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Biomedical subjects

R B Johns

Publications and source records attributed to R B Johns.

At least 19 recordsLinked to original sources
PubMed1995

Synthesis and biological characterisation of a series of iberiotoxin analogues.

We report here the synthesis of iberiotoxin (IbTX), a 37-amino acid peptide containing three disulfide bridges, and a series of mono-looped analogues. All syntheses were conducted using Fmoc chemistry. Synthesis of IbTX gave a product which was indistinguishable from a reference sample in both its physico-chemical properties and its biological activity. A series of three mono-looped analogues, in which four of the six cysteines were replaced by alanine, were synthesised to give [Ala7,13,28,33]-IbTX, [Ala13,17,33,35]-IbTX and [Ala7,17,28,35]-IbTX. Oxidation of the linear form of [Ala7,17,28,35]-IbTX to form the Cys13 to Cys33 disulfide bridge proceeded more slowly than that of the other two analogues. None of these analogues was biologically active, indicating that no single loop is the mediator of channel blocking activity.

Amino Acid Sequence↗
PubMed1994

Tightening characteristics for screwed joints in osseointegrated dental implants.

The significance of tightening abutment screws and gold cylinders to osseointegrated fixtures with the correct torque is demonstrated, and a simple relationship between applied torque and screw preload is derived by use of mechanical engineering principles. The principles of a number of tightening methods are outlined and assessments made of their accuracy. The difference between optimum and design torque is highlighted. The necessity and means of achieving optimum torque to ensure a reliable joint in clinical practice is discussed.

Dental Abutments↗
PubMed1993

The synthesis and use of pp60src-related peptides and phosphopeptides as substrates for enzymatic phosphorylation studies.

A series of peptides and phosphopeptides corresponding to the auto-phosphorylation site of pp60src, -Asn-Glu-Tyr416-Thr-Ala-, were prepared by either Boc/solution or Fmoc/solid phase peptide synthesis and used as substrates to study their enzymatic phosphorylation by various casein kinases. The Tyr(P)-containing peptide, Asn-Glu-Tyr(P)-Thr-Ala, was prepared by the use of Fmoc-Tyr(PO3Bzl2)-OH in Fmoc/solid phase peptide synthesis followed by acidolytic treatment of the peptide-resin with 5% anisole/CF3CO2H. Both Asn-Glu-Tyr-Thr-Ala and Asn-Glu-Ser(P)-Thr-Ala were prepared by the Boc/solution phase peptide synthesis and employed hydrogenolytic deprotection of the protected peptides. Enzymatic phosphorylation studies established that (A) the Tyr residue acted as an unusual positive determinant for directing phosphorylation to the Thr-residue, (B) the rate of Thr-phosphorylation was markedly facilitated by a change from the Tyr-residue to the Tyr(P)-residue, and (C) a Ser(P)-residue was as effective as the Tyr(P)-residue in facilitating Thr-phosphorylation. A subsequent structure-function study using Asn-Glu-Phe-Thr-Ala, Asn-Glu-Tyr(Me)-Thr-Ala (prepared by Fmoc/solid phase peptide synthesis) and Asn-Glu-Cha-Thr-Ala (prepared by hydrogenation of Asn-Glu-Tyr-Thr-Ala) established that the rate of Thr-phosphorylation was influenced by the extent of hydrophobic-hydrophobic interactions by the aralkyl side-chain group (either aromatic or aliphatic) of the 416-residue with casein kinase-2; the rate of Thr-phosphorylation being decreased by the introduction of methyl or hydroxyl groups at the 4-position of the aromatic group (i.e. Tyr(Me) and Tyr respectively) but enhanced by the introduction of the hydrophilic phosphate group (i.e. as Tyr(P)).

Amino Acid Sequence↗
PubMed1992

A multicenter study of overdentures supported by Brånemark implants.

Nine clinical centers participated in a prospective study of overdentures supported by Brånemark implants in the maxilla or mandible. The study initially comprised 133 patients provided with 117 implants placed in the maxilla and 393 implants placed in the mandible. The preliminary results indicate a success rate in the mandible comparable with the reports on fixed prostheses. Conversely, overdenture treatment in the maxilla seems to be less favorable than previous reports of fixed restorations. The differences between the fixed and the present overdentures in the maxilla were mainly based on differences in patient selection and bone quality. A total of 32 implants was mobile and removed and another 29 implants were lost to follow-up because of patient dropout up to the first annual checkup after denture placement. A higher implant failure rate occurred in the maxilla. Mucosal reactions were also more unfavorable around implants placed in the maxilla.

Adult↗
PubMed1992

Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants.

Metal fatigue failure of the gold screw used to retain a fixed prosthesis to Brånemark osseointegrated fixtures/abutments has been analyzed theoretically. Mechanical engineering principles show the importance of appropriate preload being applied through the gold screw to the gold cylinder and abutment. The significance of the screw design and necessity of applying the correct torque to achieve a long fatigue life for the screw are also described. The consequence of misalignment of a gold cylinder to an abutment is discussed.

