Application of pulse radiolysis to the study of proteins: chymotrypsin and trypsin

Aggregation of Albumins under Reductive Radical Stress

The reactions of radicals with human serum albumin (HSA) under reductive stress conditions were studied using pulse radiolysis and photochemical methods. It was proved that irradiation of HSA solutions under reductive stress conditions results in the formation of stable protein aggregates. HSA aggregates induced by ionizing radiation are characterized by unique emission, different from the UV emission of non-irradiated solutions. The comparison of transient absorption spectra and the reactivity of hydrated electrons (eaq-) with amino acids or HSA suggests that electron attachment to disulfide bonds is responsible for the transient spectrum recorded in the case of albumin solutions. The reactions of eaq- and CO2•- with HSA lead to the formation of the same products. Recombination of sulfur-centered radicals plays a crucial role in the generation of HSA nanoparticles, which are stabilized by intermolecular disulfide bonds. The process of creating disulfide bridges under the influence of ionizing radiation is a promising method for the synthesis of biocompatible protein nanostructures for medical applications. Our Raman spectroscopy studies indicate strong modification of disulfide bonds and confirm the aggregation of albumins as well. Low-temperature measurements indicate the possibility of electron tunneling through the HSA protein structure to specific CyS-SCy bridges. The current study showed that the efficiency of HSA aggregation depends on two main factors: dose rate (number of pulses per unit time in the case of pulse radiolysis) and the temperature of the irradiated solution.

Factors governing electron capture by small disulfide loops in two-cysteine peptides

Integrated molecular orbital-molecular orbital (IMOMO) calculations on 17 short disulfide-bridged peptides (up to 16 residues, with at most five intraloop residues) were performed to elucidate some factors controlling their electron capture. These illustrative systems display contrasted behaviors, shedding light on several criteria of differentiation: size, shape, and rigidity of the disulfide-linking loop, intramolecular hydrogen bonds, etc. The geometrical malleability of disulfide radical anions, whose existence and role as intermediate have been evidenced, is discussed. The disulfide elongation (by ca. 0.7 A) upon electron capture induces "soft" structural damages for these turn structures, with a weakening or cleavage of vicinal hydrogen bond(s). On the basis of a series of six Cys-Alan-Cys peptides, it is proposed that electron affinity reflects the topological frustration of these short and highly constrained structures. Results for a series of amino acid mutations are analyzed for the Cys-Xxx-Yyy-Cys motif, common to redox enzymes of the thioredoxin superfamily.

Reactions of reducing radicals with ribonuclease

Radiation-induced reactions of hydrated electrons, formate- and ethanol radicals with ribonuclease were studied by pulse radiolysis and by electrophoresis. Initially formate radicals react rapidly and very specifically with the disulphide bonds of ribonuclease. This reaction leads to aggregation by intermolecular S-S-interchange, the process being more effective at pH 4, since formation and decay of S-S-.-radical anions increases with decreasing pH. With high doses additional unreducible aggregates are formed. Radical formation at the positively charged histidine residues seems to be involved. Hydrated electrons do not react as selectively as the formate radicals, but with several sites in native ribonuclease. Thus with low doses unreducible aggregates are formed. Electrophoresis shows that reaction of the electrons causes fragmentation of the peptide chain, when OH-radicals are scavenged. Very weak transient spectra and very little degradation result on reaction of ethanol radicals with ribonuclease.

The reduction of flavocytochrome b2 by carboxylate radicals. A pulse radiolysis study

The reaction of flavocytochrome b2 with carboxylate radicals has been studied by the pulse-radiolysis technique. The absorbance difference spectra observed 2, 10 and 90 mus after the pulse were very similar to the (reduced-minus-oxidized) difference spectrum of the cytochrome b2 core, a deflavoderivative prepared from flavocytochrome b2. The heme b2 group was reduced in a bimolecular reaction with a rate constant of (2.1 +/- 0.2) X 10(8) M-1 X s-1. Simulation data allowed us to assign this reduction mainly to a direct reaction of CO-2 on heme b2 without flavin involvement. However, this heme b2 reduction was accelerated in a flavocytochrome b2 sample of low molar activity. This observation seemed to reflect modifications of the heme b2 environment, in particular a closer contact with the solvent. Moreover, in such samples the flavin became reducible to the semiquinone state as deduced from absorbance difference spectra in the 440-490 nm region. The agreement between experimental and computed time-courses allowed to estimate the reaction rate of bound flavin to be equal to 2 X 10(8) M-1 X s-1 in this studied sample. Thus the reactivity of bound heme b2 and flavin within flavocytochrome b2 with CO-2 seems to be sensitive to the physical alterations of the polypeptide chain.

Reactions of formate and ethanol radicals with bovine serum albumin studied by electrophoresis

Radiation-induced reactions of formate and ethanol radicals with bovine serum albumin were studied by electrophoresis. The reaction products were separated as reduced and unreduced SDS-protein-complexes on polyacrylamide gel. Quantitative evaluation of the densitometric measurements gives information about the reaction mechanisms. Initially formate radicals react rapidly and very selectively with the disulphide groups which then form partly intermolecular S-S-bonds. This formation of reducible aggregates causes a massive loss of serum albumin. With increasing dose unreducible aggregates are formed. Formate radicals show a high efficiency in both degradation processes, which are not caused by ethanol radicals. Formate radicals are much more effective in degrading serum albumin than ethanol radicals. Both radicals show a third degradation process which has low G-values and is presumably caused by H-abstraction.

Radiation-induced reduction reactions with bovine serum albumin

The reduction of bovine serum albumin (BSA) by hydrated electrons, formate and ethanol radicals leads to the formation of S-S-radicals. The reaction of the hydrated electrons becomes faster, if BSA was preirradiated in the presence of formate or ethanol, but the rate does not change if radiolysis is done in the presence of t-butanol or without scavenger. The reduction of BSA by formate radicals is 10 times slower but gives a higher yield of S-S-radicals than by hydrated electrons. The rate and yield of the reduction by the formate radicals are increased by preirradiation of BSA. The reduction by ethanol radicals is five times slower than by formate radicals and gives lower yields of S-S-radicals which are increased by preirradiation. All rates and yields of the reduction reactions are decreased by changing the pH from 6.3 to 7.3.

Applications of pulse radiolysis to protein chemistry

Since its introduction, pulse radiolysis has been an important technique for examining the properties of organic and inorganic radicals, and for enumerating those reactions responsible for cellular damage by ionizing radiation. Biochemists, and biophysicists outside the area of radiation biology appear, perhaps for historical reasons, to have an incomplete appreciation of the technique's potential. Protein chemists in particular, have been only dimly aware of the numerous reports of, and the significant results obtained from pulse radiolysis studies of proteins. Our purpose here is to bring some of these results together in order to emphasize the power and usefulness of pulse radiolysis experiments both for elucidating enzyme reaction mechanisms, and for gaining information on the structure of proteins in aqueous solutions. Reviews containing related, or in part the same material to be covered here have appeared previously; for example, Land (1970), Adamset al.(1972a), Shafferman & Stein (1975), Adams & Wardman (1977). This review updates these earlier works, but more importantly approaches the topic of protein pulse radiolysis with a different emphasis.