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Biomedical subjects

S A White

Publications and source records attributed to S A White.

At least 19 recordsLinked to original sources
PubMed2001

A case-control comparison of the results of renal transplantation from heart-beating and non-heart-beating donors.

INTRODUCTION: The decline in heart-beating brainstem dead organ donors has necessitated the search for other organ sources. In the field of renal transplantation one alternative source currently available, but little used, is that of kidneys from non-heart-beating donors (NHBD). Reticence to use NHBD kidneys is in part due to concerns over the effect that warm ischemic may have subsequent graft function. Presented here are the results of the NHBD renal transplants at the Leicester transplant unit, and compared with matched heart-beating donor transplants as a case control analysis. METHODS: In order to analyze any differences in graft performance between the two organ sources, the confounding effect of other variables known to influence the outcome of renal transplantation was minimized by matching NHBD and HBD transplants for the following criteria: donor age and sex, first or re-transplant, anastomosis and cold times, tissue match and PRA sensitisation. Transplant performance was assessed primarily by graft survival, the statistical evaluation of which was by log rank analysis of Kaplan-Meier curves. RESULTS: 72 NHBD and 192 HBD transplants were performed over an eight year period. Of the 192 HBD transplants, 105 matched one or more of the NHBD by the criteria outlined above, and thus constituted the control group for comparison. There was no significant difference in overall graft survival between the two groups. The 5 year survival for the NHBD was 73% compared with 65% for HBD kidneys. When death with a functioning graft is treated as censored data, then these figures become 75% and 81% respectively, again without statistical significance. CONCLUSION: NHBD kidneys are a valuable additional source of organs for transplantation, with long-term survival, comparable to transplants from HBD.

Acute Disease↗
PubMed2001

The heterotrimer of the membrane-peripheral components of transhydrogenase and the alternating-site mechanism of proton translocation.

Transhydrogenase undergoes conformational changes to couple the redox reaction between NAD(H) and NADP(H) to proton translocation across a membrane. The protein comprises three components: dI, which binds NAD(H); dIII, which binds NADP(H); and dII, which spans the membrane. Experiments using isothermal titration calorimetry, analytical ultracentrifugation, and small angle x-ray scattering show that, as in the crystalline state, a mixture of recombinant dI and dIII from Rhodospirillum rubrum transhydrogenase readily forms a dI(2)dIII(1) heterotrimer in solution, but we could find no evidence for the formation of a dI(2)dIII(2) tetramer using these techniques. The asymmetry of the complex suggests that there is an alternation of conformations at the nucleotide-binding sites during proton translocation by the complete enzyme. The characteristics of nucleotide interaction with the isolated dI and dIII components and with the dI(2)dIII(1) heterotrimer were investigated. (a) The rate of release of NADP(+) from dIII was decreased 5-fold when the component was incorporated into the heterotrimer. (b) The binding affinity of one of the two nucleotide-binding sites for NADH on the dI dimer was decreased about 17-fold in the dI(2)dIII(1) complex; the other binding site was unaffected. These observations lend strong support to the alternating-site mechanism.

Binding Sites↗
PubMed2001

The crystal structure of an asymmetric complex of the two nucleotide binding components of proton-translocating transhydrogenase.

BACKGROUND: Membrane-bound ion translocators have important functions in biology, but their mechanisms of action are often poorly understood. Transhydrogenase, found in animal mitochondria and bacteria, links the redox reaction between NAD(H) and NADP(H) to proton translocation across a membrane. Linkage is achieved through changes in protein conformation at the nucleotide binding sites. The redox reaction takes place between two protein components located on the membrane surface: dI, which binds NAD(H), and dIII, which binds NADP(H). A third component, dII, provides a proton channel through the membrane. Intact membrane-located transhydrogenase is probably a dimer (two copies each of dI, dII, and dIII). RESULTS: We have solved the high-resolution crystal structure of a dI:dIII complex of transhydrogenase from Rhodospirillum rubrum-the first from a transhydrogenase of any species. It is a heterotrimer, having two polypeptides of dI and one of dIII. The dI polypeptides fold into a dimer. The loop on dIII, which binds the nicotinamide ring of NADP(H), is inserted into the NAD(H) binding cleft of one of the dI polypeptides. The cleft of the other dI is not occupied by a corresponding dIII component. CONCLUSIONS: The redox step in the transhydrogenase reaction is readily visualized; the NC4 atoms of the nicotinamide rings of the bound nucleotides are brought together to facilitate direct hydride transfer with A-B stereochemistry. The asymmetry of the dI:dIII complex suggests that in the intact enzyme there is an alternation of conformation at the catalytic sites associated with changes in nucleotide binding during proton translocation.

