Search PubMed⌕ Search

Biomedical subjects

D W Johnson

Publications and source records attributed to D W Johnson.

At least 37 records · Page 2Linked to original sources
PubMed2001

The self-assembly of a [Ga4L6](12-) tetrahedral cluster thermodynamically driven by host-guest interactions.

The guest-induced synthesis of a [Ga4L6](12-) tetrahedral metal-ligand cluster resulting from a predictive design strategy is described. Each of the six dicatecholamide ligands spans an edge of the molecular tetrahedron with four Ga(III) ions at the vertices. Small cationic species not only were found to occupy the large void volume (ca. 300-400 A(3)) inside this cluster but also are necessary thermodynamically to drive cluster assembly via formation of a host-guest complex. NMe4(+), NEt4(+), and NPr4(+) all suit this purpose, and in addition the cluster exhibits a preference in the binding of these three guests: NEt4(+) is bound 300 times more strongly than NPr4(+), which is in turn bound 4 times more strongly than NMe4(+), as determined by 1H NMR spectroscopy. The K6(NEt4)6[Ga4L6] cluster was characterized by NMR spectroscopy, high- (Fourier transform ion cyclotron resonance, FT-ICR) and low-resolution electrospray ionization (ESI) mass spectrometry, elemental analysis, and single-crystal X-ray diffraction. The binding of the NEt4(+) guest molecule was confirmed in the solid state structure, which reveals that the molecule contains large channels in the solid state. As this result exemplifies, it is suggested that guest molecules will play an increasing role in the formation of larger, predesigned metal-ligand clusters.

Journal Article↗
PubMed2001

Rational design and assembly of M(2)M'(3)L(6) supramolecular clusters with C(3h) symmetry by exploiting incommensurate symmetry numbers.

A rational approach to heterometallic cluster formation is described that uses incommensurate symmetry requirements at two different metals to control the stoichiometry of the assembly. Critical to this strategy is the proper design and synthesis of hybrid ligands with coordination sites selective toward each metal. The phosphino-catechol ligand 4-(diphenylphosphino)benzene-1,2-diol (H(2)L) possesses both hard catecholate and soft phosphine donor sites and serves such a role, using soft (C(2)-symmetric) and hard (C(3)-symmetric) metal centers. The ML(3) catecholate complexes (M = Fe(III), Ga(III), Ti(IV), Sn(IV)) have been prepared and characterized as C(3)-symmetry precursors for the stepwise assembly (aufbau) of heterometallic clusters. While the single-crystal X-ray structure of the Cs(2)[TiL(3)] salt shows a C(1) mer-configuration in the solid -state, room-temperature solution NMR data of this and related complexes are consistent with either exclusive formation of the C(3)-fac-isomer with all PPh(2) donor sites syn to each other or facile fac/mer isomerization. Coordination of these [ML(3)](2)(-) (M = Ti(IV), Sn(IV)) metallaligands via their soft P donor sites to C(2)-symmetric PdBr(2) units gives exclusively pentametallic [M(2)Pd(3)Br(6)L(6)](4)(-) (M = Ti, Sn) clusters. These clusters have been fully characterized by spectral and X-ray structural data as C(3h) mesocates with Cs(+) or protonated 1,4-diazabicyclo[2.2.2]octane (DABCO.H(+)) cations incorporated into deep molecular clefts. Exclusive formation of this type of supramolecular species is sensitive to the nature of the counterions. Alkali cations such as K(+), Rb(+), and Cs(+) give high-yield formation of the respective clusters while NEt(3)H(+) and NMe(4)(+) yield none of the desired products. Extension of the aufbau assembly to produce related [M(2)Pd(3)Cl(6)L(6)](4)(-), [M(2)Pd(3)I(6)L(6)](4)(-), and [M(2)Cr(3)(CO)(12)L(6)](4)(-) (M = Ti, Sn) clusters has also been realized. In addition to this aufbau approach, self-assembly of several of these [M(2)Pd(3)Br(6)L(6)](4)(-) clusters from all eleven components (two M(IV), three PdBr(2), six H(2)L) was also accomplished under appropriate reaction conditions.

