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Biomedical subjects

R H White

Publications and source records attributed to R H White.

At least 19 recordsLinked to original sources
PubMed2001

Biosynthesis of the phosphodiester bond in coenzyme F(420) in the methanoarchaea.

The biochemical route for the formation of the phosphodiester bond in coenzyme F(420), one of the methanogenic coenzymes, has been established in the methanoarchaea Methanosarcina thermophila and Methanococcus jannaschii. The first step in the formation of this portion of the F(420) structure is the GTP-dependent phosphorylation of L-lactate to 2-phospho-L-lactate and GDP. The 2-phospho-L-lactate represents a new natural product that was chemically identified in Methanobacterium thermoautotrophicum, M. thermophila, and Mc. jannaschii. Incubation of cell extracts of both M. thermophila and Mc. jannaschii with [hydroxy-(18)O, carboxyl-(18)O(2)]lactate and GTP produced 2-phospho-L-lactate with the same (18)O distribution as found in both the starting lactate and the lactate recovered from the incubation. These results indicate that the carboxyl oxygens are not involved in the phosphorylation reaction. Incubation of Sephadex G-25 purified cell extracts of M. thermophila or Mc. jannaschii with 7,8-didemethyl-8-hydroxy-5-deazariboflavin (Fo), 2-phospho-L-lactate, and GTP or ATP lead to the formation of F(420)-0 (F(420) with no glutamic acids). This transformation was shown to involve two steps: (i) the GTP- or ATP-dependent activation of 2-phospho-L-lactate to either lactyl(2)diphospho-(5')guanosine (LPPG) or lactyl(2)diphospho-(5')adenosine (LPPA) and (ii) the reaction of the resulting LPPG or LPPA with Fo to form F(420)-0 with release of GMP or AMP. Attempts to identify LPPG or LPPA intermediates by incubation of cell extracts with L-[U-(14)C]lactate, [U-(14)C]2-phospho-L-lactate, or [8-(3)H]GTP were not successful owing to the instability of these compounds toward hydrolysis. Synthetically prepared LPPG and LPPA had half-lives of 10 min at 50 degrees C (at pH 7.0) and decomposed into GMP or AMP and 2-phospho-L-lactate via cyclic 2-phospho-L-lactate. No evidence for the functioning of the cyclic 2-phospho-L-lactate in the in vitro biosynthesis could be demonstrated. Incubation of cell extracts of M. thermophila or Mc. jannaschii with either LPPG or LPPA and Fo generated F(420)-0. In summary, this study demonstrates that the formation of the phosphodiester bond in coenzyme F(420) follows a reaction scheme like that found in one of the steps of the DNA ligase reaction and in the biosynthesis of coenzyme B(12) and phospholipids.

Adenosine Triphosphate↗
PubMed2001

The first examples of (S)-2-hydroxyacid dehydrogenases catalyzing the transfer of the pro-4S hydrogen of NADH are found in the archaea.

Reduction of 2-oxoacids to the corresponding (S)-2-hydroxyacids is an important transformation in biochemistry. To date all (S)-2-hydroxyacid dehydrogenases belonging to the L-lactate/L-malate dehydrogenase family have been found to transfer the pro-4R hydrogen of either NADH or NADPH to C-2 of the 2-oxoacid substrates during their reduction. Here, we report that recombinantly generated (S)-2-hydroxyacid dehydrogenases present in the methanoarchaea Methanococcus jannaschii and Methanothermus fervidus use the pro-4S hydrogen of NADH to reduce a series of 2-oxoacids to the corresponding (S)-2-hydroxyacids. This information as well as the low sequence identity between these archaeal enzymes and the L-lactate/L-malate family of enzymes indicate that these enzymes are not evolutionary related and therefore constitute a new class of (S)-2-hydroxyacid dehydrogenases.

Alcohol Oxidoreductases↗
PubMed2001

Warfarin therapy for an octogenarian who has atrial fibrillation.

In North America, atrial fibrillation is associated with at least 75 000 ischemic strokes each year. Most of these strokes occur in patients older than 75 years of age. The high incidence of stroke in very elderly persons reflects the increasing prevalence of atrial fibrillation that occurs with advanced age, the high incidence of stroke in elderly patients, and the failure of physicians to prescribe antithrombotic therapy in most of these patients. This failure is related to the increased risk for major hemorrhage with advanced age, obfuscating the decision to institute stroke prophylaxis with antithrombotic therapy. This case-based review describes the risk and benefits of prescribing antithrombotic therapy for a hypothetical 80-year-old man who has atrial fibrillation and hypertension, and it offers practical advice on managing warfarin therapy. After concluding that the benefits of warfarin outweigh its risks in this patient, we describe how to initiate warfarin therapy cautiously and how to monitor and dose the drug. We then review five recent randomized, controlled trials that document the increased risk for stroke when an international normalized ratio (INR) of less than 2.0 is targeted among patients with atrial fibrillation. Next, we make the case that cardioversion is not needed for this asymptomatic patient with chronic atrial fibrillation. Instead, we choose to leave the patient in atrial fibrillation and to control his ventricular rate with atenolol. Later, when the INR increases to 4.9, we advocate withholding one dose of warfarin and repeating the INR test. Finally, when the patient develops dental pain, we review the analgesic agents that are safe to take with warfarin and explain why warfarin therapy does not have to be interrupted during a subsequent dental extraction.

