G117C MelB, a mutant melibiose permease with a changed conformational equilibrium.

Replacement of the glycine at position 117 by a cysteine in the melibiose permease creates an interesting phenotype: while the mutant transporter shows still transport activity comparable to the wild type its pre steady-state kinetic properties are drastically altered. The transient charge displacements after substrate concentration jumps are strongly reduced and the fluorescence changes disappear. Together with its maintained transport activity this indicates that substrate translocation in G117C melibiose permease is not impaired but that the initial conformation of the mutant transporter differs from that of the wild type permease. A kinetic model for the G117C melibiose permease based on a rapid dynamic equilibrium of the substrate free transporter is proposed. Implications of the kinetic model for the transport mechanism of the wild type permease are discussed.

Desflurane inhibits platelet function in vitro similar to halothane.

BACKGROUND AND OBJECTIVE: The effects of the volatile anaesthetic desflurane on platelet activation in vitro were studied and compared to those of halothane. METHODS: Platelet-rich plasma was exposed to 2 MAC of desflurane or halothane, or air only and stimulated by platelet agonists ADP (2.5, 5 and 10 micromol L(-10) and collagen (10 microg m(L-1)). Platelet response was measured by Born aggregometry (maximum aggregation response, area under the curve) and flow cytometry (mean channel fluorescence, percentage of CD62P-positive cells, index of platelet activation for positive platelets). RESULTS: Aggregation response was significantly reduced in platelets exposed to desflurane or halothane; the inhibitory effect was more pronounced when the areas under the curve were analysed: values ranged from 37.5% to 73.3% of control samples for ADP stimulation and 77.1% to 79.8% for collagen stimulation. CD62P expression before and after stimulation with receptor agonists was not statistically different in platelets exposed to desflurane, halothane or air. CONCLUSIONS: By impairing platelet aggregation while not affecting alpha-degranulation desflurane has a differential effect on various aspects of platelet activation similar to halothane. Our results are compatible with the hypothesis of an impairment of platelet thromboxane receptor signalling by halothane. We suggest a similar mechanism for desflurane.

A DNA-porphyrin minor-groove complex at atomic resolution: the structural consequences of porphyrin ruffling.

The crystal structure of a B-type DNA hexanucleotide duplex complexed with the porphyrin molecule nickel-[tetra-N-methyl-pyridyl] porphyrin has been solved by multiwavelength anomalous diffraction phasing and refined to an R factor of 11.5% at a resolution of 0.9 A. The structure has been solved and refined as two crystallographically independent duplexes, stacked end to end. Contrary to expectation, the porphyrin molecule is not intercalated into the duplex but is stacked onto the ends of the two-duplex stack. The porphyrin molecule is highly buckled as a consequence of the nickel coordination, which produces large changes in local DNA structure. A second mode of porphyrin binding is apparent as a consequence of crystal packing, which places the ligand in the minor groove of an adjacent duplex. This structure thus provides, to our knowledge, the first atomic visualization of minor-groove binding for a porphyrin molecule. The geometry of groove binding provides a ready explanation for porphyrin-induced DNA strand cleavage at deoxyribose residues.