Effect of subunit interactions on enzymatic activity of glutathione S-transferases: a radiation inactivation study

Effects of high-energy electrons and gamma rays directly on protein molecules

High-energy electrons and gamma rays ionize molecules at random along their trajectories. In each event, chemical bonds are ruptured, releasing radiolytic products that diffuse away. A solution of macromolecules is mostly water whose principal radiation products are H(+) and OH(-). These can diffuse to and react with macromolecules; this indirect action of radiation is responsible for 99.9% of the damage to proteins. In frozen samples, the ionizations still occur randomly and water is still the principle molecular target, but diffusion of radiation products is limited to only a very small distance. At very low temperatures, essentially all the radiation damage to macromolecules is due to primary ionizations occurring directly in those molecules. Therefore, proteins in frozen solutions are only 10(-3) to 10(-4) as sensitive to radiation as in the liquid state. Every molecule that suffered a direct ionization is destroyed; the only surviving molecules are those that escaped ionization. The survival of frozen proteins after irradiation is a direct measure of the mass of the active structures and independent of the presence of other proteins.

Radiation target analysis indicates that phenylalanine hydroxylase in rat liver extracts is a functional monomer

The minimal enzymatically functional form of purified rat hepatic phenylalanine hydroxylase (PAH) is a dimer of identical subunits. Radiation target analysis of PAH revealed that the minimal enzymatically active form in crude extracts corresponds to the monomer. The 'negative regulation' properties of the tetrahydrobiopterin cofactor in both crude and pure samples implicates a large multimeric structure, minimally a tetramer of PAH subunits. Preincubation of the samples with phenylalanine prior to irradiation abolished this inhibition component without affecting the minimal functional unit target sizes of the enzyme in both preparations. The characteristics of rat hepatic PAH determined by studies of the purified enzyme in vitro may not completely represent the properties of PAH in vivo.

The active streptococcal hyaluronan synthases (HASs) contain a single HAS monomer and multiple cardiolipin molecules

The functional sizes of the two streptococcal hyaluronan synthases (HASs) were determined by radiation inactivation analysis of isolated membranes. The native enzymes in membranes from Group A Streptococcus pyogenes HAS and Group C Streptococcus equisimilis HAS were compared with the recombinant proteins expressed in Escherichia coli membranes. Based on their amino acid sequences, the masses of these four proteins as monomers are approximately 48 kDa. In all cases, loss of enzyme activity was a simple single exponential function with increasing radiation dose. The functional sizes calculated from these data were identical for the four HASs at approximately 64 kDa. In contrast, the sizes of the proteins estimated by the loss of antibody reactivity on Western blots were essentially identical at 41 kDa for the four HAS species, consistently lower than the functional size by approximately 23 kDa. Matrix-assisted laser desorption time of flight mass spectrometry analysis of purified S. pyogenes HAS-H6 and S. equisimilis HAS-H6 gave masses that differed by <0.07% from the predicted monomer sizes, which confirms that neither protein is posttranslationally modified or covalently attached to another protein. Ongoing studies indicate that the purified HAS enzymes require cardiolipin (CL) for maximal activity and stability. When irradiated membranes were detergent solubilized and the extracts were incubated with exogenous CL, the residual level of HAS activity increased. Consequently, the calculated functional size decreased by approximately 23 kDa to the expected size of the HAS monomer. The approximately 23-kDa larger size of the functional HAS enzyme, compared with the HAS monomer, is due, therefore, to CL molecules. We propose that the active streptococcal HA synthases are monomers in complex with approximately 16 CL molecules.

Radiation effects on the native structure of proteins: fragmentation without dissociation

Several proteins (avidin, carboxypeptidase B, glucose-6-phosphate dehydrogenase, glutamate dehydrogenase, maltase, and peroxidase) composed of one to six subunits were irradiated in the frozen state. Each irradiated protein was examined by size-exclusion chromatography (SEC) and by denaturing gel electrophoresis (SDS-PAGE). All these proteins eluted from SEC as a single peak even though SDS-PAGE showed cleavage of the polypeptide backbone of the monomers. Thus, fragmentation of the subunits did not result in dissociation of the oligomeric structure.

Structural studies of a human pi class glutathione S-transferase. Photoaffinity labeling of the active site and target size analysis

The glutathione S-transferases (GSTs; EC 2.5.1.18) are a family of dimeric proteins that catalyze reactions between glutathione (GSH) and various electrophiles. A partial cDNA for human GST pi was obtained and the open reading frame completed. The completed cDNA was cloned, and GST pi protein was expressed in bacteria. Cloned enzyme was purified and had the same kinetic constants, molecular mass, pI value, and N-terminal sequence as placental GST pi except that some of the polypeptides had N-terminal methionines. A radiolabeled azido derivative of GSH, S-(p-azidophenacyl)-[3H]glutathione, was used to photoaffinity-label the active site of the cloned enzyme. Labeled enzyme did not bind to a GSH-agarose affinity column. Labeling was prevented in the presence of S-hexylglutathione, and noncovalently-bound azido affinity label was a competitive inhibitor towards 1-chloro-2,4-dinitrobenzene and GSH. These results suggest that the azido label was binding at the active site of the enzyme. Photoaffinity-labeled enzyme was trypsinized, and two labeled peptides were purified and sequenced. One peptide corresponded to residues 183-188, whereas the other corresponded to residues 183-186. These residues appear to form part of the hydrophobic (H-site) binding region of human GST pi that has not been shown previously. Cloned enzyme was subjected to radiation inactivation to assess the importance of subunit interactions in the maintenance of catalytic activity. The target size of enzymatic activity (23 kDa) was not significantly different from that of the protein monomer (24 kDa). Therefore, each subunit of human GST pi appears to be capable of independent catalytic activity.