Aged↗
PubMed2001

Biosynthesis of the methanogenic cofactors.

Our current knowledge of the pathways and genes involved in the biosynthesis of the methanogenic coenzymes methanopterin, coenzyme B, methanofuran, coenzyme F420, and coenzyme M is presented. Proposed reaction mechanisms for several of the novel reactions involved in the pathways are presented.

Coenzymes↗
PubMed2001

Identification of coenzyme M biosynthetic 2-phosphosulfolactate phosphatase. A member of a new class of Mg(2+)-dependent acid phosphatases.

Coenzyme M (CoM; 2-mercaptoethanesulfonic acid) is the terminal methyl carrier in methanogenesis. Methanogenic archaea begin the production of this essential cofactor by sulfonating phosphoenolpyruvate to form 2-phospho-3-sulfolactate. After dephosphorylation, this precursor is oxidized, decarboxylated and then reductively thiolated to form CoM. A thermostable phosphosulfolactate phosphohydrolase (EC 3.1.3.-) catalyzing the second step in CoM biosynthesis, was identified in the hyperthermophilic euryarchaeon Methanococcus jannaschii. The predicted ORF MJ1140 in the genome of M. jannaschii encodes ComB, a Mg2+-dependent acid phosphatase that is specific for 2-hydroxycarboxylic acid phosphate esters. Recombinantly expressed purified ComB efficiently hydrolyzes rac-2-phosphosulfolactate, (S)-2-phospholactate, phosphoglycolate and both enantiomers of 2-phosphomalate. In contrast to previously studied phosphoglycolate phosphatases, ComB has a low pH optimum for activity, a narrow substrate specificity and an amino acid sequence dissimilar to any biochemically characterized protein. Like other phosphatases that function via covalent phosphoenzyme intermediates, ComB can catalyze a transphosphorylation reaction. Homologs of comB are identified in all available cyanobacterial genome sequences and in genomes from phylogenetically diverse bacteria and archaea; most of these organisms lack homologs of other CoM biosynthetic genes. The broad and disparate distribution of comB homologs suggests that the gene has been recruited frequently into new metabolic pathways.

Acid Phosphatase↗
PubMed2001

Anticoagulation in the elderly.

Oral anticoagulation therapy has demonstrated benefit in the treatment and prevention of a variety of thromboembolic disorders. Most individuals who receive oral anticoagulant therapy are elderly patients with nonvalvular atrial fibrillation and acute or recurrent venous thromboembolism. Anticoagulation in elderly patients poses unique challenges for the practicing clinician because they are simultaneously at higher risk for recurrent thromboembolism and major bleeding, including catastrophic intracranial hemorrhage. The pharmacology of warfarin in the elderly is reviewed, including important drug interactions and current dosing recommendations for elderly patients. Evidence of the benefits and risks of oral anticoagulation therapy are reviewed for patients with atrial fibrillation and venous thromboembolism. This information should enable practitioners to better assess the relative risks and benefit of oral anticoagulation therapy to guide treatment decisions in the elderly.

Age Factors↗
PubMed2001

Methanococcus jannaschii generates L-proline by cyclization of L-ornithine.

Cell extracts of Methanococcus jannaschii have been shown to readily convert L-ornithine to L-proline. This cyclization reaction proceeds with the loss of only the C-2 nitrogen, as has been documented for ornithine cyclodeaminase (EC 4.3.1.12). Since no gene homologous to that coding for ornithine cyclodeaminase is present in the genome of M. jannaschii, these results indicate that proline biosynthesis in M. jannaschii is accomplished by a previously unrecognized enzyme.

Ammonia-Lyases↗
PubMed2001

Anticoagulation therapy.

