Mutational analysis of potential zinc-binding residues in the active site of the enterococcal D-Ala-D-Ala dipeptidase VanX

VanX, one of the five proteins required for the vancomycin-resistant phenotype in clinically pathogenic Enterococci, is a zinc-containing d-Ala-d-Ala dipeptidase. To identify potential zinc ligands and begin defining the active site residues, we have mutated the 2 cysteine, 5 histidine, and 4 of the 28 aspartate and glutamate residues in the 202 residue VanX protein. Of 10 mutations, 3 cause inactivation and greater than 90% loss of zinc in purified enzyme samples, implicating His116, Asp123, and His184 as zinc-coordinating residues. Homology searches using the 10 amino acid sequence SxHxxGxAxD, in which histidine and aspartate residues are putative zinc ligands, identified the metal coordinating ligands in the N-terminal domain of the murine Sonic hedgehog protein, which also exhibits an architecture for metal coordination identical to that observed in thermolysin from Bacillus thermoproteolyticus. Furthermore, this 10 amino acid consensus sequence is found in the Streptomyces albus G zinc-dependent N-acyl-d-Ala-d-Ala carboxypeptidase, an enzyme catalyzing essentially the same d-Ala-d-Ala dipeptide bond cleavage as VanX, suggesting equivalent mechanisms and zinc catalytic site architectures. VanX residue Glu181 is analogous to the Glu143 catalytic base in B. thermoproteolyticus thermolysin, and the E181A VanX mutant has no detectable dipeptidase activity, yet maintains near-stoichiometric zinc content, a result consistent with the participation of the residue as a catalytic base.     

Asterlingulatosides C and D, cytotoxic triterpenoid saponins from Aster lingulatus

Over the last few decades, European courses in which 'classical' tropical medicine (i.e. parasitology, entomology, and clinical aspects of tropical diseases) is taught have largely become anachronisms. Most countries in the tropics have their own medical schools and few have much need of expatriate doctors. There do, however, appear to be other ways in which Europeans may still help improve health in the tropics. One is to control the quality of the expatriate nurses who are still in demand in the tropics (often as cheaper, generally less demanding substitutes for doctors). This may be achieved by only training the best, insisting they spend some time after graduation gaining maturity before they leave for the tropics, ensuring they realise that their clinical skills will probably be inferior to their local counterparts in the tropics, and encouraging them to continue studying once abroad. The second way Europeans may help is to change the bias of their courses from teaching to training. There seems little doubt that the intellectual personalities of the brightest young men and women in the tropics often develop far better when they spend a period in a foreign environment, especially when they are allowed to function within a team, with trainees and trainers from other countries. Some 'international' courses may offer such benefits, especially if closely tailored to the needs of the trainees and their home countries.     

Construction and characterization of a radio-iodinatable mutant of recombinant human CD4

Drug-selected intrachromosomal gene amplification by breakage-fusion-bridge (BFB) cycles is well documented in mammalian cells, but factors governing this mechanism are not clear. Here, we show that only some clastogenic drugs induce drug resistance through intrachromosomal amplification. We strictly correlate triggering of BFB cycles to induction of fragile site expression. We demonstrate a dual role for fragile sites in intrachromosomal amplification: a site telomeric to the selected gene is involved in initiation, while a centromeric site defines the size and organization of early amplified units. The positions of fragile sites relative to boundaries of amplicons found in human cancers support the hypothesis that fragile sites play a key role in the amplification of at least some oncogenes during tumor progression.     

The diagnosis and management of gall bladder in private patients in Harare

Gall bladder disease is getting more and more common in Africans. Some present with typical right upper quadrant pain but some are found on routine examination of the abdomen. The diagnosis is now very easy with ultrasound scan of the abdomen. The treatment used to be laparotomy and cholecystectomy but now is with laparoscopic surgery which leaves very little scarring. The results of the Harare study show chronic cholecystitis with cholesterol stones and very few bilirubinate stones.     