Amino Acid Sequence↗
PubMed2001

The unusual transhydrogenase of Entamoeba histolytica.

We have expressed and purified a protein fragment from Entamoeba histolytica. It catalyses transhydrogenation between analogues of NAD(H) and NADP(H). The characteristics of this reaction resemble those of the reaction catalysed by a complex of the NAD(H)- and NADP(H)-binding subunits of proton-translocating transhydrogenases from bacteria and mammals. It is concluded that the complete En. histolytica protein, which, along with similar proteins from other protozoan parasites, has an unusual subunit organisation, is also a proton-translocating transhydrogenase. The function of the transhydrogenase, thought to be located in organelles which do not have the enzymes of oxidative phosphorylation, is not clear.

Animals↗
PubMed2001

Human islet cell transplantation--future prospects.

BACKGROUND: Islet transplantation has the potential to cure diabetes mellitus. Nevertheless despite successful reversal of diabetes in many small animal models, the clinical situation has been far more challenging. The aim of this review is to discuss why insulin-independence after islet allotransplantation has been so difficult to achieve. METHODS: A literature review was undertaken using Medline from 1975 to July 2000. Results reported to the International Islet Transplant Registry (ITR) up to December 1998 were also analysed. RESULTS: Up to December 1998, 405 islet allotransplants have been reported the ITR. Of those accurately documented between 1990 and 1998 (n = 267) only 12% have achieved insulin-independence (greater than 7 days). However with refined peri-transplant protocols insulin independence at 1 year can reach 20%. CONCLUSIONS: There are many factors which can explain the failure of achieving insulin-independence after islet allotransplantation. These include the use of diabetogenic immunosuppressive agents to abrogate both islet allo-immunity and auto-immunity, the critical islet mass to achieve insulin-independence and the detrimental effects of transplanting islets in an ectopic site. However recent evidence most notably from the Edmonton group demonstrates that islet allotransplantation still has great potential to become an established treatment option for diabetic patients.

Animals↗
PubMed2001

Pancreas resection and islet autotransplantation for end-stage chronic pancreatitis.

OBJECTIVE: To assess the safety and efficacy of islet autotransplantation (IAT) combined with total pancreatectomy (TP) to prevent diabetes. SUMMARY BACKGROUND DATA: There have been recent concerns regarding the safety of TP and IAT. This is thought to be related to the infusion of large volumes of unpurified pancreatic digest into the portal vein. Minimizing the volume of islet tissue by purifying the pancreatic digest has not been previously evaluated in terms of the postoperative rate of death and complications, pain relief, and insulin independence. METHOD: During a 54-month period, 24 patients underwent pancreas resection with IAT. Islets were isolated using collagenase and a semiautomated method of pancreas digestion. Where possible, islets were purified on a density gradient and COBE processor. Islets were embolized into the portal vein, within the spleen and portal vein, or within the spleen alone. The total median volume of digest was 9.9 mL. RESULTS: The median number of islets transplanted was 140,419 international islet equivalents per kilogram. The median increase in portal pressure was 8 mmHg. Early complications included duodenal ischemia, a wedge splenic infarct, partial portal vein thrombosis, and splenic vein thrombosis. Intraabdominal adhesions were the main source of long-term problems. Eight patients developed transient insulin independence. Three patients were insulin-independent as of this writing. Patients had significantly decreased insulin requirements and glycosylated hemoglobin levels compared with patients undergoing TP alone. Of the patients alive and well as of this writing, four had failed to gain relief of their abdominal pain and were still opiate-dependent. CONCLUSION: Combined TP and IAT can be a safe surgical procedure. Unfortunately, almost all patients were still insulin-dependent, but they had decreased daily insulin requirements and glycosylated hemoglobin levels compared with patients undergoing TP alone. A prospective randomized study is therefore needed to assess the long-term benefit of TP and IAT on diabetic complications.