Journal Article↗
PubMed2001

Recurrences of cytomegalovirus retinitis in a human immunodeficiency virus-infected patient, despite potent antiretroviral therapy and apparent immune reconstitution.

We describe a 42-year-old man with human immunodeficiency virus infection who developed multiple recurrences of cytomegalovirus (CMV) retinitis despite receiving highly active antiretroviral therapy and having apparent immune reconstitution as evidenced by CD4(+) T lymphocyte counts of > 200 cells/mm(3). Laboratory investigation during one recurrence of retinitis confirmed that there was active CMV replication in the plasma and vitreous fluid. In addition, lymphoproliferative responses to CMV antigens were absent despite evidence of reactivity to Candida antigen and pokeweed mitogen. The clinical significance of this case and of other recently reported cases is discussed.

AIDS-Related Opportunistic Infections↗
PubMed2001

Analysis of alcohols, as dimethylglycine esters, by electrospray ionization tandem mass spectrometry.

Dimethylglycine (DMG) esters are new derivatives for the rapid, sensitive and selective analysis of primary and secondary alcohols, in complex mixtures, by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Their development was inspired by the use of the complementary dimethylaminoethyl esters for the trace, rapid analysis of fatty acids. DMG esters are simply prepared by heating a dichloromethane solution of the imidazolide of dimethylglycine, containing triethylamine, and an alcohol. DMG esters of long-chain fatty alcohols, isoprenoidal alcohols and hydroxy-acids are analysed by electrospray ionization tandem mass spectrometry with a precursor ion of m/z 104 scan. Diols, glyceryl esters, glyceryl ethers and some sterols are analysed by a neutral loss of 103 Da scan. Trimethylglycine (TMG) ester iodides, prepared by alkylation of DMG esters with methyl iodide, are more sensitive derivatives for molecules containing secondary alcohol groups, such as cholesterol and gibberellic acid. They are analysed by a precursor ion of m/z 118 scan. DMG or TMG derivatives were shown to be at least comparable and sometimes an order of magnitude more sensitive than N-methylpyridyl ether derivatives for ESI-MS/MS analysis of the different classes of alcohols. Applications of these derivatives for the diagnosis of inherited disorders and the analysis of natural products are presented.

Alcohols↗
PubMed2001

Analysis of amino acids as formamidene butyl esters by electrospray ionization tandem mass spectrometry.

Amino acid formamidene butyl esters are optimally prepared by heating amino acids with dimethylformamide dimethylacetal (DMF-DMA) for 2 minutes at 65 degrees C and then with n-butanol/hydrogen chloride for 15 minutes at 65 degrees C. The formamidene butyl esters of simple alpha-amino acids and beta-amino acids afford between 1.1 and 20 times the signal intensity of the corresponding butyl esters during electrospray ionization tandem mass spectral analysis. Complex alpha-amino acids, such as ornithine, arginine and citrulline, and gamma-amino acids are better analyzed as butyl esters. Secondary alpha-amino acids, such as proline and sarcosine, give a mixture of two iminium salts with the DMF-DMA derivatization method. A derivative incorporating two molecules of the amino acid predominates at higher derivatization temperatures. Contrary to a previous report, N-formylamino acids were not detected. The presence of secondary amino acids in amino acid mixtures, derivatized as formamidene butyl esters, affords derivatives that incorporate two different amino acids. The new formamidene butylation method is unlikely to replace the butylation procedure used by neonatal blood spot screening programs for amino acid disorders, since a much poorer response was obtained with formamidene butyl esters of arginine and citrulline, important in the diagnosis of arginase deficiency and citrullinaemia.

Alkylation↗
PubMed2001

Characterization of urinary sulfatides in metachromatic leukodystrophy using electrospray ionization-tandem mass spectrometry.

