Functional orientation of the acyltransferase domain in a module of the erythromycin polyketide synthase

Modular polyketide synthases (PKSs), such as the 6-deoxyerythronolide B synthase (DEBS), catalyze the biosynthesis of structurally complex and medicinally important natural products. These large multienzymes are organized into a series of functional units known as modules. Each dimeric module contains two catalytically independent clusters of active sites homologous to those of vertebrate fatty acid synthases. Earlier studies have shown that modules consist of head-to-tail homodimers in which ketosynthase (KS) and acyl carrier protein (ACP) domains are contributed by opposite subunits to form a catalytic center. Here, we probe the functional topology of the acyltransferase (AT) domain which transfers the methylmalonyl moiety of methylmalonyl-CoA onto the phosphopantetheine arm of the ACP domain. Using a bimodular derivative of DEBS, the AT domain of module 2 (AT2) was inactivated by site-directed mutagenesis. Heterodimeric protein pairs were generated in vitro between the inactivated AT2 (AT2 degrees) polypeptide and an inactive KS1 (KS1 degrees) or KS2 (KS2 degrees) protein. Both of these hybrid proteins supported polyketide synthesis, suggesting that AT2 can perform its function from either subunit. The apparent catalytic rate constants for each of the two hybrid protein pairs, KS1 degrees/AT2 degrees and KS2 degrees/AT2 degrees, were identical, indicating that no significant kinetic preference exists for a particular AT2-ACP2 combination. These results suggest that the AT domain can be shared between the two clusters of active sites within the same dimeric module. Such a novel structural organization might provide a functional advantage for the efficient biosynthesis of polyketides.     

Adverse effects of wide calcium:phosphorus ratios on supplemental phytase efficacy for weanling pigs fed two dietary phosphorus levels

Ninety-six weanling pigs (initial BW = 9.3 kg, initial age = 37 d) were used in a 4-wk experiment to evaluate the response to three Ca: total (t) P ratios (1.2:1, 1.6:1, or 2.0:1) fed in combination with two P levels (.07 or .16% available that correspond to .36 or .45% tP) and two phytase levels (PY; 700 or 1,050 units/kg of diet). A 3 x 2 x 2 factorial arrangement of treatments was employed using a corn-soybean meal diet. Performance, serum mineral concentrations and alkaline phosphatase (ALP) activity, Ca and P digestibility and excretion, and bone mechanical measurements were examined. Average daily gain (P < .001), average daily feed intake (P < .01), and gain:feed (P < .05) were decreased linearly as the Ca:tP ratio became wider. The digestibility of P and Ca were decreased (P < .001) linearly as the Ca:tP ratio became wider. The digestibility of P (P < .001) and fecal P excretion (P < .01) were increased at the higher level of P. Increasing PY from 700 to 1,050 units (U)/kg of diet increased (P < .05) P digestibility and decreased (P < .01) P excretion but did not improve bone measurements. Shear force, stress and energy, and percentage of ash of both metacarpal and 10th rib linearly decreased (P < .001 to .05) as the Ca:tP ratio became wider, and bone measurements were generally greater for pigs fed the higher P level. Serum Ca concentration increased (P < .01) and the P concentration decreased (P < .001) as the Ca:tP ratio increased, but Mg, Zn, and ALP activity were not influenced by the Ca:tP ratio. Serum Ca and P concentrations were affected by PY supplementation over the 4-wk trial, but serum Mg and Zn concentrations were not affected by dietary treatments. Adverse effects of a wide Ca:tP ratio were greater at the low P diet for all responses. In addition, the activity of supplemental PY in diets seemed to be decreased as the Ca:tP ratio became wider and this negative effect of Ca:tP ratio seemed greater at the low P level, and seemed to parallel the effects of Ca:tP ratio on performance, P digestibility, bone, and serum measurements. Narrowing the dietary Ca:total P ratio from 2.0:1 to 1.2:1 led to an approximate 16% increase in phytase efficacy for improving performance, digestibility, bone measurements, and serum Ca levels.     

1,4-Cyclohexanecarboxylates: potent and selective inhibitors of phosophodiesterase 4 for the treatment of asthma

Evaluation of a variety of PDE4 inhibitors in a series of cellular and in vivo assays suggested a strategy to improve the therapeutic index of PDE4 inhibitors by increasing their selectivity for the ability to inhibit PDE4 catalytic activity versus the ability to compete for high affinity [3H]rolipram-binding sites in the central nervous system. Use of this strategy led ultimately to the identification of cis-4-cyano-4-[3-(cyclopentyloxy)-4-methoxyphenyl]cyclohexane-1-carboxyl ic acid (1, SB 207499, Ariflo), a potent second-generation inhibitor of PDE4 with a decreased potential for side effects versus the archetypic first generation inhibitor, (R)-rolipram.     