Kinetic mechanism of octopus hepatopancreatic glutathione transferase in reverse micelles

Octopus glutathione transferase (GST) was enzymically active in aerosol-OT [sodium bis-(2-ethylhexyl)sulphosuccinate]/iso-octane reverse micelles albeit with lowered catalytic constant (kcat). The enzyme reaction rate was found to be dependent on the [H2O]/[surfactant] ratio (omega(o)) of the system with maximum rate observed at omega(o) 13.88, which corresponded to vesicles with a core volume of 64 nm3. According to the physical examinations, a vesicle of this size is barely large enough to accommodate a monomeric enzyme subunit. Dissociation of the enzyme in reverse micelles was confirmed by cross-linking of the associated subunits with glutaraldehyde and separation of the monomers and dimers with electrophoresis in the presence of SDS. The kinetic properties of the enzyme were investigated by steady-state kinetic analysis. Both GSH and 1-chloro-2,4-dinitrobenzene (CDNB) showed substrate inhibition and the Michaelis constant for CDNB was increased by 36-fold to 11.05 mM in reverse micelles. Results on the initial-velocity and product-inhibition studies indicate that the octopus GST conforms to a steady-state sequential random Bi Bi mechanism. The results from a log kcat versus pH plot suggest that amino acid residues with pKa values of 6.56 0.07 and 8.81 0.17 should be deprotonated to give optimum catalytic function. In contrast, the amino acid residue with a pKa value of 9.69 0.16 in aqueous solution had to be protonated for the reaction to proceed. We propose that the pKa1 (6.56) is that for the enzyme-bound GSH, which has a pKa value lowered by 1.40-1.54 pH units compared with that of free GSH in reverse micelles. The most probable candidate for the observed pKa2 (8.81) is Tyr7 of GST. The pKa of Tyr7 is 0.88 pH unit lower than that in aqueous solution and is about 2 pH units below the normal tyrosine. This tyrosyl residue may act as a base catalyst facilitating the dissociation of enzyme-bound GSH. The possible interaction of GST with plasma membrane in vivo is discussed.

The mathematics of radiation target analyses

Radiation target theory has been extended to complex biochemical systems. Mathematical analyses are presented for multiple forms of biological active proteins, for the presence of large inhibitors or activators, for compounds which regulate rate or affinity and for multiple-step reactions. Several predictions of these models have been verified experimentally.

Radiation inactivation of human gamma-interferon: cellular activation requires two dimers

gamma-Interferon (IFN-gamma) is a 17-kDa broad-spectrum cytokine which exerts its effects on a variety of target cells through its interaction with the IFN-gamma receptor. Although physicochemical studies of Escherichia coli-derived IFN-gamma, as well as its crystal structure, demonstrate that it is a homodimer in solution (M(r) 34,000), previous radiation inactivation studies yielded a functional size for IFN-gamma of 63-73 kDa in an antiviral assay. To understand the relationship between the solution form of IFN-gamma and the moiety that actually binds to the cellular receptor and activates cells, we examined irradiated nonradioactive and 32P-labeled IFN-gamma for its migration in SDS/polyacrylamide gels (to determine its physical integrity), its binding to cells, its reactivity in an ELISA, and its antiviral activity. The functional size of IFN-gamma differed in the assays, being 22 +/- 2 kDa for the physical destruction of IFN-gamma, 56 +/- 2 kDa for the cellular binding assay, 45-50 kDa for reactivity in the ELISA, and 72 +/- 6 kDa for antiviral activity. The results from the binding assays constitute direct evidence that IFN-gamma binds to its cellular receptor as a dimer. However, for antiviral activity, the functional mass is equivalent to a tetramer. This is consistent with models involving ligand-induced receptor dimerization, whereby two dimers acting in concert (equivalent to the target size of a tetramer) are required to activate cells in the antiviral assay.

X-ray crystal structures of cytosolic glutathione S-transferases. Implications for protein architecture, substrate recognition and catalytic function

Crystal structures of cytosolic glutathione S-transferases (EC 2.5.1.18), complexed with glutathione or its analogues, are reviewed. The atomic models define protein architectural relationships between the different gene classes in the superfamily, and reveal the molecular basis for substrate binding at the two adjacent subsites of the active site. Considerable progress has been made in understanding the mechanism whereby the thiol group of glutathione is destabilized (lowering its pKa) at the active site, a rate-enhancement strategy shared by the soluble glutathione S-transferases.

Damage to proteins due to the direct action of ionizing radiation

Proteins exposed to ionizing radiation suffer both reversible and irreversible effects. Reversible effects are defined as those which disappear in a short period of time after the removal of the radiation field and without further treatment of the sample. Irreversible effects are those which cause a permanent alteration in the structure of a protein.