Metachromatic leukodystrophy is an inherited disorder characterized by a deficiency of the lysosomal enzyme arylsulfatase A and the subsequent accumulation of sulfatide in neural and visceral tissues. Clinical diagnosis is usually confirmed by in vitro analysis of arylsulfatase A activity, but may be complicated in cases of arylsulfatase A pseudodeficiency and sphingolipid activator protein deficiency. Large quantities of sulfatide can be detected in the urinary sediment of affected individuals and its measurement can aid in diagnosis. A number of complex methods have been described for the measurement of urinary sulfatide excretion. We have developed a rapid, sensitive, and specific mass spectrometric method for determining urinary sulfatide concentration of metachromatic leukodystrophy patients. Sulfatides are extracted from urine and then directly analyzed using electrospray ionization-tandem mass spectrometry. A sulfatide internal standard has been employed for quantification. The assay has demonstrated significant elevations in the concentrations of several hydroxy and nonhydroxy molecular species of sulfatide in the urine of metachromatic leukodystrophy patients compared to age-matched controls. Analysis of urinary sulfatides in arylsulfatase A pseudodeficiency patients showed a mild elevation in some individuals when related to urinary phosphatidylcholine.

Adolescent↗
PubMed2001

The relationship of a clonal outbreak of Enterococcus faecium vanA to methicillin-resistant Staphylococcus aureus incidence in an Australian hospital.

Australian isolates of vancomycin-resistant enterococci (VRE) have been widely scattered geographically, predominantly polyclonal and of the VanB phenotype. Forty-nine VRE were isolated from 47 patients in our hospital from October 1996 to December 1999. Forty-four of these VRE were Enterococcus faecium with a vanA glycopeptide resistance genotype. Four isolates were pathogenic. Thirty-five VRE were from an outbreak in the Renal and Infectious Diseases Units over a four-month period. Pulsed-field gel electrophoresis (PFGE) demonstrated that 41 of the 49 VRE were indistinguishable or closely related. Enhanced environmental cleaning, strict contact isolation of colonized patients and reducing inpatient admissions terminated the epidemic. Cohorting of methicillin-resistant Staphylococcus aureus (MRSA)-positive patients was restricted because VRE patients occupied the isolation facilities. This resulted in a statistically significant increase in MRSA infections across the hospital. VRE epidemics have the ability to influence the epidemiology of other nosocomial pathogens when infection control resources are exhausted.

Australia↗
PubMed2001

A prospective crossover trial comparing intermittent intravenous and continuous oral iron supplements in peritoneal dialysis patients.

BACKGROUND: Concomitant iron supplementation is required in the great majority of erythropoietin (Epo)-treated patients with end-stage renal failure. Intravenous (i.v.) iron supplementation has been demonstrated to be superior to oral iron therapy in Epo-treated haemodialysis patients, but comparative data in iron-replete peritoneal dialysis (PD) patients are lacking. METHODS: A 12-month, prospective, crossover trial comparing oral and i.v. iron supplementation was conducted in all Princess Alexandra Hospital PD patients who were on a stable dose of Epo, had no identifiable cause of impaired haemopoiesis other than uraemia, and had normal iron stores (transferrin saturation >20% and serum ferritin 100-500 mg/l). Patients received daily oral iron supplements (210 mg elemental iron per day) for 4 months followed by intermittent, outpatient i.v. iron infusions (200 mg every 2 months) for 4 months, followed by a further 4 months of oral iron. Haemoglobin levels and body iron stores were measured monthly. RESULTS: Twenty-eight individuals were entered into the study and 16 patients completed 12 months of follow-up. Using repeated-measures analysis of variance, haemoglobin concentrations increased significantly during the i.v. phase (108+/-3 to 114+/-3 g/l) compared with each of the oral phases (109+/-3 to 108+/-3 g/l and 114+/-3 to 107+/-4 g/l, P<0.05). Similar patterns were seen for both percentage transferrin saturation (23.8+/-2.3 to 30.8+/-3.0%, 24.8+/-2.1 to 23.8+/-2.3%, and 30.8+/-3.0 to 26.8+/-2.1%, respectively, P<0.05) and ferritin (385+/-47 to 544+/-103 mg/l, 317+/-46 to 385+/-47 mg/l, 544+/-103 to 463+/-50 mg/l, respectively, P=0.10). No significant changes in Epo dosages were observed throughout the study. I.v. iron supplementation was associated with a much lower incidence of gastrointestinal disturbances (11 vs 46%, P<0.05), but exceeded the cost of oral iron treatment by 6.5-fold. CONCLUSIONS: Two-monthly i.v. iron infusions represent a practical alternative to oral iron and can be safely administered to PD patients in an outpatient setting. Compared with daily oral therapy, 2-monthly i.v. iron supplementation in PD patients was better tolerated and resulted in superior haemoglobin levels and body iron stores.

