Solutions of bovine β-lactoglobulin (β-Lg) A−C were heated at temperatures between 50 and 90 °C for 12.5 min at pH 6.7 or 7.4, and the products were analyzed by alkaline, sodium dodecyl sulfate (SDS), and two-dimensional (2D) (alkaline and then SDS) polyacrylamide gel electrophoresis (PAGE). Results from the pH 6.7 samples that were ∼70% denatured showed that the proportion of β-Lg that was in very large aggregates was β-Lg C > β-Lg B > β-Lg A. 2D PAGE showed that there were a large number of unexpected intermediate products, especially from β-Lg A. These and other results, including the dissociation of disulfide-bonded dimers from trimers and tetramers by SDS, indicate that (1) β-Lg dimers could be important intermediates in the further aggregation of β-Lg, (2) hydrophobically driven associations occur within the aggregates, (3) the mechanism of β-Lg aggregation is not simple, and (4) differences in variant protein behavior are explainable in terms of net negative charge and specific amino acid substitutions. Keywords: Thermal denaturation; electrophoresis; aggregate formation; hydrophobically associated aggregates; disulfide-linked aggregates; β-lactoglobulin variants; two-dimensional polyacrylamide gel electrophoresis

Dilute solutions of beta-lactoglobulin (beta-Lg) A, B, and C were heated in phosphate buffer at temperatures between 40 and 94 degrees C for 10 min, cooled, and analyzed using near-UV and far-UV circular dichroism (CD). The decrease in near-UV CD intensity at 293 nm (Deltaepsilon(293)) could be analyzed in terms of a two-state model, and the stability was beta-Lg C > beta-Lg A > beta-Lg B on the basis of the midpoint temperatures for samples heated at pH 6.7 and 7.4. However, the slopes of the curves at the midpoint temperature for variant A were generally less than those for beta-Lg B and beta-Lg C, indicating that the substitution of Val (beta-Lg A) for Ala (beta-Lg B or beta-Lg C) at position 118 had altered the entropic contribution to unfolding of the protein. The changes in CD at 270 nm (Deltaepsilon(270)), an index of significant alteration to disulfide bond dihedral angles, occurred at higher temperatures than those for the Deltaepsilon(293) results. The far-UV CD showed some small changes as a consequence of heat treatment, and the shifts at 205 nm ([theta](205)) fitted a two-state model. Plotting the changes in both Deltaepsilon(293) and [theta](205) against the loss of nativelike and sodium dodecyl sulfate-monomeric protein (assessed by polyacrylamide gel electrophoresis) showed a strong 1:1 relationship between Deltaepsilon(293) or [theta](205) and the loss of nativelike beta-Lg. These results indicated that the initial irreversible stage in the heat-induced aggregation of beta-Lg (nativelike monomer to unfolded monomer) altered the chirality of the environment of Trp(19) and modified the secondary structure of beta-Lg slightly. The differences in the behavior of variants A-C were explicable on the basis of generalized electrostatic and hydrophobicity effects as well as specific amino acid effects.

Dilute solutions of beta-lactoglobulin (beta-Lg) A, B, and C were heated at temperatures between about 40 and 94 degrees C for 10 min, cooled, and analyzed using Trp fluorescence and extrinsic fluorescence spectra of the probe 1,8-anilinonaphthalene sulfonate (ANS). Thiol availabilities using 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) were determined using a separate set of samples. The normalized ANS fluorescence emission intensity and the thiol availability results showed a 1:1 relationship with the loss of nativelike but not SDS-monomeric protein, as determined by PAGE analysis. The normalized Trp emission intensity results did not show a comparable 1:1 relationship with the loss of nativelike protein, indicating that the Trp intensity arose from consequential disulfide bond reorganization and not the initial unfolding reaction. The results were also analyzed in terms of two-state models, and the midpoint temperatures (T(mid)) for the proteins were generally beta-Lg C > beta-Lg A > beta-Lg B, and the slopes at the midpoint temperatures for the A variant were generally less than those for the B and C variants indicating that beta-Lg A may denature by a different mechanism from that of beta-Lg B or beta-Lg C. The T(mid) parameters derived from the ANS fluorescence intensity results were similar to those for thiol availability and both were lower than the T(mid) values for Trp emission intensity showing that creation of an ANS binding site on a beta-Lg molecule was linked to the irreversible exposure of a thiol group and the loss of native beta-Lg but preceded the decrease in Trp(61) fluorescence quenching. These results for the differences between the behavior of the A and B or the C variants involved the creation of a destabilizing cavity by the Val(118)Ala (A --> B) substitution and the changed charge distribution within the CD loop caused by the Asp(64)Gly (A --> B) substitution.