Alteration of Ca2+ dependence of neurotransmitter release by disruption of Ca2+ channel/syntaxin interaction

Presynaptic N-type calcium channels interact with syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) through a binding site in the intracellular loop connecting domains II and III of the alpha1 subunit. This binding region was loaded into embryonic spinal neurons of Xenopus by early blastomere injection. After culturing, synaptic transmission of peptide-loaded and control cells was compared by measuring postsynaptic responses under different external Ca2+ concentrations. The relative transmitter release of injected neurons was reduced by approximately 25% at physiological Ca2+ concentration, whereas injection of the corresponding region of the L-type Ca2+ channel had virtually no effect. When applied to a theoretical model, these results imply that 70% of the formerly linked vesicles have been uncoupled after action of the peptide. Our data suggest that severing the physical interaction between presynaptic calcium channels and synaptic proteins will not prevent synaptic transmission at this synapse but will make it less efficient by shifting its Ca2+ dependence to higher values.     

Nonopioid analgesics shorten the duration of postoperative ileus

Morphine inhibits propagating and stimulates nonpropagating colon contractions in monkeys and humans. The use of morphine or other opioids that inhibit propulsive contractions prolongs postoperative ileus. In contrast, ketorolac tromethamine, a nonsteroidal analgesic, has no effect on colon contractions in monkeys. In 14 patients having elective abdominal operations, bipolar electrodes were implanted on the right (n = 13) and left (n = 10) colon. Group A (n = 8) received ketorolac, 30 mg IM q6h, for pain relief. Group B (n = 6) needed supplemental morphine, 2-10 mg IV or IM, plus ketorolac to control their pain. Myoelectric activity was recorded from each subject on postop Days 1-5 and analyzed by computer for electrical control activity (ECA), short and long electrical response activity (ERA), and propagation of long ERA. There was a difference between the two groups in return of propagated long ERA bursts that correlated with clinical recovery from postoperative ileus. Postoperative analgesia with ketorolac resulted in faster resolution of ileus compared to morphine plus ketorolac because opioid-induced motor abnormalities in the colon were avoided.     

The propeptide of the vitamin K-dependent carboxylase substrate accelerates formation of the gamma-glutamyl carbanion intermediate

Vitamin K-dependent carboxylase catalyzes the post-translational gamma-carboxylation of 9-12 glutamyl residues of several blood coagulation proteins. Carboxylase purified from Chinese hamster ovary (CHO) cells as a recombinant FLAG-carboxylase fusion protein [Sugiura, I., et al. (1996) J. Biol. Chem. 271, 17837-17844] was utilized with pentapeptide substrate FL[3H-R,S]EAL with high specific radioactivity to probe the timing of glutamyl Cgamma-3H cleavage relative to Cgamma-COO- bond formation by 14CO2 incorporation rates. Studies were conducted over a range of NaH14CO3 concentrations to assess uncoupling of gamma-glutamyl carbanion formation and over a range of concentrations of ProPT18, the 18-residue peptide corresponding to the -18 to -1 propeptide region of prothrombin known to affect the catalytic efficiency of carboxylase. At saturation, ProPT18 accelerates Cgamma-3H cleavage 11-13-fold and Cgamma-14CO2- formation 6-7-fold, converting a Cgamma-3H cleavage/Cgamma-14CO2- formation ratio of 1.2-1.4 in the absence of ProPT18 to 2.3-2.8 in its presence, a relative increase in and uncoupling of Cgamma-3H cleavage from C-C bond formation. When the HCO3- concentration was varied, the V/K3H+/V/K14CO2 ratios rose as HCO3- fractional saturation dropped to a ratio of 9.3-10.8/l at low bicarbonate, indicating an uncoupling of nine out of ten gamma-glutamyl carbanion formations from carboxylative capture, consistent with prior reports on microsomal enzyme [Larson, A. E., et al. (1981) J. Biol. Chem. 256, 11032-11035]. These results with pentapeptide substrate FLEAL validate reversible gamma-glutamyl carbanion formation by pure carboxylase and indicate the ProPT18 increase in catalytic efficiency is in selective lowering of an energy barrier preceding the gamma-glutamyl carbanion intermediate.     

Annotated bibliography: studies evaluating decision-support interventions for patients

As the size of the older population grows and mortality rates continue to decline, an unprecedented number of women will live to very old age. Recent research has provided a better understanding of the impact of disability in the older population, risk factors for disability, and the consequences of disability. Older women have consistently been found to have higher prevalence rates of disability than men of the same age. This difference does not result from women developing disability more often than men, but rather surviving longer with their disabilities. This effect may be explained at least in part by the differences in the diseases underlying disability in older women and men. Interventions that can reduce the burden of disability in the aging population are now being explored. In the next century, it will be increasingly important to develop new prevention and treatment strategies that address the functional consequences of chronic disease in the population of women living to older and older ages.