Dental Abutments↗
PubMed1990

Phosphorylation of src-phosphopeptides by casein kinases-1 and -2: favourable effect of phosphotyrosine.

The synthetic phosphotyrosyl tridecapeptide H-Arg-Leu-Ile-Glu-Asp-Asn-Glu-Tyr(P)-Thr-Ala-Arg-Gln-Gly-OH, reproducing a major phosphoacceptor site of protein tyrosine kinases of the src-family, can be phosphorylated at Thr-9 by both casein kinases -1 and -2. Its shorter derivative H-Asn-Glu-Tyr(P)-Thr-Ala-OH is not affected by casein kinase-1 while representing a substrate as good as the tridecapeptide for casein kinase-2. The unphosphorylated analogue H-Asn-Glu-Tyr-Thr-Ala-OH, however, is a much poorer substrate, and no significant phosphorylation could be observed of its O-methyl ether derivative H-Asn-Glu-Tyr(Me)-Thr-Ala-OMe. These data on one side corroborate the concept that casein kinase-1 recognizes residues located on the C-terminal edge of acidic stretches, providing, on the other, the evidence that phosphotyrosyl side chains can act as specificity determinants for casein kinase-2.

Amino Acid Sequence↗
PubMed1989

Synthetic fragments of beta-casein as model substrates for liver and mammary gland casein kinases.

The octapeptide Glu-Ser-Leu-Ser-Ser-Ser-Glu-Glu, corresponding to the 14-21 sequence of bovine beta-casein A2 and 11 shorter and/or modified derivatives were synthesized and used as model substrates for three casein kinases: rat liver casein kinases 2 and 1 and a casein kinase isolated from the golgi-enriched fraction of lactating mammary gland (GEF-casein kinase). Casein kinase-2 readily phosphorylates the octapeptide at its Ser-4 residue with a Vmax value comparable to those obtained with protein substrates and Km values of 85 microM and 11 microM in the absence and presence of polylysine, respectively. These are the most favourable kinetic parameters reported so far with peptide substrates of casein kinase-2. Stepwise shortening of the octapeptide from its N terminus promotes both a gradual decrease of Vmax and an increase of Km, this being especially dramatic in passing from the hexapeptide Leu-Ser-Ser-Ser-Glu-Glu (Km 210 microM) to the pentapeptide Ser-Ser-Ser-Glu-Glu (Km 2630 microM). The tetrapeptide Ser-Ser-Glu-Glu is the shortest derivative still phosphorylated by casein kinase-2, albeit very slowly, and the tripeptides Ser-Glu-Glu and Glu-Leu-Ser were not substrates at all. Furthermore, the pentapeptide Ser-Ser-Ser-Glu-Glu was found to be a better substrate than Ser-Ser-Ala-Glu-Glu, Ser-Ala-Ser-Glu-Glu and Ser-Ala-Ala-Glu-Glu by virtue of its lower Km value. These data, while confirming that the motif Ser-Xaa-Xaa-Glu is specifically recognized by casein kinase-2, strongly suggest that additional local structural features can improve the phosphorylation efficiency of serine-containing peptides which are devoid of the large acidic clusters recurrent in many phosphorylation sites of casein kinase 2. In particular, predictive structural analysis as well as NMR and C18 reverse-phase HPLC elution profile data support the hypothesis that a beta-turn conformation is responsible for the remarkable suitability of the octapeptide Glu-Ser-Leu-Ser-Ser-Ser-Glu-Glu and some of its shorter derivatives to phosphorylation mediated by casein kinase-2. While neither the peptide Glu-Ser-Leu-Ser-Ser-Ser-Glu-Glu nor any of its derivatives were affected by casein kinase-1, a rapid phosphorylation of the octapeptide by GEF-casein kinase at Ser-5 (not Ser-4) was obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1988

Partially dephosphorylated phosphopeptide AcSer(P)-Ser(P)-Ser(P) is an excellent substrate for casein kinase-2.

The synthetic phosphopeptide AcSer(P)-Ser(P)-Ser(P), reproducing a recurrent feature of casein and other phosphoproteins, once partially dephosphorylated by acid phosphatase, serves as an efficient substrate for casein kinase-2. Previous dephosphorylation beyond 30% hinders subsequent phosphorylation and the entirely dephosphorylated peptide is not a substrate at all. The kinetic constants of the partially dephosphorylated phosphopeptide are much more favourable than those of the synthetic peptides SEEEAA, SSEE and SEE, the latter one being totally inert. Optimal phosphorylation occurs at pH values that ensure complete ionization of the phosphoseryl side chains. These data provide incontrovertible demonstration that phosphoserine can replace carboxylic amino acids as specificity determinant for CK-2, being more effective than glutamic acid itself.