Administration, Oral↗
PubMed2001

Rapid and quantitative analysis of unconjugated C(27) bile acids in plasma and blood samples by tandem mass spectrometry.

A subgroup of peroxisomal disorders, peroxisome biogenesis defects (PBD), can be differentiated by elevated levels of C(27) bile acids in plasma and bile. Patients with peroxisomal disorders, who lack the ability to chain-shorten the C(27) bile acid intermediates into C(24) bile acids, show elevated levels of C(27) bile acids, notably 3 alpha,7 alpha-dihydroxy-5 beta-cholest-26-oic acid and 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oic acid. C(27) bile acids are normally estimated against other bile acid standards, by time-consuming gas chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry methods, in plasma (minimum of 50 microl). In this article we describe the quantitation of unconjugated di- and trihydroxy C(27) bile acids in 5-microl plasma samples and 3-mm blood spots, using deuterium-labeled internal standards. The synthesis of (2)H(3)-labeled di- and trihydroxycoprostanic acids is described. The sample preparation and analysis by electrospray tandem mass spectrometry (ES-MS/MS) takes less than 1 h and features dimethylaminoethyl ester derivatives. The levels of the di- and trihydroxy bile acids are significantly higher in PBD patients than in age-matched control subjects for both plasma and blood spots collected at birth (some stored for up to 18 years). Excellent correlation is observed between the C(26:0)/C(22:0) very long chain fatty acid (VLCFA) ratio and the levels of trihydroxy C(27) bile acids in plasma from PBD patients. The ES-MS/MS method can be used to rapidly screen for PBD patients in plasma samples with elevated C(26:0)/C(22:0) VLCFA ratios and in archived collections of neonatal blood spots. - Johnson, D. W., H. J. ten Brink, R. C. Schuit, and C. Jakobs. Rapid and quantitative analysis of unconjugated C(27) bile acids in plasma and blood samples by tandem mass spectrometry. J. Lipid Res. 2001. 42: 9;-16.

Age Factors↗
PubMed2001

A rapid screening procedure for cholesterol and dehydrocholesterol by electrospray ionization tandem mass spectrometry.

The mono-(dimethylaminoethyl) succinyl (MDMAES) ester is a new derivative for rapid, mild, and sensitive electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis of cholesterol and dehydrocholesterol. It is an order of magnitude more sensitive than the previous most practical alternative, the N-methylpyridyl ether derivative. The MDMAES derivative was used to develop a rapid screening procedure for the biochemical diagnosis of Smith-Lemli-Opitz syndrome (SLOS) by measuring the dehydrocholesterol/cholesterol ratio in plasma (5 microl) and plasma spotted onto filter paper. Details of the synthesis of [25,26,26,26,27,27,27-(2)H7]-7-dehydrocholesterol, used as a standard for quantitation, are included. The measurement of total sterols as MDMAES esters, after base hydrolysis of plasma, afforded a dehydrocholesterol/cholesterol ratio of 0.05-2.95 for SLOS patient samples (n = 5) compared with 0.001-0.003 for normal adult controls (n = 20). Direct hexane extraction of plasma without base hydrolysis enabled the measurement of free sterols with a total sample analysis time of <1 h. The free dehydrocholesterol/cholesterol ratio was 0.10-4.47 for SLOS patient samples (n = 5) and 0.003-0.011 for normal adult controls (n = 20).

Adult↗
PubMed2001

Oral versus intravenous iron supplementation in peritoneal dialysis patients.

