Simulated dipeptide recognition by vancomycin

The antimicrobial activity of vancomycin and related glycopeptide antibiotics is due to stereospecific recognition of polypeptide components in bacterial cell walls. To better understand how these antibiotics recognize polypeptide determinants, we have developed dynamic models of the complexes formed by the vancomycin aglycon and two different dipeptide ligands, Ac-D-ala-D-ala and Ac-D-ala-gly. Molecular dynamics simulations of the two complexes, initially conditioned with distance constraints derived from two-dimensional nuclear magnetic resonance (NMR) studies, are conformationally stable and propagate in a manner consistent with the NMR-derived constraints after the constraints are removed. Free energy calculations accurately predict the relative binding affinity of these two complexes and help validate the simulation models for detailed structural analysis. Although the two ligands adopt similar conformations when bound to the antibiotic, there are clear differences in the configuration of intermolecular hydrogen bonds, the overall shape of the antibiotic, and other structural features of the two complexes. This analysis illustrates how complex structural and dynamic factors interrelate and contribute to differences in binding affinity.     

Sociodemographic factors and the variation in syphilis rates among US counties, 1984 through 1993: an ecological analysis

OBJECTIVES: Syphilis in the United States is focally distributed, with high incidence rates in the South and in metropolitan areas nationwide. In this study an ecological analysis, using the county as the unit of analysis, was performed to generate hypotheses about community-level determinants of syphilis rates. METHODS: Bivariate rank correlations and multivariate, backward stepwise elimination linear regressions were performed. Mean annual incidence of primary- and secondary-stage syphilis in a county was the dependent variable, and county sociodemographic characteristics (from census data) were the independent variables. RESULTS: In the multivariate regression model, sociodemographic characteristics accounted for 71% of the variation in syphilis rates among counties. With other factors accounted for, the most highly correlated characteristics were percentage non-Hispanic Black population, county location in the South, percentage of the population that was urban, percentage Hispanic population, and percentage of births to women younger than 20 years. CONCLUSIONS: Most of the variation in syphilis rates among counties is accounted for by sociodemographic characteristics. Identification and remediation of modifiable health determinants for which these factors are markers are needed to improve the health status of these populations.     

Mutational analysis of the potential phosphorylation sites for protein kinase C on the CCK(A) receptor

1. Many G protein-coupled receptors contain potential phosphorylation sites for protein kinase C (PKC), the exact role of which is poorly understood. In the present study, a mutant cholecystokininA (CCK(A)) receptor was generated in which the four consensus sites for PKC action were changed in an alanine. Both the wild-type (CCK(A)WT) and mutant (CCK(A)MT) receptor were stably expressed in Chinese hamster ovary (CHO) cells. 2. Binding of [3H]-cholecystokinin-(26-33)-peptide amide (CCK-8) to membranes prepared from CHO-CCK(A)WT cells and CHO-CCK(A)MT cells revealed no difference in binding affinity (Kd values of 0.72 nM and 0.86 nM CCK-8, respectively). 3. The dose-response curves for CCK-8-induced cyclic AMP accumulation and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) formation were shifted to the left in CHO-CCK(A)MT cells. This leftward shift was mimicked by the potent inhibitor of protein kinase activity, staurosporine. However, the effect of staurosporine was restricted to CHO-CCK(A)WT cells. This demonstrates that attenuation of CCK-8-induced activation of adenylyl cyclase and phospholipase C-beta involves a staurosporine-sensitive kinase, which acts directly at the potential sites of PKC action on the CCK(A) receptor in CCK-8-stimulated CHO-CCK(A)WT cells. 4. The potent PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), evoked a rightward shift of the dose-response curve for CCK-8-induced cyclic AMP accumulation in CHO-CCK(A)WT cells but not CHO-CCK(A)MT cells. This is in agreement with the idea that PKC acts directly at the CCK(A) receptor to attenuate adenylyl cyclase activation. 5. In contrast, TPA evoked a rightward shift of the dose-response curve for CCK-8-induced Ins(1,4,5)P3 formation in both cell lines. This demonstrates that high-level PKC activation inhibits CCK-8-induced Ins(1,4,5)P3 formation also at a post-receptor site. 6. TPA inhibition of agonist-induced Ca2+ mobilization was only partly reversed in CHO-CCK(A)MT cells. TPA also inhibited Ca2+ mobilization in response to the G protein activator, Mas-7. These findings are in agreement with the idea that partial reversal of agonist-induced Ca2+ mobilization is due to the presence of an additional site of PKC inhibition downstream of the receptor and that the mutant receptor itself is not inhibited by the action of PKC. 7. The data presented demonstrate that the predicted sites for PKC action on the CCK(A) receptor are the only sites involved in TPA-induced uncoupling of the receptor from its G proteins. In addition, the present study unveils a post-receptor site of PKC action, the physiological relevance of which may be that it provides a means for the cell to inhibit phospholipase C-beta activation by receptors that are not phosphorylated by PKC.     