Adult↗
PubMed2000

Rapid determination of gemifloxacin in human plasma by high-performance liquid chromatography-tandem mass spectrometry.

A method was developed for the determination of gemifloxacin (I) in human plasma using high-performance liquid chromatography-tandem mass spectrometry. Prior to analysis, the protein in plasma samples was precipitated with acetonitrile containing [13C2H3] gemifloxacin (II) to act as an internal standard. The supernatant was injected onto a PLRP-S column without any further clean-up. The mass spectrometer was operated in positive ion mode, employing a heat assisted nebulisation. electrospray interface. Ions were detected in multiple reaction monitoring (MRM) mode. The assay requires 50 microl of plasma and is precise and accurate within the range 10-5,000 ng/ml. The average within-run and between-run coefficients of variation were <11% at 10 ng/ml and greater concentrations. The average accuracy of validation standards was generally within +/-7% of the nominal concentration. There was no evidence of instability of I in human plasma following three complete freeze-thaw cycles and samples can safely be stored for at least 6 months at -20 degrees C. The method proved very robust and was successfully applied to the analysis of clinical samples from patients dosed with gemifloxacin.

Anti-Infective Agents↗
PubMed2000

Protein-protein recognition, hydride transfer and proton pumping in the transhydrogenase complex.

BACKGROUND: Membrane-bound ion pumps are involved in metabolic regulation, osmoregulation, cell signalling, nerve transmission and energy transduction. How the ion electrochemical gradient interacts with the scalar chemistry and how the catalytic machinery is gated to ensure high coupling efficiency are fundamental to the mechanism of action of such pumps. Transhydrogenase is a conformationally coupled proton pump linking a proton gradient to the redox reaction between NAD(H) and NADP(H). The enzyme has three components; dI binds NAD(H), dII spans the membrane and dIII binds NADP(H). RESULTS: The first crystal structure of a transhydrogenase dI component (from Rhodospirillum rubrum) has been determined at 2.0 A resolution. The monomer comprises two domains. Both are involved in dimer formation, and one has a Rossmann fold that binds NAD+ in a novel mode. The two domains can adopt different conformations. In the most closed conformation, the nicotinamide ring is expelled from the cleft between the two domains and is exposed on the outside of the protein. In this conformation it is possible to dock the structure of dI/NAD+ with that of a dIII/NADP+ complex to provide the first insights into the molecular basis of the hydride-transfer step. CONCLUSIONS: Analysis of the model of the dI/dIII complex identifies residues potentially involved in dI/dIII interaction and shows how domain motion in dI results in a shift in position of the nicotinamide ring of NAD+. We propose that this movement is responsible for switching between the forbidden and allowed states for hydride transfer during proton pumping.

Molecular Sequence Data↗
PubMed2000

The high-resolution structure of the NADP(H)-binding component (dIII) of proton-translocating transhydrogenase from human heart mitochondria.

