Stabilization of Na,K-ATPase by ionic interactions

The effect of ions on the thermostability and unfolding of Na,K-ATPase from shark salt gland was studied and compared with that of Na,K-ATPase from pig kidney by using differential scanning calorimetry (DSC) and activity assays. In 1 mM histidine at pH 7, the shark enzyme inactivates rapidly at 20 °C, as does the kidney enzyme at 42 °C (but not at 20 °C). Increasing ionic strength by addition of 20 mM histidine, or of 1 mM NaCl or KCl, protects both enzymes against this rapid inactivation. As detected by DSC, the shark enzyme undergoes thermal unfolding at lower temperature (T_m ≈ 45 °C) than does the kidney enzyme (T_m ≈ 55 °C). Both calorimetric endotherms indicate multi-step unfolding, probably associated with different cooperative domains. Whereas the overall heat of unfolding is similar for the kidney enzyme in either 1 mM or 20 mM histidine, components with high mid-point temperatures are lost from the unfolding transition of the shark enzyme in 1 mM histidine, relative to that in 20 mM histidine. This is attributed to partial unfolding of the enzyme due to a high hydrostatic pressure during centrifugation of DSC samples at low ionic strength, which correlates with inactivation measurements. Addition of 10 mM NaCl to shark enzyme in 1 mM histidine protects against inactivation during centrifugation of the DSC sample, but incubation for 1 h at 20 °C prior to addition of NaCl results in loss of components with lower mid-point temperatures within the unfolding transition. Cations at millimolar concentration therefore afford at least two distinct modes of stabilization, likely affecting separate cooperative domains. The different thermal stabilities and denaturation temperatures of the two Na,K-ATPases correlate with the respective physiological temperatures, and may be attributed to the different lipid environments.     

Incidence, antibiotic resistance and clonal relations of MRSA strains isolated from a Romanian university hospital

Our aim was to estimate the frequency and characteristics ofmethicillin-resistant Staphylococcus aureus (MRSA) strains occurring in a Romanian teaching hospital. We retrospectively studied isolates from infected or colonized patients treated at the intensive care and surgical units during January 2004-December 2005. The antibiotic susceptibility of MRSA strains and the presence of mecA gene were determined. Consecutively occurring strains isolated through a three-month period were typed using pulsed field gel electrophoresis. A total of 423 S. aureus strains were identified, methicillin-resistance was detected in 211 (49.9%) strains. Most of them were multiresistant. One of the MRSA genotypes identified by PFGE was commonly recovered from patients treated in the intensive care unit. According to our results, MRSA strains were frequently isolated pathogens in our hospital and there is an urgent need to enhance infection control efforts.     

Bacteriorhodopsin's M412 intermediate contains a 13-cis, 14-s-trans, 15-anti-retinal Schiff base chromophore

The structure of the retinal chromophore about the C = N and C14-C15 bonds in bacteriorhodopsin's M412 intermediate has been determined by analyzing resonance Raman spectra of 2H and 13C isotopic derivatives. Normal mode calculations on 13-cis-retinal Schiff bases demonstrate that the C15-D rock and N-CLys stretch are strongly coupled for C = N-syn chromophores and weakly coupled for C = N-anti chromophores. When the Schiff base geometry is anti, the C15-D rock appears as a localized resonance Raman active mode at approximately 980 cm-1, which is moderately sensitive to 13C substitution at positions 14 and 15 (approximately 7 cm-1) and insensitive to 13C substitution at the epsilon position of lysine. When the Schiff base geometry is syn, in-phase and out-of-phase combinations of the C15-D rock and N-CLys stretch are predicted at approximately 1060 and approximately 910 cm-1, respectively. The in-phase mode is more sensitive to 13C substitution at positions 14 and 15 (approximately 15 cm-1) and at the epsilon position of lysine (approximately 4 cm-1). Calculations and comparison with experimental data on dark-adapted bacteriorhodopsin indicate that the in-phase mode at approximately 1060 cm-1 carries the majority of the resonance Raman intensity. M412 exhibits a C15-D rock at 968 cm-1 that shifts 8 cm-1 when 13C is added at positions 14 and 15 and is insensitive to 13C substitution at the epsilon-position of lysine. This demonstrates that M412 contains a C = N-anti Schiff base.(ABSTRACT TRUNCATED AT 250 WORDS)     

Halorhodopsin and sensory rhodopsin contain a C6-C7 s-trans retinal chromophore.

Halorhodopsin (HR) and sensory rhodopsin (SR) have been regenerated with retinal analogues that are covalently locked in the 6-s-cis or 6-s-trans conformations. Both pigments regenerate more completely with the locked 6-s-trans retinal and produce analogue pigments with absorption maxima (577 nm for HR and 592 nm for SR) nearly identical to those of the native pigments (577 and 587 nm). This indicates that HR and SR bind retinal in the 6-s-trans conformation. The opsin shift for the locked 6-s-trans analogue in HR is 1,200 cm-1 less than that for the native chromophore (5,400 cm-1). The opsin shift for the 6-s-trans analogue in SR is 1,100 cm-1 less than that for the native retinal (5,700 cm-1). This demonstrates that approximately 20% of the opsin shift in these pigments arises from a protein-induced change in the chromophore conformation from twisted 6-s-cis in solution to planar 6-s-trans in the protein. The reduced opsin shift observed for the locked 6-s-cis analogue pigments compared with the locked 6-s-trans pigments may be due to a positive electrostatic perturbation near C7.