Energetics of protein thermodynamic cooperativity: contributions of local and nonlocal interactions

The respective roles of local and nonlocal interactions in the thermodynamic cooperativity of proteins are investigated using continuum (off-lattice) native-centric Gō-like models with a coarse-grained C_α chain representation. We study a series of models in which the (local) bond- and torsion-angle terms have different strengths relative to the (nonlocal) pairwise contact energy terms. Conformational distributions in these models are sampled by Langevin dynamics. Thermodynamic cooperativity is characterized by the experimental criteria requiring the van't Hoff to calorimetric enthalpy ratio ΔH_vH/ΔH_cal≈1 (the calorimetric criterion), as well as a two-state-like variation of the average radius of gyration upon denaturation. We find that both local and nonlocal interactions are critical for thermodynamic cooperativity. Chain models with either much weakened local conformational propensities or much weakened favorable nonlocal interactions are significantly less cooperative than chain models with both strong local propensities and strong favorable nonlocal interactions. These findings are compared with results from a recently proposed lattice model with a local-nonlocal coupling mechanism; their relationship with experimental measurements of protein cooperativity and chain compactness is discussed.     

Pharmacokinetics of human recombinant tissue-type plasminogen activator, administered intra-abdominally, in a rat peritonitis model

Human recombinant tissue-type plasminogen activator (rtPA), administered intraperitoneally, may promote intra-abdominal fibrinolysis in peritonitis, thereby preventing adhesion and abscess formation. The pharmacokinetics of a single intraperitoneal dose of 0.5 or 2.0 mg/ml human rtPA were assessed in rats with fecal peritonitis and related to their endogenous tPA and plasminogen activator inhibitor (PAI) activity. Endogenous PAI activity was strongly elevated both in peritoneal fluid and in plasma, whereas tPA activity was only slightly increased in the peritoneal fluid. Both doses of rtPA significantly enhanced fibrinolytic activity of the peritoneal fluid up to 24 h in a dose-dependent manner. However, tPA activity was lower than calculated from the tPA antigen levels and the specific activity of rtPA in the fluid of rats with peritonitis. Plasma tPA activity was low at both doses. A single dose of 0.5 or 2.0 mg/ml rtPA can raise fibrinolytic activity in the abdominal cavity up to 24 h. High endogenous peritoneal PAI levels may adversely effect intra-abdominal use of human rtPA in peritonitis.     

Measurement of battery noise and resistor-current noise at subaudio frequencies

Using a low-noise low-frequency parametric convertor as a preamplifier, narrowband noise measurements have been carried out on wirewound resistors, two types of dry Leclanché batteries and two types of carbon resistor. Wire-wound resistors and one of the types of battery exhibited no measurable excess noise. An ordinary carbon resistor was found to produce 108 times its thermal noise at 3 c/s when passing 4A. The noise power produced by the second type of battery was linearly related to its current.     

SHORT AND LONG FATIGUE CRACK GROWTH IN A SiC REINFORCED ALUMINIUM ALLOY

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.     

Design of peptides, proteins, and peptidomimetics in chi space

Peptide and protein biological activities depend on their three dimensionals structures in the free state and when interacting with their receptors/acceptors. The backbone conformations such as alpha-helix, beta-sheet, beta-turn, and so forth provide critical templates for the three-dimensional structure, but the overall shape and intrinsic stereoelectronic properties of the peptide or protein important for molecular recognition, signal transduction, enzymatic specificity, immunomodulation, and other biological effects depend on arrangement of the side chain groups in three-dimensional chi space (their chi 1, chi 2, etc. torsional angles). In this paper we explore approaches to the de novo design of polypeptides and peptidomimetics with biased or specific conformational/topographical properties in chi space. We consider computational and experimental methods that can be used to examine the effects of specific structural modifications in constraining side chain groups of amino acid residues and their similarities in chi space to the natural amino acids to evaluate what sort of mimetics are likely to mimic normal amino acids. We then examine some of the asymmetric synthetic methods that are being developed to obtain the amino acid mimetics. Finally, we consider selected examples in the literature where these specialized amino acids have been incorporated in biologically active peptides and the specific insights they have provided regarding the topographical requirements for bioactive peptide potency, selectivity, and other biochemical and pharmacological properties. Constraints in chi space show great promise as useful tools in peptide, protein, and peptidomimetic de novo design of structures and pharmacophores with specific stereostructural, biochemical and biological properties.     

Tuberculin skin testing of hospitalized patients

OBJECTIVES: We sought to define the prevalence of tuberculin skin test (TST) positivity in a group of newly hospitalized patients, to identify risk factors for positive tests, and to examine the impact of testing on infection control practices. DESIGN: Unblinded cohort study over 5 days in July 1992. SETTING: A 1,000-bed university-affiliated hospital. PATIENTS: All patients admitted (excluding obstetric patients and newborns) were interviewed. Patients without a history of tuberculosis (TB) or a positive TST were offered a TST with Candida and tetanus controls. RESULTS: Of 346 patients offered the test, 21 (6%) had a prior history of TB or a positive TST, and 36 (10%) declined to participate; 279 of the remaining 289 completed the study. Anergy was demonstrated in 94 (33.7%) of 279 patients. New positive TSTs were identified in 19 (10.3%) of 185 nonanergic patients. Of the 19 TST-positive patients, 6 (32%) had infiltrates on chest radiographs and were evaluated for active TB. One patient was treated empirically for active TB, and five received isoniazid prophylaxis. Risk factors for a new positive TST included age (odds ratio [OR], 1.56 per decade of life; P = .021), African American race (OR, 4.81; P = .008), alcohol abuse (OR, 5.53; P = .005), and peptic ulcer disease (OR, 4.53; P = .017). Risk factors for anergy included admission to a surgical service (OR, 2.1; P = .006), current use of steroids (OR, 2.65; P = .005), and human immunodeficiency virus (HIV) infection (OR, undefined; P = .034). CONCLUSIONS: Despite a high rate of anergy, routine tuberculin skin testing identified a substantial number of patients with TB infection who might otherwise have gone unrecognized.     

Development of survival responsiveness to brain-derived neurotrophic factor, neurotrophin 3 and neurotrophin 4/5, but not to nerve growth factor, in cultured motoneurons from chick embryo spinal cord

During embryonic development, most neuronal populations undergo a process usually referred to as naturally occurring neuronal death. For motoneurons (MTNs) of the lumbar spinal cord of chick embryos, this process takes place in a well defined period of time, between embryonic days 6 and 10 (E6-E10). Neurotrophins (NTs) are the best characterized family of neurotrophic factors and exert their effects through activation of their specific Trk receptors. In vitro and in vivo studies have demonstrated that rodent motoneurons survive in response to BDNF, NT3, and NT4/5. In contrast, the trophic dependencies of chicken motoneurons have been difficult to elucidate, and various apparently conflicting reports have been published. In the present study, we describe how freshly isolated motoneurons from E5.5 chick embryos did not respond to any neurotrophin in vitro. Yet, because motoneurons were maintained alive in culture in the presence of muscle extract, they developed a delayed specific survival response to BDNF, NT3, and NT4/5 that is clearly dose-dependent, reaching saturation at doses of 100 pg/ml. This trophic response correlated with increasing expression of the corresponding functional receptors TrkB and TrkC. Moreover, TrkB receptor is able to become autophosphorylated and to activate classical intracellular signaling pathways such as the extracellular signal-regulated protein kinase when it is stimulated with its cognate ligand BDNF. Therefore, our results reconcile the reported differences between in vivo and in vitro studies on the ability of chicken MTNs to respond to some members of the neurotrophin family of trophic factors.