Limited hydrolysis of bovine plasma albumin at neutral and alkaline pH catalyzed by associated proteinases

Ferguson plot analysis of multiple intermediate species of thermally unfolded bovine serum albumin

Ferguson plot was used to characterize the multiple intermediate species of bovine serum albumin (BSA) upon thermal unfolding. Differential scanning calorimetry showed an irreversible melting of BSA in Tris-HCl and phosphate buffers with a mid-transition temperature, Tm, of ∼68 °C. Thermally unfolded BSA was analyzed by agarose native gel electrophoresis stained by Coomassie blue and SYPRO Orange staining as a function of pH or protein concentration. SYPRO Orange was used to stain unfolded proteins. BSA heated at 70 and 80 °C, i.e., above the Tm, formed multiple intermediate species, which depended on the pH between 7.0 and 8.0, protein concentration and which buffer was used. These intermediate species were analyzed by Ferguson plot, which showed that BSA heated at 60 °C had a similar size to the native BSA, indicating that they are either native or native-like state consistent with no SYPRO Orange staining. The intermediate species observed at higher temperatures with the mobility less than that of the native BSA showed a steeper Ferguson plot and were stained by SYPRO Orange, indicating that these species had a larger hydrodynamic size than the native BSA and were unfolded.

On the heterogeneity of serum albumin: effect of sodium dodecyl sulphate on agarose chromatography of the protein

The chromatographic behaviour of serum albumin changes if it is dissolved in an aqueous solution of sodium dodecyl sulphate (SDS) and is left at room temperature or is heated. Instead of emerging as one peak from an agarose column it emerges as more than one peak and the protein in each when rechromatographed elutes in more than one peak. Electropherograms made in the presence of SDS of the protein in these peaks show multiple banding from proteins migrating with a wide range of molecular weights. The chromatographic and electrophoretic behaviour of serum albumin in the presence of SDS suggests that the protein is not a simple monomer and alternative explanations for this behaviour are discussed.

Spin-label studies on rat liver and heart plasma membranes: do probe-probe interactions interfere with the measurement of membrane properties?

The structures of purified rat liver and heart plasma membranes were studied with the 5-nitroxide stearic acid spin probe, I(12,3). ESR spectra were recorded with a 50 gauss field sweep, and also with a new technique which "expands" the spectrum by (1) recording pairs of adjoining peaks with a smaller field sweep and (2) superposing the common peaks. The hyperfine splittings measured from the "expanded" spectra were significantly more precise than those obtained from the "unexpanded" spectra. Both procedures were used to study the effects of various I(12,3) probe concentrations on the spectra of liver and heart membranes, as well as the effects of temperature and CaCl2 additions on the spectra of liver membranes, and revealed the following: The polarity-corrected order parameters of liver (31 degrees) and heart (22 degrees) membranes were found to be independent of the probe concentration, if experimentally-determined low I(12,3)/lipid ratios were employed. The absence of obvious radical-interaction broadening in the unexpanded spectra indicated that "intrinsic" membrane properties may be measured at these low probe/lipid ratios. Here, "intrinsic" properties are defined as those which are measured when probe-probe interactions are negligible, and do not refer to membrane behavior in the absence of a perturbing spin label. At higher I(12,3)/lipid ratios, the order parameters of liver and heart membranes were found to substantially decrease with increasing probe concentration. The increase in the "apparent" fluidity of both membrane systems is attributed to enhanced radical interactions; however, an examination of these spectra (without reference to "low" probe concentration spectra) might incorrectly suggest that radical interactions were absent. For the membrane concentrations employed in these studies, the presence of "liquid-lines" (or "fluid components") in the unexpanded ESR spectra was a convenient marker of high probe concentrations. A thermotropic phase separation was observed in liver membranes between 19 degrees and 28 degrees. Addition of CaCl2 to liver plasma membrane [labelled with "low" I(12,3) concentrations] increased the rigidity of the membrane at 31 degrees and 37 degrees, without inducing a segregation of the probe in the bilayer. Previously reported data are discussed in relation to these results, and suggested minimal criteria for performing membrane spin label studies are included.