Comparisons of highly virulent H5N1 influenza A viruses isolated from humans and chickens from Hong Kong

Genes of an influenza A (H5N1) virus from a human in Hong Kong isolated in May 1997 were sequenced and found to be all avian-like (K. Subbarao et al., Science 279:393-395, 1998). Gene sequences of this human isolate were compared to those of a highly pathogenic chicken H5N1 influenza virus isolated from Hong Kong in April 1997. Sequence comparisons of all eight RNA segments from the two viruses show greater than 99% sequence identity between them. However, neither isolate's gene sequence was closely (>95% sequence identity) related to any other gene sequences found in the GenBank database. Phylogenetic analysis demonstrated that the nucleotide sequences of at least four of the eight RNA segments clustered with Eurasian origin avian influenza viruses. The hemagglutinin gene phylogenetic analysis also included the sequences from an additional three human and two chicken H5N1 virus isolates from Hong Kong, and the isolates separated into two closely related groups. However, no single amino acid change separated the chicken origin and human origin isolates, but they all contained multiple basic amino acids at the hemagglutinin cleavage site, which is associated with a highly pathogenic phenotype in poultry. In experimental intravenous inoculation studies with chickens, all seven viruses were highly pathogenic, killing most birds within 24 h. All infected chickens had virtually identical pathologic lesions, including moderate to severe diffuse edema and interstitial pneumonitis. Viral nucleoprotein was most frequently demonstrated in vascular endothelium, macrophages, heterophils, and cardiac myocytes. Asphyxiation from pulmonary edema and generalized cardiovascular collapse were the most likely pathogenic mechanisms responsible for illness and death. In summary, a small number of changes in hemagglutinin gene sequences defined two closely related subgroups, with both subgroups having human and chicken members, among the seven viruses examined from Hong Kong, and all seven viruses were highly pathogenic in chickens and caused similar lesions in experimental inoculations.     

Interleukin (IL)-10, IL-12, and interferon-gamma production in primary and memory immune responses to varicella-zoster virus

Varicella immunization provided the opportunity to examine the kinetics of interleukin (IL)-10, IL-12 and interferon (IFN)-gamma production elicited during primary in vivo sensitization with proteins of varicella-zoster virus (VZV), a common human herpesvirus. VZV-specific IFN-gamma release and T cell proliferation were elicited by immunization and persisted through 15 months of follow-up. The induction of VZV-specific T cells and IgG antibodies was accompanied by transient increases in IL-10 and IL-12 production. T cell proliferation to VZV was significantly lower in adults at 15 months than in vaccinated children or naturally immune subjects and correlated with lower IFN-gamma responses in individual vaccinees. After primary immunity was induced, continued IL-12 production was not necessary to maintain the predominant Th1-type response elicited by VZV. Cytokine profiles observed during primary in vivo sensitization to VZV suggest that parallel increases in IFN-gamma and IL-10 may be important in the induction of immunity to some viral pathogens.     

Glucose transporter (GLUT-4) is targeted to secretory granules in rat atrial cardiomyocytes

The insulin-responsive glucose transporter GLUT-4 is found in muscle and fat cells in the trans-Golgi reticulum (TGR) and in an intracellular tubulovesicular compartment, from where it undergoes insulin-dependent movement to the cell surface. To examine the relationship between these GLUT-4-containing compartments and the regulated secretory pathway we have localized GLUT-4 in atrial cardiomyocytes. This cell type secretes an antihypertensive hormone, referred to as the atrial natriuretic factor (ANF), in response to elevated blood pressure. We show that GLUT-4 is targeted in the atrial cell to the TGR and a tubulo-vesicular compartment, which is morphologically and functionally indistinguishable from the intracellular GLUT-4 compartment found in other types of myocytes and in fat cells, and in addition to the ANF secretory granules. Forming ANF granules are present throughout all Golgi cisternae but only become GLUT4 positive in the TGR. The inability of cyclohexamide treatment to effect the TGR localization of GLUT-4 indicates that GLUT-4 enters the ANF secretory granules at the TGR via the recycling pathway and not via the biosynthetic pathway. These data suggest that a large proportion of GLUT-4 must recycle via the TGR in insulin-sensitive cells. It will be important to determine if this is the pathway by which the insulin-regulatable tubulo-vesicular compartment is formed.     

Lesions in the dentate hilum and CA2/CA3 regions of the rat hippocampus produce cognitive deficits that correlate with site-specific glial activation

The aim of the present experimental investigation was to study the morphological and dimensional changes of bone, augmented at titanium implants by a membrane technique, taking place after membrane removal. In 12 rabbits, screw-shaped titanium implants were inserted in the tibial metaphyses in such a way that 5 threads became uncovered with bone. Surgery was performed on 2 occasions in order to retrieve specimens with different follow-up times. An e-PTFE barrier and a titanium device were used to provide space for bone formation. In 1 tibia of each rabbit, the membranes and spacers were removed after 8 weeks of healing, and the implants followed for 16 more weeks. Impressions were taken at day 0 and after 8 and 24 weeks of healing and plaster models were produced. In the contralateral tibiae, implants were inserted either 16 or 8 weeks prior to sacrifice. Measurements were made on the plaster models in 3 dimensions at 35 points around each implant in a coordinate measuring machine. Specimens taken 8, 16 and 24 weeks after insertion were analysed by means of light microscopical morphometry. The coordinate measurements showed that, in mean, 1.92 mm of bone had been formed during the first 8 weeks. A statistically significant loss of the height of the newly formed bone (0.70 mm) and thereby reduction of bone volume was found 24 weeks postoperatively. The volume decrease of the newly formed bone was more pronounced beside the implants than over the implant body. The histology showed that woven bone had been formed at the implants after 8 weeks. Further bone formation and remodelling and a net increase of mineralized bone were seen. The degree of bone-implant contact and bone area in the threads increased with time. The present study showed that coordinate measurements on plaster models, obtained from the experimental areas, in combination with histology, form a useful technique to study long-term changes of augmented bone. It was found that bone formed by a barrier membrane technique, decreased in volume during a 16-week follow-up period after barrier removal. Less dimensional changes were observed for the bone formed over the implant body, indicating that a solid surface may have a stabilizing effect on the augmented bone.