BACKGROUND: Transhydrogenase, located in the inner membranes of animal mitochondria and the cytoplasmic membranes of bacteria, couples the transfer of reducing equivalents between NAD(H) and NADP(H) to proton pumping. The protein comprises three subunits termed dI, dII and dIII. The dII component spans the membrane. The dI component, which contains the binding site for NAD(+)/NADH, and the dIII component, which has the binding site for NADP(+)/NADPH, protrude from the membrane. Proton pumping is probably coupled to changes in the binding affinities of dIII for NADP(+) and NADPH. RESULTS: The first X-ray structure of the NADP(H)-binding component, dIII, of human heart transhydrogenase is described here at 2.0 A resolution. It comprises a single domain resembling the classical Rossmann fold, but NADP(+) binds to dIII with a reversed orientation. The first betaalphabetaalphabeta motif of dIII contains a Gly-X-Gly-X-X-Ala/Val 'fingerprint', but it has a different function to that in the classical Rossmann structure. The nicotinamide ring of NADP(+) is located on a ridge where it is exposed to interaction with NADH on the dI subunit. Two distinctive features of the dIII structure are helix D/loop D, which projects from the beta sheet, and loop E, which forms a 'lid' over the bound NADP(+). CONCLUSIONS: Helix D/loop D interacts with the bound nucleotide and loop E, and probably interacts with the membrane-spanning dII. Changes in ionisation and conformation in helix D/loop D, resulting from proton translocation through dII, are thought to be responsible for the changes in affinity of dIII for NADP(+) and NADPH that drive the reaction.

Amino Acid Sequence↗
PubMed2000

Sub-clinical acute rejection detected using protocol biopsies in patients with delayed graft function.

Acute rejection in renal transplants is difficult to diagnose when patients have delayed graft function (DGF) in the early post-transplant period. In this study protocol, renal transplant biopsies were performed in an attempt to detect sub-clinical acute rejection episodes. Eighty-three patients were eligible for the study, of whom 33 had DGE All had protocol renal transplant biopsies performed under ultrasound control at 7 days post-transplant, and those with DGF had further biopsies weekly until the graft functioned. All histologically confirmed acute rejection episodes were treated. Sub-clinical acute rejection was detected in 6/33 (18%) patients with DGF compared to 2/50 (4%) in the other patients (P < 0.05). Borderline rejection was present in 4/33 (12%) and 4/50 (8%) patients, respectively. Because of the high detection rate of sub-clinical acute rejection and the low morbidity of renal transplant biopsies, their use is recommended in patients with DGF.

Acute Disease↗
PubMed2000

The feasibility of spleen-preserving pancreatectomy for end-stage chronic pancreatitis.

BACKGROUND: It is considered difficult to preserve the spleen at the time of distal or total pancreas resection for chronic pancreatitis (CP). The aim of this study was to assess the feasibility of preserving the spleen in patients requiring total or completion pancreatectomy for CP. METHODS: All patients having total or completion pancreatectomy for CP were evaluated postoperatively in terms of morbidity, mortality, and pain relief. To assess splenic vascularity, all patients underwent abdominal ultrasound and power doppler imaging to assess splenic perfusion and the patency of the remaining splenic vessels. RESULTS: Of 35 patients having total pancreatectomy, the spleen was preserved in 30 patients (19 women, 11 men; median age 40 years). The etiology of CP was mainly idiopathic (n = 14) or alcohol related (n = 12). All patients presented with chronic abdominal pain (median 5 years) requiring opiate-derived analgesia for pain relief. Fifteen patients (50%) had undergone previous therapeutic intervention for pain relief. The spleen was preserved with either an intact splenic artery and vein in 19 patients and or the short gastric vessels (n = 11). The mean duration of the procedure was 7 hours (range 5 to 11) and mean blood loss was 1,090 mL. The 30-day mortality was 3.8% (n = 1). Five patients had splenic complications (17%). These included splenectomy (n = 2), intrasplenic collection (n = 2), and a wedge splenic infarct (n = 1). Two of these complications were related to intrasplenic islet autotransplants. Follow-up with abdominal ultrasound and power doppler scanning showed no other abnormalities; blood flow was demonstrable in all patients with intact splenic arteries and vein (n = 19). The mean hospital stay was 25 days. Of the 24 patients who were beyond 6 months' follow-up, 82% (n = 20) have complete relief of pain, and 4 still require opiate analgesia. CONCLUSIONS: Spleen-preserving pancreatectomy is a feasible procedure for chronic pancreatitis, providing complete pain relief in 80% of patients. When the splenic artery and vein cannot be preserved, there is a minimal risk of splenic complications that may require further treatment; but for the majority of patients, splenectomy is avoided.

Adult↗