Mutagenesis of structural half-cystine residues in human thioredoxin and effects on the regulation of activity by selenodiglutathione

A human thioredoxin cDNA was modified to optimize Escherichia coli expression and subcloned into the plasmid pACA, a vector for T7 RNA polymerase-directed expression. The substitution of structural (noncatalytic) half-cystines in human thioredoxin (hTrx) was made by site-directed mutagenesis. The recombinant wild-type (wt) hTrx and its mutant C61S, C72S, and C61S/C72S were expressed and purified to homogeneity. Characterization of the wt and mutant hTrx was done with respect to redox activity with thioredoxin reductase (TR), tryptophan fluorescence, and effects of incubation with GS-Se-SG, which is believed to be the major metabolite of inorganic selenium compounds in mammalian tissues. The Km and kcat of wild-type hTrx for human placenta thioredoxin reductase (HP-TR) at pH 7.0 were 2.0 microM and 2800 min-1, respectively. The mutant proteins C61S, C72S, and C61S/C72S had Km and kcat values similar to those of the wt thioredoxin. Tryptophan fluorescence measurements showed that the wt and mutant proteins had similar stability to a denaturing agent. Incubation of fully reduced thioredoxin with 0.1 molar equivalent of GS-Se-SG resulted in continued oxidation of SH groups. After 3.5 h only 0.5 of initially 4.6 SH groups/thioredoxin remained. With the oxidized protein, a pronounced lag phase in thioredoxin reductase-dependent insulin disulfide reduction was present. Disulfide-linked dimers of the protein were present. The results clearly showed that noncatalytic cysteine residues in hTrx were oxidized accompanied by dimerization and inactivation. The activities of the mutant proteins C72S and C61S/C72S were unchanged after 3 h of incubation with GS-Se-SG. No dimer appeared of the C72S thioredoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of crystallin polypeptides in rat lenses during the development of galactose-induced cataract

Some striking differences in relative polypeptide abundance of crystallins were observed in normal and galactose-induced cataractous lenses of rat by means of SDS-PAGE. In the cataractous lenses a prominent band appeared at about 25 kDa and the aA chain increased markedly, whereas the relative amount of the 31 kDa band decreased substantially. These alterations are similar to the changes observed during the incubation of young mouse lenses in glucose-free medium.

In vivo animal models of body composition in aging

We developed several techniques that provide data on body elemental composition from in vivo measurements in rats. These methods include total body potassium by whole-body counting of endogenous 40K; total body calcium (TBCa), sodium and chloride by in vivo neutron activation analysis and total body phosphorus (TBP) and nitrogen (TBN) by photon activation analysis. These elements provide information on total body fat, total body protein and skeletal mass. Measurements were made in 6-, 12- and 24-month-old rats. TBN increased slightly between 6 and 12 months but was significantly lower by 24 months, indicating a substantial loss in total body protein. Working at the National Synchrotron Light Source, we studied rat femurs by computed microtomography (CMT), and the elemental profile of the femur cortex by synchrotron-radiation induced X-ray emission (SRIXE). Although there were no significant changes in TBCa and TBP, indices of skeletal mass, CMT revealed a marked increase in the size and number of cavities in the endosteal region of the femur cortex with increasing age. The SRIXE analysis of this cortical bone revealed a parallel decrease in the endosteal Ca/P ratio. Thus, there are major alterations in bone morphology and regional elemental composition despite only modest changes in total skeletal mass.

High-energy x-ray beam characterization using photonuclear reaction

Spectral changes in the primary photon beam of a high-energy medical electron accelerator have been monitored using photonuclear reactions. The beam across the radiation field was measured using induced activity in Teflon cylinder and copper foil samples at several off-axis angles, at different SSDs in air and in a solid water phantom. The induced activities of the positron emitters were measured using a coincidence detection system. The experimental results are in agreement with Monte Carlo calculations and Schiff's theory.

