Structure and mechanism of arylamine N-acetyltransferases.

Arylamine N-acetyltransferases (NATs) are a family of phase II drug-metabolising enzymes which are important in the biotransformation of various aromatic and heterocyclic amines and hydroxylamines, arylhydrazines and arylhydrazides. NATs are present in a wide range of eukaryotes and prokaryotes. Humans have two functional NAT isoforms, both of which are highly polymorphic. The pharmacogenetics of NATs is an area which has been extensively studied. The determination of the X-ray crystal structure of NAT from Salmonella typhimurium led to the identification of the catalytically essential triad of residues: Cys-His-Asp, which is present in all functional NAT enzymes. Recent co-crystallisation data and in silico docking studies of NAT from Mycobacterium smegmatis with substrates and inhibitors have aided the identification of important contact residues within the active site. The X-ray crystal structures of four prokaryotic NAT proteins have now been determined, and these have been used to generate structural models of eukaryotic NATs, providing valuable insight into their active-site architecture. In addition to aiding crystallographic experiments, recent progress in the production of recombinant prokaryotic and eukaryotic NATs has allowed comparative studies of the kinetics and activity profiles of these enzymes. In this review we present an overview of recent structural and activity studies on NAT enzymes, and we outline how in silico methods may be used to predict NAT protein-ligand interactions based on the current knowledge.

Kinetic resolution of alpha-acetoxy carboxylic acids with homochiral SuperQuats.

Preliminary investigations focused on the kinetic resolution of (+/-)-alpha-acetoxy mandelic carboxylic acid chloride with isopropyl SuperQuat and isopropyl Evans' homochiral auxiliaries. The addition of 2 equiv. of the acid chloride to the lithium salts of the auxiliaries (SuperQuat and Evans') at -100 degrees C in the THF afforded the corresponding N-acyl auxiliaries in high chemical yields (95% and 89%) and d.e.'s of 66% and 40%, respectively. Using the same mandelic acid chloride but incorporating SuperQuat auxiliaries derived from (S)-phenylglycine and (S)-phenylalanine yielded the corresponding N-acyl SuperQuats in 86% and 90% and d.e. 's of 66% and 30%, respectively. Substituting the phenyl group within the alpha-acetoxy mandelic acid chloride for a n-butyl, tert-butyl and cyclohexyl group changed the d.e. significantly when kinetically resolved with isopropyl SuperQuat. The yields were all similar, i.e., approximately 90% but the d.e.'s varied considerably, with values of 20%, 50%, and 82%, respectively. Attempted kinetic resolution of alpha-methoxyphenylacetyl chloride using the lithium salt of isopropyl SuperQuat revealed a diasteroselectivity of 45%, significantly lower than that obtained with the corresponding alpha-acetoxy carboxylic acid chlorides.

Sonoelectrochemical and sonochemical effects of cavitation: correlation with interfacial cavitation induced by 20 kHz ultrasound.

Sonoelectrochemical measurements at macro-electrodes under extreme conditions with a very short distance between ultrasonic horn tip and electrode and different ultrasound intensity levels are shown to result in violent cavitation detected in form of current peaks superimposed on the average limiting current. Analysis of the current data obtained for the oxidation of ferrocene in dimethylformamide (0.1 M NBu4PF6) at a 4 mm diameter Pt disc electrode and for the reduction of Ru(NH3)6(3+) in aqueous 0.1 M KCl at a 6 mm diameter Pt disc electrode consistently indicate a change of the physicochemical nature of sonoelectrochemical processes under extreme conditions. The sonoelectrochemical measurement of the rate constant for the carbon bromide bond cleavage of a 3-bromobenzophenone radical anion electrogenerated at a glassy carbon electrode in dimethylformamide solution in the presence of power ultrasound is shown to yield evidence for a breakdown of the conventional mass transport model of a planar diffusion layer under extreme conditions. The change can be correlated to the number of current data points deviating more than 10% from the mean of the current due to violent cavitation processes superimposed onto the average limiting current. Further, a study of the sonochemical destruction of aqueous dilute cyanide solution (in 0.1 M NaOH) demonstrates a correlation between the electrochemically detected cavitation violence and the sonochemical activity. Factors that govern the violence of interfacial cavitation appear to be directly proportional to the factors that make cavitation in the bulk solution chemically efficient.

