Are cervical spine radiograph examinations useful in patients with low clinical suspicion of cervical spine fracture? An experience with 254 cases.

Trauma patients with low clinical suspicion of cervical spine fracture are often examined with a plain X-ray cervical spine series rather than with cervical spine computed tomography (CT). The authors have been concerned by the absence of fractures in the group of patients examined with plain X-ray. The objective of this investigation was to determine the usefulness of plain X-ray examinations in suspected cases of cervical spine fracture compared to CT. A retrospective review was performed of all trauma patients undergoing imaging for suspected cervical spine fracture in our Emergency Department over a one-year period (January 1, 2007 to December 31, 2007). During the study period, 254 cervical spine plain X-ray and 3,080 cervical spine CT examinations were performed. Of the 254 plain X-ray examinations, 237 were interpreted as negative for fracture, 11 were suboptimal examinations, and six were interpreted as possible fractures (later ruled out by further imaging). Of the 3,080 CT examinations, 2,884 were interpreted as negative for fracture and 196 as positive. The overall positivity rates for acute cervical spine fracture were 0.0% in plain X-ray and 6.4% in CT examinations. These data confirm the authors' concern that plain X-ray imaging for patients with low clinical suspicion for cervical spine trauma in our hospital may have too low a yield to justify its use. However, the 6.4% positivity rate in the group of patients selected for CT examination justifies its use in this group.

Cytochrome b mutations that modify the ubiquinol-binding pocket of the cytochrome bc1 complex and confer anti-malarial drug resistance in Saccharomyces cerevisiae.

Atovaquone is a new anti-malarial agent that specifically targets the cytochrome bc1 complex and inhibits parasite respiration. A growing number of failures of this drug in the treatment of malaria have been genetically linked to point mutations in the mitochondrial cytochrome b gene. To better understand the molecular basis of atovaquone resistance in malaria, we introduced five of these mutations, including the most prevalent variant found in Plasmodium falciparum (Y268S), into the cytochrome b gene of the budding yeast Saccharomyces cerevisiae and thus obtained cytochrome bc1 complexes resistant to inhibition by atovaquone. By modeling the variations in cytochrome b structure and atovaquone binding with the mutated bc1 complexes, we obtained the first quantitative explanation for the molecular basis of atovaquone resistance in malaria parasites.

Molecular basis for atovaquone resistance in Pneumocystis jirovecii modeled in the cytochrome bc(1) complex of Saccharomyces cerevisiae.

Atovaquone is a substituted hydroxynaphthoquinone that is widely used to prevent and clear Plasmodium falciparum malaria and Pneumocystis jirovecii pneumonia. Atovaquone inhibits respiration in target organisms by specifically binding to the ubiquinol oxidation site at center P of the cytochrome bc(1) complex. The failure of atovaquone treatment and mortality of patients with malaria and P. jirovecii pneumonia has been linked to the appearance of mutations in the cytochrome b gene. To better understand the molecular basis of atovaquone resistance, we have introduced seven of the mutations from atovaquone-resistant P. jirovecii into the cytochrome b gene of Saccharomyces cerevisiae and thus obtained cytochrome bc(1) complexes resistant to inhibition by atovaquone. In these enzymes, the IC(50) for atovaquone increases from 25 nm for the enzyme from wild-type yeast to >500 nm for some of the mutated enzymes. Modeling of the changes in cytochrome b structure and atovaquone binding with the mutated bc(1) complexes provides the first quantitative explanation for the molecular basis of atovaquone resistance.

Molecular basis for atovaquone binding to the cytochrome bc1 complex.

Atovaquone is a substituted 2-hydroxynaphthoquinone that is used therapeutically to treat Plasmodium falciparum malaria, Pneumocystis carinii pneumonia, and Toxoplasma gondii toxoplasmosis. It is thought to act on these organisms by inhibiting the cytochrome bc1 complex. We have examined the interaction of atovaquone with the bc1 complex isolated from Saccharomyces cerevisiae, a surrogate, nonpathogenic fungus. Atovaquone inhibits the bc1 complex competitively with apparent Ki = 9 nm, raises the midpoint potential of the Rieske iron-sulfur protein from 285 to 385 mV, and shifts the g values in the EPR spectrum of the Rieske center. These results indicate that atovaquone binds to the ubiquinol oxidation pocket of the bc1 complex, where it interacts with the Rieske iron-sulfur protein. A computed energy-minimized structure for atovaquone liganded to the yeast bc1 complex suggests that a phenylalanine at position 275 of cytochrome b in the bovine bc1 complex, as opposed to leucine at the equivalent position in the yeast enzyme, is responsible for the decreased sensitivity of the bovine bc1 complex (Ki = 80 nm) to atovaquone. When a L275F mutation was introduced into the yeast cytochrome b, the sensitivity of the yeast enzyme to atovaquone decreased (Ki = 100 nm) with no loss in activity, confirming that the L275F exchange contributes to the differential sensitivity of these two species to atovaquone. These results provide the first molecular description of how atovaquone binds to the bc1 complex and explain the differential inhibition of the fungal versus mammalian enzymes.