Esophageal Stricture Caused by Actinomyces in a Patient with No Apparent Predisposing Factors.

Actinomyces species are Gram positive anaerobic or microaerophilic bacteria that are part of the human flora in the oropharyngeal, gastrointestinal, and genitourinary tract. In the presence of a mucosal injury, they can become pathogenic and infect the underlying tissue without respect for tissue planes, leading to abscesses, fistulas, and sinus tracts. Through contiguous and hematogenous spread, virtually any organ can become infected. The presentation can be myriad and often mimics tumors, tuberculosis, or other more common infections and inflammatory conditions. While the cervicofacial region is the most common site of infection, involvement of the esophagus is unusual. Esophageal actinomycosis mostly occurs in patients with compromised immunity or prior esophageal injuries. Occurrence in immunocompetent individuals without risk factors is exceedingly rare. We report a case of esophageal actinomycosis complicated by an esophageal stricture in a patient with no apparent predisposing conditions.

A delayed presentation of splenic laceration and hemoperitoneum following an elective colonoscopy: A rare complication with uncertain risk factors.

Splenic laceration is a rare yet often underreported complication of colonoscopy that is infrequently discussed with the patient during the consent process. Most cases present within 48 h after the inciting colonoscopy; a delayed presentation is rare. We present a case of splenic laceration with hemoperitoneum that manifested 5 days after the initial colonoscopy. The patient was treated conservatively. Traditionally perceived risk factors such as intra-abdominal adhesions, splenomegaly, anticoagulation use, biopsy, polypectomy, a technically challenging procedure, and anesthesia assistance have not been clearly shown to increase the incidence of splenic injury following a colonoscopy. Since the risk factors of splenic injury remain unclear, the clinical presentation is nonspecific, and the consequences can be serious, the endoscopist should make an effort to inform the patient of this rare complication before the procedure.

Mechanism of substrate specificity in 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidases.

5'-Methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) nucleosidase (MTAN) plays a key role in the methionine-recycling pathway of bacteria and plants. Despite extensive structural and biochemical studies, the molecular mechanism of substrate specificity for MTAN remains an outstanding question. Bacterial MTANs show comparable efficiency in hydrolyzing MTA and SAH, while the plant enzymes select preferentially for MTA, with either no or significantly reduced activity towards SAH. Bacterial and plant MTANs show significant conservation in the overall structure, and the adenine- and ribose-binding sites. The observation of a more constricted 5'-alkylthio binding site in Arabidopsis thalianaAtMTAN1 and AtMTAN2, two plant MTAN homologues, led to the hypothesis that steric hindrance may play a role in substrate selection in plant MTANs. We show using isothermal titration calorimetry that SAH binds to both Escherichia coli MTAN (EcMTAN) and AtMTAN1 with comparable micromolar affinity. To understand why AtMTAN1 can bind but not hydrolyze SAH, we determined the structure of the protein-SAH complex at 2.2Å resolution. The lack of catalytic activity appears to be related to the enzyme's inability to bind the substrate in a catalytically competent manner. The role of dynamics in substrate selection was also examined by probing the amide proton exchange rates of EcMTAN and AtMTAN1 via deuterium-hydrogen exchange coupled mass spectrometry. These results correlate with the B factors of available structures and the thermodynamic parameters associated with substrate binding, and suggest a higher level of conformational flexibility in the active site of EcMTAN. Our results implicate dynamics as an important factor in substrate selection in MTAN.