The vast majority of erythropoietin (EPO)-treated peritoneal dialysis (PD) patients require iron supplementation. Most authors and clinical practice guidelines recommend primary oral iron supplementation in PD patients because it is more practical and less expensive. However, numerous studies have clearly demonstrated that oral iron therapy is unable to maintain EPO-treated PD patients in positive iron balance. Once patients become iron-deficient, intravenous iron administration has been found to more effectively augment iron stores and hematologic response than does oral therapy. We recently performed a prospective, cross-over trial in 28 iron-replete PD patients and showed that twice-monthly outpatient iron polymaltose infusions (200 mg) were a practical and safe alternative to oral iron. That treatment produced significant increases in hemoglobin concentration and body iron stores. The additional expense of intravenous iron therapy was completely offset by reductions in EPO dosage. Careful monitoring of iron stores is important in patients receiving intravenous iron supplementation in view of epidemiologic links with infection and cardiovascular disease. Nevertheless, a growing body of evidence suggests that, as has been found for hemodialysis patients, intravenous iron therapy is superior to oral iron supplementation in EPO-treated PD patients.

Administration, Oral↗
PubMed2000

Novel surveillance and cure of a donor-transmitted lymphoma in a renal allograft recipient.

BACKGROUND: In this report we describe a malignant lymphoma of donor origin inadvertently transplanted into two renal allograft recipients, despite standard comprehensive donor screening. The successful clearance of the tumor from both patients and a novel method of surveillance are detailed. METHODS: Initial management consisted of withdrawal of immunosuppression to promote rejection of the allograft and the transplanted tumor in both patients, followed by graft removal. Peripheral blood microchimerism was assessed in both recipients using nested polymerase chain reaction to detect the DYZ3 gene on the Y chromosome (donor male, recipients female). RESULTS: Although microchimerism was detected on day 6 after transplantation and day 1 after explantation, repeat peripheral blood examination at 1, 3, and 6 months after explantation demonstrated no microchimerism. Both patients remain well at 12 months and have been relisted for transplantation. CONCLUSION: Despite inadvertent transplantation of a previously undiagnosed malignancy of donor origin, the recipients' immune response was able to eliminate donor tumor cells after the withdrawal of immunosuppression. Repeated surveillance of peripheral blood from both recipients, using a novel application of the technique of nested polymerase chain reaction to amplify donor DNA, demonstrated no persistence of donor cells, supporting effective eradication of the donor malignancy.

Aged↗
PubMed2000

A comparison of the effects of dialysis and renal transplantation on the survival of older uremic patients.

BACKGROUND: Patients over age 60 constitute half of all new patients accepted into the renal replacement therapy programs in Australia. However, the optimal treatment of their end-stage renal disease remains controversial. The aim of the present study was to compare survival for dialysis and renal transplantation in older patients who were rigorously screened and considered eligible for transplantation. METHODS: The study cohort consisted of 174 consecutive patients over 60 who were accepted on to the Queensland cadaveric renal transplant waiting list between January 1, 1993 and December 31, 1997. Follow-up was terminated on October 1, 1998. Data were analyzed on an intention-to-transplant basis using a Cox regression model with time-varying explanatory variables. An alternative survival analysis was also performed, in which patients no longer considered suitable for transplantation were censored at the time of their removal from the waiting list. RESULTS: There were 67 patients receiving a renal transplant, whereas the other 107 continued to undergo dialysis. These two groups were well matched at baseline with respect to age, gender, body mass index, renal disease etiology, comorbid illnesses, and dialysis duration and modality. The overall mortality rate was 0.096 per patient-year (0.131 for dialysis and 0.029 for transplant, P<0.001). Respective 1-, 3- and 5-year survivals were 92%, 62%, and 27% for the dialysis group and 98%, 95%, and 90% (P<0.01) for the transplant group. Patients in the transplant group had an adjusted hazard ratio 0.16 times that of the dialysis group (95% confidence interval 0.06-0.42). If patients were censored at the time of their withdrawal from the transplant waiting list, the adjusted hazard ratio was 0.24 (95% confidence interval 0.09-0.69). CONCLUSIONS: Renal transplantation seems to confer a substantial survival advantage over dialysis in patients with end-stage renal failure who are rigorously screened and considered suitable for renal transplantation.

Aged↗