Despite refinements and standardization in the use of anticoagulants, many problems remain for clinicians. Dr. Crowther describes appropriate starting and maintenance doses of warfarin, factors accounting for inter- and intra-observer variability and importantly, the management of the over-anticoagulated patients and bleeding patients. Dr. White compares unfractionated heparin (UFH) and low molecular weight heparin (LMWH) and addresses whether there truly are differences in the efficacy and safety of different LMWH's for both arterial and venous indications. Dr. Ortel discusses the management of the problem patient who requires anticoagulants, the management of heparin-induced thrombocytopenia, the pregnant patient, the obese patient, patients who have renal insufficiency and/or liver disease, patients with malignant disease, and other challenging patient populations.

Anticoagulants↗
PubMed2000

Predictors of rehospitalization for symptomatic venous thromboembolism after total hip arthroplasty.

BACKGROUND: Recent studies have shown that symptomatic venous thromboembolism after total hip arthroplasty most commonly develops after the patient is discharged from the hospital. Risk factors associated with these symptomatic thromboembolic events are not well defined. METHODS: Using administrative data from the California Medicare records for 1993 through 1996, we identified 297 patients 65 years of age or older who were rehospitalized for thromboembolism within three months after total hip arthroplasty. We compared demographic, surgical, and medical variables potentially associated with the development of thromboembolism in these patients and 592 unmatched controls. RESULTS: A total of 89.6 percent of patients with thromboembolism and 93.8 percent of control patients were treated with pneumatic compression, warfarin, enoxaparin, or unfractionated heparin, alone or in combination. In addition, 22.2 percent and 29.7 percent, respectively, received warfarin after discharge. A body-mass index (the weight in kilograms divided by the square of the height in meters) of 25 or greater was associated with rehospitalization for thromboembolism, with an odds ratio of 2.5 (95 percent confidence interval, 1.8 to 3.4). In a multivariate model, the only prophylactic regimens associated with a reduced risk of thromboembolism were pneumatic compression in patients with body-mass indexes of less than 25 (odds ratio, 0.3; 95 percent confidence interval, 0.2 to 0.6) and warfarin treatment after discharge (odds ratio, 0.6; 95 percent confidence interval, 0.4 to 1.0). CONCLUSIONS: In patients who underwent total hip arthroplasty, a body-mass index of 25 or greater was associated with subsequent hospitalization for thromboembolism. Pneumatic compression in patients with a body-mass index of less than 25 and prophylaxis with warfarin after discharge were independently protective against thromboembolism.

Aged↗
PubMed2000

Management and dosing of warfarin therapy.

When initiating warfarin therapy, clinicians should avoid loading doses that can raise the International Normalized Ratio (INR) excessively; instead, warfarin should be initiated with a 5-mg dose (or 2 to 4 mg in the very elderly). With a 5-mg initial dose, the INR will not rise appreciably in the first 24 hours, except in rare patients who will ultimately require a very small daily dose (0.5 to 2.0 mg). Adjusting a steady-state warfarin dose depends on the measured INR values and clinical factors: the dose does not need to be adjusted for a single INR that is slightly out of range, and most changes should alter the total weekly dose by 5% to 20%. The INR should be monitored frequently (eg, 2 to 4 times per week) immediately after initiation of warfarin; subsequently, the interval between INR tests can be lengthened gradually (up to a maximum of 4 to 6 weeks) in patients with stable INR values. Patients who have an elevated INR will need more frequent testing and may also require vitamin K1. For example, a nonbleeding patient with an INR of 9 can be given low-dose vitamin K1 (eg, 2.5 mg phytonadione, by mouth). Patients who have an excessive INR with clinically important bleeding require clotting factors (eg, fresh-frozen plasma) as well as vitamin K1.

Administration, Oral↗
PubMed2000

A population-based study of the effectiveness of inferior vena cava filter use among patients with venous thromboembolism.

BACKGROUND: There are few population-based data regarding the effectiveness of inferior vena cava filter use in the prevention of symptomatic pulmonary embolism. OBJECTIVE: To determine the 1-year cumulative incidence of rehospitalization for venous thrombosis or pulmonary embolism among patients with thromboembolism treated with a vena cava filter compared with the incidence in a control population with thromboembolism. PATIENTS AND METHODS: Population-based retrospective analysis of linked hospital discharge abstracts in California. From January 1, 1991, through December 30, 1995, 3632 patients were treated with a filter and 64,333 controls were admitted with a principal diagnosis of venous thromboembolism. RESULTS: Filter-treated patients had significantly greater comorbidity, with a higher frequency of previous pulmonary embolism, recent major bleeding, malignant neoplasm, and stroke. Patients who initially manifested pulmonary embolism were significantly more likely to be rehospitalized for pulmonary embolism than patients with an initial diagnosis of venous thrombosis alone, among filter-treated patients (relative risk, 6.72; 95% confidence interval, 3.61-12.49) and controls (relative risk, 5.30; 95% confidence interval, 4.61-6.10). Risk-adjusted proportional hazards modeling showed no significant difference between filter-treated patients and controls in the relative hazard of rehospitalization for pulmonary embolism. However, filter placement was associated with a significantly higher relative hazard of rehospitalization for venous thrombosis among patients who initially manifested pulmonary embolism (relative hazard, 2.62; 95% confidence interval, 2.09-3.29), but not among those who presented with venous thrombosis (relative hazard, 1.14; 95% confidence interval, 0.92-1.43). CONCLUSIONS: Insertion of a vena cava filter was not associated with a significant reduction in the 1-year incidence of rehospitalization for pulmonary embolism. Use of a filter was associated with a higher incidence of rehospitalization for venous thrombosis, but only among patients who initially manifested pulmonary embolism. A prospective clinical study is needed to determine the efficacy of filter use among patients with pulmonary embolism who do not meet strict guidelines for insertion of a vena cava filter.

