of the liberation of K+ ions from the stock of K+ ions electrostatically bound to the polyanion. The initial G(K+) is 36 and hence 6 times higher than the G value of OH radicals in N2O saturated solutions. At a poly U concentration of 60 mg l-1 half of the ion release occurred at 12 J kg-1 and nearly all ions are released at 40 J kg-1. The liberation of K+ is explained to be a consequence of the formation of chain breaks leading to an increase of the degree of dissociation. The rate of the ion release was studied under pulse radiolysis conditions. Because of the high G-value of counterion liberation and the use of conductivity as analytical quantity the method is very sensitive. With 6 mg l-1 poly U the rate could be measured even at a dose per pulse of 0.25 J kg-1. The kinetics of the ion release can be described in terms of two parallel first order reactions of comparable contribution with an average rate constant of 0.8 s-1 at 20 degrees C, 60 mg l-1 poly U and pH = 6.8 with a small contribution of slower components. In more acidic solutions, besides K+ ions H+ are also liberated since at low pH values bound K+ is replaced by H+. The rate of the ion release was found to increase with increasing replacement of K+ by H+ (kobs = 100 s-1 at pH = 3.4, 60 mg l-1 poly U and T = 18 degrees C). With potassium ion free polyuridylic acid the corresponding rate constant amounted to 220 s-1, nearly independent of pH. From the temperature dependence activation parameters for the ion release were derived (Ea = 57 kJ mol-1, A = 1.0 X 10(10) s-1 at pH = 6.8). Addition of p-benzoquinone at pH = 3.7 and dithiothreitol (DTT) at pH 6.8 were found to decrease the size of the conductivity changes and to increase the rate. The results show that p-benzoquinone and DTT react with poly U radicals and that these reactions prevent chain breaks and ion liberation. It is concluded that the rate determining step of the conductivity increase is the formation of strand breaks by a cleavage of poly U radicals and that this reaction is pH dependent. The pH dependence and the observed value for the activation energy was found to be in agreement with the behaviour of a model system for the earlier postulated C-4' mechanism for strand break formation of polynucleotides and DNA.

Adrenergic and cholinergic interactions in rat gastric somatostatin and gastrin release

The interactions of adrenergic and cholinergic influences on the gastric D cell were studied using an isolated perfused rat stomach in vitro. The sevenfold increase in the release of somatostatin-like immunoreactivity (SLI) in response to isoproterenol (4 . 10(-8) M) was dose-dependently inhibited by acetylcholine (10(-5) to 10(-8) M) whereas gastrin levels increased in a dose-dependent manner. Both inhibition of stimulated SLI and augmentation of gastrin release were completely abolished by atropine (10(-6) M). Isoproterenol (8 . 10(-9) M)-induced stimulation of SLI secretion was not altered by atropine. Antral exclusion completely eliminated gastrin secretion but basal and beta-adrenergic stimulated SLI release was not influenced. It is concluded that (1) cholinergic agonism reverses the stimulatory action of adrenergic agonists on the D cell, and (2) SLI from the rat stomach in vitro originates almost exclusively in the fundic region.

Plasma-immunoreactive trypsin in chronic renal failure

Plasma-immunoreactive trypsin (IRT) was measured by radioimmunoassay in 52 patients with chronic renal failure. 29 were on conservative treatment, 14 on-regular haemodialysis, and 9 on regular haemofiltration. IRT levels were elevated in those with raised plasma creatinine and there was a significant positive correlation to plasma creatinine concentrations (r = 0.853; p less than 0.001). IRT levels were not significantly altered by haemodialysis or haemofiltration. It is concluded that the kidney plays an important role in the metabolism of plasma trypsin. Since plasma IRT levels may be elevated in patients with chronic renal failure, the IRT determination may be misleading in diagnosing acute and chronic pancreatitis in the presence of kidney disease.