Co(II)-substituted Haemophilus influenzae ß-carbonic anhydrase: spectral evidence for allosteric regulation by pH and bicarbonate ion.

Cobalt(II)-substituted Haemophilus influenzae ß-carbonic anhydrase (HICA) has been produced by overexpression in minimal media supplemented with CoCl(2), enabling kinetic, structural, and spectroscopic characterization. Co(II)-substituted HICA (Co-HICA) has comparable catalytic activity to that of wild-type enzyme with k(cat)=82±19 ms(-1) (120% of wild-type). The X-ray crystal structure of Co-HICA was determined to 2.5Å resolution, and is similar to the zinc enzyme. The absorption spectrum of Co-HICA is consistent with four-coordinate geometry. pH-dependent changes in the absorption spectrum of Co-HICA, including an increase in molar absorptivity and a red shift of a 580 nm peak with decreasing pH, correlate with the pH dependence of k(cat)/K(m). The absence of isosbestic points in the pH-dependent absorption spectra suggest that more than two absorbing species are present. The addition of bicarbonate ion at pH 8.0 triggers spectral changes in the metal coordination sphere that mimic that of lowering pH, supporting its hypothesized role as an allosteric inhibitor of HICA. Homogeneously (99±1% Co) and heterogeneously (52±5% Co) substituted Co-HICA have distinctly different colors and absorption spectra, suggesting that the metal ions in the active sites in the allosteric dimer of Co-HICA engage in intersubunit communication.

Evidence for a bicarbonate "escort" site in Haemophilus influenzae beta-carbonic anhydrase .

The Haemophilus influenzae beta-carbonic anhydrase (HICA) allosteric site variants V47A and G41A were overexpressed and purified to homogeneity. These variants have k(cat)/K(m) values similar to that of the wild-type enzyme and exhibit a similar dramatic decrease in catalytic activity at pH <8.0. However, both HICA-G41A and -V47A were serendipitously found to bind sulfate ion or bicarbonate ion near pairs of Glu50 and Arg64 residues located on the dimerization interface. In the case of HICA-V47A, bicarbonate ions simultaneously bind to both the dimerization interface and the allosteric sites. For HICA-G41A, two of 12 chains in the asymmetric unit bind bicarbonate ion exclusively at the dimerization interface, while the remaining 10 chains bind bicarbonate ion exclusively at the allosteric site. We propose that the new anion binding site along the dimerization interface of HICA is an "escort" site that represents an intermediate along the ingress and egress route of bicarbonate ion to and from the allosteric binding site, respectively. The structural evidence for sulfate binding at the escort site suggests that the mechanism of sulfate activation of HICA is the result of sulfate ion competing for bicarbonate at the escort site, preventing passage of bicarbonate from the bulk solution to its allosteric site.

Just a mirage: heterotopic intrauterine and twin ectopic pregnancy mimicked by mirror imaging on ultrasound.

Heterotopic pregnancies are rare and are usually diagnosed by transvaginal ultrasound. Despite the routine use of sonography in early pregnancy, artifact created by mirror imaging can drastically and erroneously alter medical decision making by interfering with image interpretation. A heterotopic pregnancy with a single intrauterine gestation and twin left adnexal ectopic gestational sacs was observed on ultrasound in a woman presenting with abdominal pain. During laparoscopy, an ectopic pregnancy was not identified, and subsequent intraoperative ultrasound reproduced the heterotopic pregnancy through manipulation of bowel, confirming mirror image artifact. This phenomenon is rarely seen in obstetric imaging; therefore, lack of awareness can lead to false diagnosis of heterotopic pregnancy. Techniques to verify correct diagnosis should be used to resolve potential mirror artifact before proceeding with surgical management.

Magnetic resonance characteristics and susceptibility weighted imaging of the brain in gadolinium encephalopathy.

BACKGROUND AND PURPOSE: To report the brain imaging features on magnetic resonance imaging (MRI) in inadvertent intrathecal gadolinium administration. METHODS: A 67-year-old female with gadolinium encephalopathy from inadvertent high dose intrathecal gadolinium administration during an epidural steroid injection was studied with multisequence 3T MRI. RESULTS: T1-weighted imaging shows pseudo-T2 appearance with diffusion of gadolinium into the brain parenchyma, olivary bodies, and membranous labyrinth. Nulling of cerebrospinal fluid (CSF) signal is absent on fluid attenuation recovery (FLAIR). Susceptibility-weighted imaging (SWI) demonstrates features similar to subarachnoid hemorrhage. CT may demonstrate a pseudo-cerebral edema pattern given the high attenuation characteristics of gadolinium. CONCLUSION: Intrathecal gadolinium demonstrates characteristic imaging features on MRI of the brain and may mimic subarachnoid hemorrhage on susceptibility-weighted imaging. Identifying high dose gadolinium within the CSF spaces on MRI is essential to avoid diagnostic and therapeutic errors.

Nodular fasciitis in the axillary tail of the breast.

Nodular fasciitis is a benign proliferation of myofibroblasts which presents clinically as a rapidly growing mass with nonspecific features on imaging and high cellular activity on histopathology. Nodular fasciitis can be mistaken for malignant fibrous lesions such as soft tissue sarcoma or breast carcinoma when located within breast tissue. This presents a problem for appropriate treatment planning as the natural history of nodular fasciitis is spontaneous regression. We present the mammographic, sonographic, computed tomography, and histopathologic characteristics of nodular fasciitis in a 68 year female initially presenting with a rapidly enlarging right axillary mass.