Quenching of intrinsic tryptophan fluorescence in membranes of rat pituitary cells

Tryptophan fluorescence of mitochondrial complex III reconstituted in phosphatidylcholine bilayers

The tryptophan intrinsic fluorescence of mitochondrial complex III reconstituted in phosphatidylcholine bilayers was examined at different temperatures. Absorption and emission maxima occur at 277 and 332 nm, irrespective of temperature or lipid:protein ratio even if there are indications (from fluorescence quenching) of protein conformational changes as a function of lipid:protein ratio. Low values of Trp fluorescence quantum yield in complex III (0.008-0.010) are probably due to the neighborhood of the heme groups. The temperature-dependent decrease of fluorescence intensity is nonlinear; the corresponding Arrhenius plots show "breaks" or discontinuities that could be interpreted as thermally dependent changes in protein conformation. However, no temperature-dependent changes in fluorescence quenching have been observed that may be related to protein conformational changes. In addition, Arrhenius plots of the fluorescence intensity of simple molecules, such as Trp or 1-anilino-8-naphthalene sulfonate in the presence of aqueous phospholipid dispersions, also show breaks in the same temperature range. Stern-Volmer plots of acrylamide and iodide quenching were also nonlinear, indicating large differences in quenching constants for the various tryptophanyl residues. The quenching results also suggest that, at high lipid:protein ratios, the microviscosity of the protein matrix is higher than that in lipid-poor systems. Comparison of quenching efficiencies of iodide and acrylamide suggest that no significant fraction of the fluorophores occurs in the neighborhood of charged residues.

Conformational changes in rat liver chromatin after liver regeneration

N-Pyrenemaleimide, a fluorescent probe that specifically labels histone H3 of rat liver chromatin in situ, was used to monitor the accessibility of histone H3 in chromatin isolated from rat liver at different times during degeneration. At times of maximum DNA synthesis (18--24 h after hepatectomy), the accessibility of the probe was found to be markedly (40--50%) increased. This increase is abolished, however, by treatment of the chromatin fibres with high salt (2 M-NaCl) or detergent. Tryptophan fluorescence was also enhanced at points of maximum DNA synthesis, suggesting that some non-histone tryptophan-containing protein was being synthesized. The polarization of the labelled histone H3 is not markedly altered, suggesting that fibre aggregation or dissociation does not occur. Mononucleosomes extracted from sham-operated and hepatectomized animals did not exhibit any difference in binding to the probe. Also, analysis of the chromatin protein by electrophoresis on detergent- and acid/urea/ Triton-X-100-containing polyacrylamide gels showed no detectable difference in histone H3 : 1, H3 : 2 or H3 : 3 subclasses.

The in situ labeling of histone H3 in chromatin by a fluorescent probe

A sulfhydryl-specific fluorescent probe, N-3-pyrene maleimide, has been shown to label with high efficiency the sulfhydryl groups of histone H3 in nonsheared chromatin. The probe labels chromatin preparations obtained by mild homogenization or nuclease treatment of rat liver and mouse thymocyte, but not chick erythrocyte nuclei. Mononucleosomes from all nuclear preparations are labeled by the probe. The reaction is inhibited by prior reaction of the chromatin with N-ethyl maleimide. The reaction kinetics show fast and slow components representing reactions with cysteinyl sulfhydryl groups and lysyl epsilon-amino groups, respectively. Dissociation of the chromatin by urea (6 M) or sodium dodecyl sulfate (SDS) increases the fluorescence intensity (2-3 fold) and is maximal at approx. 0.01-0.02% (w/v) SDS. Histones extracted from the labelled chromatin show that approx. 80-90% of the label is associated with the histone fraction and column chromatography of this fraction shows that the label is primarily associated with histone H3. Labelling of the isolated histone fractions shows significant labelling only of histone H3. The intrinsic fluorescence of tryptophan is quenched by the labelled histone H3, but not by iodide, suggesting that non-histone (tryptophan-containing) proteins lie in close proximity to the labelled histone H3 but are not immediately accessible to external solvent. The labelled chromatin exhibits fluorescence anistropy, the anistropy parameter being 0.19 +/- 0.003 for chromatin, 0.05 +/- 0.01 for mononucleosomes and 0.0 for isolated histone H3. This demonstrates the restriction placed on the label's mobility by the chromatin fiber. The formation of a superhelix at 60-100 mM NaCl has been monitored with the probe. An increase in fluorescence intensity at 80 mM NaCl is observed with intact chromatin (but not H-1 depleted chromatin) followed by dissociation of the octamer in 1.50-2.0 M salt accompanied by a large increase in labelling.