An 18 amino acid amphiphilic helix forms the membrane-anchoring domain of the Escherichia coli penicillin-binding protein 5

Small (10 residue) C-terminal deletions of PBP5 cause release of this Inner membrane protein into the periplasm, indicating disruption of the membrane binding domain. To define the extent of the membrane anchoring domain, oligonucleotide-directed mutagenesis was used to introduce both single amino acid changes and novel restriction sites into the DN A, allowing subsequent construction of precise internal deletions. The 10 C-terminal amino acid residues possess very weak membrane anchoring potential. By extending the sequence to 18 residues membrane binding equivalent to that of authentic PBP5 was achieved. A proline substitution in this region, breaking a potential α-helix, also disrupts the membrane binding domain. These results are discussed with respect to the amphi-philicity of the C-terminal sequence when arranged in an α-helix.     

In-vivo effects of ivermection onRhipicephalus appendiculatus: the influence of tick feeding patterns and drug pharmacokinetics

Sublethal effects seen amongstRhipicephalus appendiculatus feeding on ivermectin-treated rabbits were diverse and dependent both on drug dose, pharmacokinetics and tick feeding patterns: changes in drug formulation, the time of infestation relative to treatment, and the tick instar used, profoundly influenced acaricidal activity. Death was a sequel to paralysis only if tick feeding was interrupted for sufficient time to produce irreversible dehydration. Concurrent pharmacokinetic investigations revealed that, for the larvae ofR. appendiculatus, the mean critical lethal dose of ivermectin imbibed over a 5-day engorgement period was 3500 g/kg. This quantity of ivermectin was achieved in the blood-meals of larvae feeding on rabbits treated subcutaneously with a single dose of Ivomec injection (MSD)^*800 g/kg, provided infestation took place within 24 h of treatment. At lower drug doses, or if larval infestations were delayed for>24 h post-treatment, the quantity of circulating ivermectin (and thus imbibed by the tick larvae) fell below 3500 g/kg and an increasing percentage of larvae successfully engorged and detached. More than 90% of such larvae moulted to the nymphal stage. Nymphae and larvae exhibited similar susceptibility to ivermectin on treated rabbits which could be explained by similar feeding patterns. However, adult female and male ticks were markedly less susceptible and interpretation of ivermectin-induced effects was more complex.     

Failure to detect changes in sarcolemma permeability in amphibian muscle following severe cellular damage

1. Isolated amphibian hearts and pectoris cutaneous muscles were exposed either to DNP or to caffeine, thereby producing severe myofilament damage. 2. No accompanying change in sarcolemma permeability was detected by monitoring either CK or LDH release or Procion yellow entry in the heart, or by Procion entry in amphibian skeletal muscle. 3. The findings are in contrast with mammalian cardiac and skeletal muscles, and confirm that the pathways leading to myofilament degradation and to the breakdown in sarcolemma organization are separate.     

Alterations in alpha 1-adrenoceptor stimulation of isolated atria from experimental diabetic rats

This purpose of this investigation was to determine the influence of experimental diabetes (3 months) on the responsiveness of rat isolated atria to alpha 1-adrenoceptor stimulation by phenylephrine. Diabetes was chemically induced with streptozotocin (65 mg/kg i.v.) in 42- to 43-day-old, nonfasted male Sprague-Dawley derived rats. Chronotropic (right atria) and inotropic (left atria) indices were recorded in response to alpha 1-adrenoceptor stimulation by phenylephrine. These experiments were performed in the presence of beta-adrenoceptor antagonism (timolol). Isolated right atria from diabetic rats demonstrated a greater increase in heart rate in response to phenylephrine than did corresponding control atria. Left atria were supersensitive (decrease in EC50 values) and hyperresponsive to alpha 1-adrenoceptor stimulation by phenylephrine when compared with stimulation of control left atria. Diabetic left atria in response to phenylephrine were observed to exchange more radioactive calcium (45Ca2+) than control left atria, whereas both diabetic and control left atria exchanged the same amount of 45Ca2+ during basal contractile conditions. Phenylephrine had no effect on 45Ca2+ efflux from either diabetic or control atria. These results indicate that 3 months of uncontrolled experimental diabetes in the rat produces an enhancement of alpha 1-adrenoceptor activation of isolated atria, and that there is an alteration in Ca2+ mobilization which may contribute to the enhanced receptor activation.     

"Metallothionein-like" metal complexes in angiosperms; their structure and function

Evidence for the presence of the metal-binding protein metallothionein, MT, in higher plants is equivocal. Although a number of MT-like metal complexes have been isolated from plants, the chemical structures of most of these compounds have not been fully elucidated. Recently a novel class of plant peptides, poly (γ-glutamylcysteinyl) glycines, (γEC)_nG, have been discovered. These peptides bind metal ions, and in the presence of such ions the amount of (γEC), G in plant cells increases. The presence of peptide bonds through the γ-carboxyl group of glutamate, rather than the α-carboxyl group, suggests that these peptides are not encoded by structural genes but are the products of biosynthetic pathways. Cells which are resistant to supra-optimal concentrations of certain metal ions over-produce (γEC)_n G. (γEC)_n G. may be functional analogues of MT. Whether or not some plants also produce MT is an important question which remains to be answered.     

Histone segregation on replicating chromatin

We have reinvestigated the mode of segregation of preexisting histones onto replicating chromosomes. Since our previous data have indicated that only histones H3 and H4 do not appear to move from their association with the DNA strand with which they are bound until the next round of replication, we have concentrated our attention on these two histones. The strategy we have employed involved density labeling of DNA and radiolabeling of the histones of interest. Subsequently, we followed the association of histones and DNA during further rounds of DNA replication. One can make predictions concerning the nature of the association between specific histones and particular DNA strands depending on the mode of deposition. The results have confirmed our previous findings that histones segregate randomly. The possibility that such a result is a consequence of turnover of radiolabel in non-histone proteins and subsequent reutilization for histone synthesis has been tested directly. This process appears to be occurring to only a very limited extent. The implications of these conclusions for chromatin structure and gene control are discussed.