Dimercaptosuccinic acid distribution in renal tubular acidosis.

A 99Tcm dimercaptosuccinic acid (DMSA) scan performed after a urinary tract infection demonstrated an unusual pattern of isotope uptake, promoting further investigations leading to a diagnosis of renal tubular acidosis secondary to nephropathic cystinosis. This is known to affect isotope imaging but a unique feature in this undiagnosed case was the uncorrected metabolic acidosis, which had further altered the distribution of the DMSA. It is noteworthy because other patients referred for imaging with renal disease may also have abnormalities of acid base balance.

Complications of the ileal pouch: is the pouchogram a useful predictor?

A series of ileal pouchograms from 25 consecutive patients has been analysed retrospectively. Ileal pouchography may demonstrate abnormalities which delay closure of the covering ileostomy. The aim was to determine whether disruption of the ileoanal anastomosis and/or leak at pouchography correlated with pelvic sepsis after ileostomy closure. Disruption of the stapled ileoanal anastomosis is a sensitive (88%) but not specific predictor (57%) for subsequent pelvic sepsis. The predictive value of a negative test is high (89%). Leak of contrast from the anastomosis is specific (81%) but not sensitive (56%) for pelvic sepsis. No significant relationship was demonstrated between width of the presacral space and the presence of pelvic sepsis. No significant relationship was demonstrated between diameter of the ileoanal anastomosis and symptoms of stricture. The presence of anastomotic disruption or leak at pouchography prior to ileostomy closure are useful predictors of potential pelvic sepsis.

Ultrasound, computed tomography and magnetic resonance imaging in patellar tendinitis.

This prospective study describes the ultrasound, computerized tomography and magnetic resonance imaging findings in 16 cases of patellar tendinitis. In all cases tendon enlargement and reduced echogenicity were visible on ultrasound. Computerized tomography demonstrated enlargement of the tendon with reduced attenuation of the central portion. Magnetic resonance imaging showed focal tendon enlargement in all patients with high signal lesions in 88% of cases. This study has shown that patellar tendinitis may be identified with all three modalities. Ultrasound is recommended as the initial investigation in the assessment of patients with this condition.

The role of indium-labelled leukocyte imaging in pyrexia of unknown origin.

Indium-111-labeled white blood cell scanning is often used in the investigation of pyrexia of unknown origin (PUO) to locate an otherwise occult source of sepsis. From a series of 166 white blood cell studies performed for sepsis, 28 cases of true PUO were identified and reviewed. The sensitivity was 60% and specificity 70%, with a positive predictive value of 38% and negative predictive value of 90%. Only 11% of studies revealed a pyogenic cause for PUO. These results are discussed and the potential role of gallium scanning in PUO is raised.

Enhancement of collapsed lung: a potential pitfall in CT interpretation.

In the presence of a pleural effusion there may be compression of adjacent lung causing segmental atelectasis. In some effusions minor degrees of atelectasis give rise to a fine rim of enhancing lung on computerized tomographic (CT) examination which should not be misinterpreted as extravasated contrast from aortic or oesophageal lumen.

Intramolecular general acid catalysis in the binding reactions of alpha 2-macroglobulin and complement components C3 and C4.

The complement system proteins C3 and C4 and the plasma protease inhibitor alpha 2-macroglobulin, when activated by limited proteolysis, can bind covalently to other macromolecules. The three proteins also exhibit an unusual internal peptide-bond cleavage reaction when denatured. The covalent binding reaction is likely to occur by a transacylation mechanism involving an internal thiolester in the three proteins. However, the activated species of these proteins are much more reactive than simple thiolesters. Studies of molecular models of the thiolester region in C3 show that an intramolecular acid catalysis mechanism can both account for the exceptional reactivity of the activated form of these proteins and provide an explanation for the denaturation-induced peptide bond cleavage.