Amino Acid Sequence↗
PubMed1985

Inhibition of intestinal citrulline synthesis causes severe growth retardation in rats.

delta-N-(phosphonacetyl)-L-ornithine (PALO) is a powerful and specific inhibitor of ornithine transcarbamylase (1), but it does not readily enter intact cells and therefore does not inhibit citrulline synthesis in intact liver or intestine. We have used the glycylglycine derivative of PALO (Gly-Gly-PALO) to evaluate the importance of intestinal citrulline synthesis in supplying arginine for growth. We have shown that the peptide derivative of PALO is selectively taken up by gut cells via the peptide permease and is released intracellularly as the free inhibitor. When administered in drinking water to 6-wk-old rat pups on an arginine-deficient diet, serum citrulline was reduced from 85 +/- 4.2 to 44 +/- 2.2 microM and arginine from 240.1 +/- 19.0 to 52.1 +/- 4.1 microM. Ornithine increased from 100 +/- 6.2 to 273.5 +/- 21.3 microM. Addition of 0.1 mM Gly-Gly-PALO to drinking water caused a rapid and complete inhibition of growth in rats on arginine-deficient diets, and this growth inhibition could be partially prevented by simultaneous administration of 1% (wt/wt) arginine to the diet and completely prevented with 1% (wt/wt) citrulline. The specificity of the effects of Gly-Gly-PALO on intestinal citrulline synthesis was shown by the inability of the drug to be taken up or to inhibit citrulline synthesis in isolated rat hepatocytes, and the oral administration of the drug had no effect on serum ammonia concentrations. The relative importance of endogenous synthesis of arginine compared with dietary arginine for growth was shown by the ability of Gly-Gly-PALO to inhibit the growth of rats maintained on standard laboratory chow containing normal levels of arginine.

Animals↗
PubMed1984

Using commercial application' software. Integrating clinic office practices.

Automation of clinic business office practices has often been accomplished by the use of the procedures obtained from the provider of automated services. With the availability of the microcomputer and the commercially available business application software, the desired automated procedures may be obtained by discretionary use of the products. By describing the integration and the operational use of business application software within a multiphysician specialty clinic, the author highlights both the problems encountered and the benefits achieved.

Ambulatory Care Facilities↗
PubMed1983

Normal-phase HPLC analysis of microbial carotenoids and neutral lipids.

A facile, reproducible HPLC procedure for the analysis of algal and bacterial carotenoids has been developed. By selection of appropriate detection wavelengths, it is possible to determine wavelength ratios that typify selected carotenoids and permit rapid distinction between coeluting pigments with different numbers of conjugated double bonds. The concurrent isolation of individual neutral lipid classes with the carotenoid analysis is also demonstrated.

Bacteria↗
PubMed1983

Phytol degradation by marine bacteria.

Microbial degradation of phytol is often proposed to be the primary source of the acyclic isoprenoid acids observed in sediments, yet only a limited number of these acids have been found in bacterial cultures grown on phytol. This study reports detailed capillary gas chromatography and gas chromatography-mass spectrometry analyses of the products resulting from growth of marine bacteria on phytol as the sole carbon source. We examined two strains of bacteria which were able to oxidize phytol to phytenic acid but were unable to further degrade phytol. The third isolate studied converted phytol to a mixture of five saturated isoprenoid acids. The C(17) isoprenoid acid produced was of particular interest, since its genesis from phytol would have involved several unusual intermediates. It is suggested that this acid is produced by bacterial metabolism of the C(18) isoprenoid ketone (produced from phytol abiologically under oxic conditions) and that its abundance is thus a sensitive indicator of sedimentary depositional conditions.

Journal Article↗
PubMed1981

trans-Monounsaturated Acids in a Marine Bacterial Isolate.

A sedimentary bacterial isolate has been shown to contain trans-monounsaturated fatty acids (6% of the total fatty acids). The ratio of trans- to cis-acids in this isolate was in the range 3.2 to 7.6. The identification of trans-monounsaturated acids in a marine bacterium implied that the trans-acids which have been reported in recent sediments could derive, in whole or part, from direct bacterial input.

Journal Article↗
PubMed1980

Mercury levels in dental students and faculty measured by neutron activation analysis.

Samples of head hair, fingernails, and toenails were taken from 61 dental students and faculty members. The mercury levels were analyzed using a simple neutron activation analysis technique which proved to be easily applicable to large numbers. Significant results were found for the head hair and fingernail groups. There may be a correlation between the higher mercury levels and increased pressure of work, which might result in decreased standards of mercury hygiene.

Faculty, Dental↗
PubMed1977

Cross-linking of amino acids by formaldehyde. Preparation and 13C NMR spectra of model compounds.

Model cross-linked systems have been prepared by reacting amino acids or alkylamines with formaldehyde and various amino acid model compounds such as 2,4-dimethylphenol (tyrosine), 3-methylindole (tryptophan) and alkylamides (glutamine, asparagine. 13C NMR spectra of the products show the resonances of the formaldehyde-derived methylene carbons in the region 45-60 ppm. Interferences occur from resonances of the alpha-amino acid methine carbons. From the data for these products and other model compounds it has been possible to predict the shifts of the residual methylene carbons in a variety of cross-linked systems. This NMR technique shows promise as a rapid non-degradative method for identification of cross-linking sites.

Amino Acids↗