1. Glucose -fructose oxidoreductase operates by a classic ping-pong mechanism with a single site for all substrates: glucose, fructose, gluconolactone and sorbitol. The K , values for these substrates were determined. The values of k,,, are 200 s-' and 0.8 s-' for the forward and reverse directions respectively.2. The overall catalytic process consists of two half-reactions with alternate reduction of NADP' and oxidation of NADPH tightly bound to the enzyme. Reduction of enzyme-NADP' by glucose and oxidation of enzyme-NADPH by gluconolactone involve single first-order processes. The values of the rate constants at saturating substrate are 2100 s-l and 8 s-' respectively; deuterium isotope effects indicate that these are for the hydrogen transfer step. Oxidation of enzyme-NADPH by fructose is first order with a limiting rate constant of at least 430 s-'. The reaction of enzyme-NADP' with sorbitol is biphasic, with rate constants for both phases less than 1 s-', This behaviour is explained by a mechanism in which the slow cyclisation of the acyclic form of fructose follows its dissociation from the enzyme.3. The rate-determining steps for the overall reaction are probably dissociation of gluconolactone in the forward direction and hydrogen transfer from sorbitol to enzyme-bound NADP' in the reverse direction.Following the discovery that the ethanol-producing bacterium Zymomonas mobilis produced sorbitol as well as ethanol when grown on sucrose or glucose/fructose mixtures [l -31, an enzyme was identified which catalysed the process [4, 51. The purified enzyme contains tightly bound NADP' and transfers hydrogens from glucose to fructose, producing gluconod-lactone and sorbitol [ 5 ] . The enzyme, named glucose -fructose oxidoreductase, is present in amounts up to 0.7% of the cytoplasmic protein; it was found to have low affinity for its substrates, but a high specific activity [ 5 ] . The presence of a gluconolactonase in Z . mobilis [5] ensures that the reaction is not reversible in vivo; thus the enzyme does not provide a route for metabolising sorbitol.Preliminary kinetic studies on the enzyme showed that the mechanism is ping pong, with a reduced NADP-enzyme intermediate. The K, for glucose was estimated to be between 6 -30 mM [4,5], highly dependent on fructose concentration, and the K , for fructose may be greater than 1 M.The ability to isolate the enzyme in large quantities, and a simple detection system for the reduced enzyme intermediate, have allowed us to carry out stopped-flow studies of the half-reactions and characterise the principal rate constants. A simple assay system for the reverse reaction has enabled us to obtain a detailed picture of the kinetic behaviour of the enzyme at pH 6.5. MATERIALS AND METHODSD-Glucose (predominantly a) was AR grade from Ajax Chemicals (Sydney, Australia). To ensure equilibration of the CI and B forms of D-glucose, stock glucose solutions were made up at least 18 h before use, unless otherwise stated below. Fructose (containing < 0.05% glucose), B-D-glucose (contain...

L-Arginine, an amino acid found in significant quantities in grape juice and wine, is known to be catabolized by some wine lactic acid bacteria. The correlation between the occurrence of arginine deiminase pathway enzymes and the ability to catabolize arginine was examined in this study. The activities of the three arginine deiminase pathway enzymes, arginine deiminase, ornithine transcarbamylase, and carbamate kinase, were measured in cell extracts of 35 strains of wine lactic acid bacteria. These enzymes were present in all heterofermentative lactobacilli and most leuconostocs but were absent in all the homofermentative lactobacilli and pediococci examined. There was a good correlation among arginine degradation, formation of ammonia and citrulline, and the occurrence of arginine deiminase pathway enzymes. Urea was not detected during arginine degradation, suggesting that the catabolism of arginine did not proceed via the arginase-catalyzed reaction, as has been suggested in some earlier studies. Detection of ammonia with Nessler's reagent was shown to be a simple, rapid test to assess the ability of wine lactic acid bacteria to degrade arginine, although in media containing relatively high concentrations (>0.5%) of fructose, ammonia formation is inhibited.