Buffer dependence of CO2 hydration catalyzed by human carbonic anhydrase I

The steady-state kinetics of CO2 hydration catalyzed by human carbonic anhydrase I (carbonate hydro-lyase, EC 4.2.1.1) has been investigated at three pH values corresponding to different parts of the pH-rate profile. Two buffer systems with similar pKa values were used at each pH. The results show that the catalyzed rates depend on the buffer concentration but also on the chemical nature of the buffer. For example, at pH 8.8 the buffer 1,2-dimethylimidazole behaves formally as a second substrate in a 'ping-pong' mechanism yielding a maximal kcat value of 2.2 x 10(5) s-1, whereas much lower rates were obtained with Taps buffers. Similarly, at pH 7.3 1-methylimidazole yields higher rates than Mops and at pH 6.3 3,5-lutidine is more efficient than Mes. Non-Michaelis-Menten kinetics were observed with all buffers except 1,2-dimethylimidazole. In addition, while the apparent buffer activation by 1,2-dimethylimidazole can be described by a single Km value of 26 mM, the Mes concentration dependence is consistent with the presence of two components of similar magnitudes with Km values of 45 mM and 0.15 mM. These results are interpreted within the framework of the 'zinc-hydroxide' mechanism in terms of multiple pathways for the rate-contributing transfer of a proton from the zinc-bound water molecule, formed during CO2/HCO3- interconversion, to the reaction medium, thus, regenerating zinc-bound OH-.

Kinetics, anion binding and mechanism of Co(II)-substituted bovine muscle carbonic anhydrase

The binding of N3- to Co(II)-substituted bovine carbonic anhydrase III was measured at various pH values by spectrophotometric titrations. The apparent Ki values were found to increase with pH in the studied range between pH 5.8 and 8.9. The inhibition of CO2 hydration by N-3 was found to be essentially uncompetitive at all investigated pH values (pH 6.3-8.9). The Ki values for the inhibition of kcat are much smaller than those obtained in the spectrophotometric titrations indicating that an enzyme form with a high affinity for N-3, presumably having a metal-bound H2O, accumulates in the steady state at saturating CO2 concentrations. Assuming that the low pH limit of Ki = 9 microM for the inhibition of kcat represents the affinity of N-3 for the Co(II)-OH2 form, a pKa value near 5 can be estimated for Co(II)-bound water from the pH dependence of N-3 binding in the absence of CO2. Measurements of time-resolved absorption spectra during CO2 hydration in the presence of a low N-3 concentration showed the transient appearance of the characteristic spectrum of the enzyme-N-3 adduct clearly demonstrating the accumulation in the steady state of an enzyme form with a high affinity for N-3. In similar experiments without inhibitor the transient formation of a spectral form corresponding to a Co(II)-OH2 species has been demonstrated. This spectral form is rather featureless lacking the absorption maxima at 618 nm and 640 nm characteristic of the Co(II)-OH- species. Our results strongly support the hypothesis that the rate-limiting step in CO2 hydration catalyzed by carbonic anhydrase III is the protolysis of metal-bound water.

A comparison of the kinetic properties of native bovine muscle carbonic anhydrase and an activated derivative with modified thiol groups

Steady-state and equilibrium kinetic properties of native bovine carbonic anhydrase III (carbonate hydrolyase, EC 4.2.1.1) and a derivative modified with methyl methanethiosulfonate were investigated. The modified enzyme has a markedly increased CO2 hydration activity compared to the native form with a 3-times higher value of kcat and a 6-10-times higher value of kcat/Km. Qualitatively, the activated enzyme shows the same kinetic behavior as native isoenzyme III. This is reflected in similar pH dependences of the kinetic parameters for CO2 hydration, similar solvent hydrogen isotope effects on these parameters, similar deviations from Michaelis-Menten kinetics for the HCO3- dehydration reaction, and similar behavior of the kinetics of CO2/HCO3- exchange at chemical equilibrium as measured by a 13C-NMR magnetization transfer technique. It is concluded that the conversion of -SH groups to -S-S-CH3 moieties does not change the catalytic mechanism, but leads to an increased rate of CO2/HCO3- interconversion as well as to an increased rate of proton transfer between the active site and the reaction medium.