Management of left-sided colonic obstruction by subtotal colectomy and ileocolic anastomosis

OBJECTIVE: To assess complications and functional results of emergency subtotal colectomy with ileocolic anastomosis for acute left-sided colonic obstruction. DESIGN: Retrospective study. SETTING: University hospital, Netherlands. SUBJECTS: 37 patients with acute left-sided colonic obstruction. INTERVENTIONS: Emergency subtotal colectomy with immediate anastomosis (n = 20), Hartmann's procedure (n = 13) or double-loop transverse colostomy (n = 4). MAIN OUTCOME MEASURES: Mortality, morbidity, duration of hospital stay, frequency of defecation, and continence. RESULTS: Morbidity after subtotal colectomy was 10% (n = 2) and mortality 0. There was one anastomotic dehiscence that required a temporary ileostomy. Mean hospital stay was 15 days (range 10-31). All had adequate continence. After 6 weeks mean frequency of defecation was 3/24 hrs (range 2-6). 9 patients died within 2 years of metastatic disease. CONCLUSIONS: Subtotal colectomy with ileocolic anastomosis is a suitable procedure for treating left-sided colonic obstruction provided that pelvic floor function is adequate and a skilled surgeon is available.     

The infrared dichroism of transmembrane helical polypeptides

Polarized attenuated total internal reflectance techniques were applied to study the infrared dichroism of the amide I transition moment in two membrane-bound peptides that are known to form oriented transmembrane helices: gramicidin A in a supported phospholipid monolayer and Ac-Lys2-Leu24-Lys2-amide (L24) in oriented multibilayers. These studies were performed to test the ability of these techniques to determine the orientation of these peptides, to verify the value of optical parameters used to calculate electric field strengths, to examine the common assumptions regarding the amide I transition moment orientation, and to ascertain the effect of surface imperfections on molecular disorder. The two peptides exhibit marked differences in the shape and frequency of their amide I absorption bands. Yet both peptides are highly ordered and oriented with their helical axes perpendicular to the membrane surface. In the alpha-helix formed by L24, there is evidence for a mode with type E1 symmetry contributing to amide I, and the amide I transition moment must be more closely aligned with the peptide C=O (< 34 degrees) than earlier studies have suggested. These results indicate that long-standing assumptions about the orientation of amide I in a peptide require some revision, but that in general, infrared spectroscopy yields reliable information about the orientation of membrane-bound helical peptides.     

Genetic variants of human T-lymphotrophic virus type II in American Indian groups

The human T-lymphotropic virus type II (HTLV-II) is found in many New World Indian groups in North and South America and may have entered the New World from Asia with the earliest migration of ancestral Amerindians over 15,000 years ago. To characterize the phylogenetic relationships of HTLV-II strains infecting geographically diverse Indian populations, we used polymerase chain reaction to amplify HTLV-II sequences from lymphocytes of seropositive Amerindians from Brazil (Kraho, Kayapo, and Kaxuyana), Panama (Guaymi), and the United States (the Navajo and Pueblo tribes of the southwestern states and the Seminoles of Florida). Sequence analysis of a 780-base pair fragment (located between the env gene and the second exons of tax/rex) revealed that Amerindian viruses clustered in the same two genetic subtypes (IIa and IIb) previously identified for viruses from intravenous drug users. Most infected North and Central American Indians had subtype IIb, while HTLV-II infected members of three remote Amazonian tribes clustered as a distinct group within subtype IIa. These findings suggest that the ancestral Amerindians migrating to the New World brought at least two genetic subtypes, IIa and IIb. Because HTLV-II strains from Amazonian Indians form a distinct group within subtype HTLV-IIa, these Brazilian tribes are unlikely to be the source of IIa viruses in North American drug users. Finally, the near identity of viral sequences from geographically diverse populations indicate that HTLV-II is a very ancient virus of man.