Effects of protective agents on membrane fluidity of freeze-dried Lactobacillus delbrueckii ssp. bulgaricus

AIMS

To evaluate the effect of protective agents upon survival of Lactobacillus delbrueckii ssp. bulgaricus during freeze-drying and storage, and selective amino acids on cell membrane fluidity.

METHODS AND RESULTS

The protective effect of amino-acids and sugars at different concentrations was studied by determining the viability of lyophilized cells after storage under air at 30 degrees C. Survival following freeze-drying was improved by all compounds. During storage, neither proline nor maltose had protective effects on lyophilized Lact. delbrueckii ssp. bulgaricus. Glutamate 5% and aspartate 5% showed similar protection capability during freeze-drying (94-95%) and after storage (92-99%). Fluorescence probes (DPH and TMA-DPH) were used to study the effect of both amino acids on membrane fluidity. Polarization decreased with increasing concentrations of glutamate or aspartate. Lowest values were observed with TMA-DPH.

CONCLUSIONS

Glutamate 5% and aspartate 5% allowed maintaining high viability rates during freeze-drying and storage of Lact. delbrueckii ssp. bulgaricus because of an increase of the membrane fluidity by inserting in the interfacial region of bacterial plasma membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results show the first evidence of the mechanisms underlying glutamate and aspartate as lyoprotectors.

Zn(2+) ions selectively induce antimicrobial salivary peptide histatin-5 to fuse negatively charged vesicles. Identification and characterization of a zinc-binding motif present in the functional domain

The salivary antimicrobial peptide histatin-5 is able to aggregate and fuse negatively charged small unilamellar vesicles, and this fusogenic activity is selectively induced by the presence of zinc ions. Circular dichroism spectroscopy shows that histatin-5, in the presence of negatively charged vesicles and zinc ions, undergoes a conformational change leading to the stabilization of an alpha-helical secondary structure. We attribute the specific action of the zinc ions to the presence of a consensus sequence, HEXXH, located in the C-terminal functional domain of histatin-5, a recognized zinc-binding motif in many proteins. Two-dimensional proton NMR spectroscopy of histatin-5 in a trifluoroethanol/water mixture (a membrane mimetic environment) has been performed and the results analyzed by means of distance geometry and restrained molecular dynamics simulations. Our results reveal that the peptide chain, including the Zn-binding consensus sequence corresponding to residues 15-19, is in a helicoidal conformation. Comparison of the chemical shifts of the individual amino acids in histatin-5 with those recently reported in other solvents indicates that trifluoroethanol/water has a structuring capability somewhere between water and dimethyl sulfoxide. The mechanism of action of this antimicrobial peptide is discussed on the basis of its structural characteristics with particular attention to the Zn-binding motif.

Calcium ion independent membrane leakage induced by phospholipase-like myotoxins

The two snake venom myotoxins ammodytin L and myotoxin II, purified respectively from Vipera ammodytes ammodytes and Bothrops asper, have phospholipase-like structures but lack an Asp-49 in the active site and are without normal phospholipase activity. The interaction of these proteins with different types of liposomes indicated that the myotoxins were able to provoke rapid and extensive release of the aqueous content of liposomes. Leakage was measured by two different methods: fluorescence dequenching of liposome-entrapped carboxyfluorescein and ESR measurement of intravesicular TEM-POcholine reduction by external ascorbate. The process was independent of Ca2+ and took place without any detectable phospholipid hydrolysis. Nonmyotoxic phospholipases tested under the same conditions were unable to induce liposome leakage, which could be detected only when Ca2+ was added to the medium and with the concomitant hydrolysis of phospholipids. The kinetics of Ca(2+)-dependent and Ca(2+)-independent leakage were completely different, indicating two different mechanisms of interaction with the lipid bilayer. Studies using diphenylhexatriene as a probe of lipid membrane organization indicated that the myotoxins gave rise to a profound perturbation of the arrangement of the lipid chains in the membrane interior, whereas interaction of Naja naja phospholipase A2 with the membrane surface did not affect lipid organization. On the basis of these results we suggest that a new type of cytolytic reaction mechanism is responsible for the effects of phospholipase-like myotoxins in vivo.

Vitamin D3 administration increases the membrane fluidity of intestinal mitochondria

The fluorescence anisotropy in the mitochondria from vitamin D-treated chicks is significantly lower than that from the vitamin D-deficient animals with the inner core probe DPH. Surface membrane fluidity, measured with the probe TMA-DPH, shows no differences between the organelles of both groups. The fluorescence studies performed in mitochondrial subfractions revealed that cholecalciferol treatment induces a decrease of lipid order parameter S (DPH) in the mitochondrial inner membrane. These results pose the question of whether vitamin D3 participates in the regulation of physiological function of the intestinal mitochondria through changes in the physical properties of the membranes.