Adult↗
PubMed2000

Temporary reversal of anticoagulation using oral vitamin K.

Brief reversal of oral anticoagulant therapy is frequently necessary prior to minor surgery or invasive procedures. We sought to determine the effect of an oral dose of 2.0 mg of vitamin K(1) on the international normalized ratio (INR) among patients with a stable therapeutic INR who were maintained on their daily dose of warfarin. We prospectively studied a convenience cohort of patients attending an anticoagulation clinic who had either just completed treatment for venous thromboembolism or were receiving prophylaxis for atrial fibrillation, cardiomyopathy, or peripheral vascular disease. Each patient received an oral dose of 2.0 mg of aqueous vitamin K(1). Serial INR measurements were taken over 1 week. There was wide variation in the INR response between patients, from no change to complete reversal of anticoagulation. The effect also varied widely over time. There was a significant inverse correlation between the fall in logarithm of the INR and the daily warfarin dose required to achieve an INR value of 2.5 (r=-0.52, p=0.011). Use of a 2.0 mg oral dose of vitamin K(1) does not reliably reverse (correct) a therapeutic INR in patients maintained on their daily dose of warfarin.

Administration, Oral↗
PubMed2000

Identification of an archaeal 2-hydroxy acid dehydrogenase catalyzing reactions involved in coenzyme biosynthesis in methanoarchaea.

Two putative malate dehydrogenase genes, MJ1425 and MJ0490, from Methanococcus jannaschii and one from Methanothermus fervidus were cloned and overexpressed in Escherichia coli, and their gene products were tested for the ability to catalyze pyridine nucleotide-dependent oxidation and reduction reactions of the following alpha-hydroxy-alpha-keto acid pairs: (S)-sulfolactic acid and sulfopyruvic acid; (S)-alpha-hydroxyglutaric acid and alpha-ketoglutaric acid; (S)-lactic acid and pyruvic acid; and 1-hydroxy-1,3,4,6-hexanetetracarboxylic acid and 1-oxo-1,3,4, 6-hexanetetracarboxylic acid. Each of these reactions is involved in the formation of coenzyme M, methanopterin, coenzyme F(420), and methanofuran, respectively. Both the MJ1425-encoded enzyme and the MJ0490-encoded enzyme were found to function to different degrees as malate dehydrogenases, reducing oxalacetate to (S)-malate using either NADH or NADPH as a reductant. Both enzymes were found to use either NADH or NADPH to reduce sulfopyruvate to (S)-sulfolactate, but the V(max)/K(m) value for the reduction of sulfopyruvate by NADH using the MJ1425-encoded enzyme was 20 times greater than any other combination of enzymes and pyridine nucleotides. Both the M. fervidus and the MJ1425-encoded enzyme catalyzed the NAD(+)-dependent oxidation of (S)-sulfolactate to sulfopyruvate. The MJ1425-encoded enzyme also catalyzed the NADH-dependent reduction of alpha-ketoglutaric acid to (S)-hydroxyglutaric acid, a component of methanopterin. Neither of the enzymes reduced pyruvate to (S)-lactate, a component of coenzyme F(420). Only the MJ1425-encoded enzyme was found to reduce 1-oxo-1,3,4,6-hexanetetracarboxylic acid, and this reduction occurred only to a small extent and produced an isomer of 1-hydroxy-1,3,4,6-hexanetetracarboxylic acid that is not involved in the biosynthesis of methanofuran c. We conclude that the MJ1425-encoded enzyme is likely to be involved in the biosynthesis of both coenzyme M and methanopterin.

Alcohol Oxidoreductases↗
PubMed2000

Identification of the gene encoding sulfopyruvate decarboxylase, an enzyme involved in biosynthesis of coenzyme M.

The products of two adjacent genes in the chromosome of Methanococcus jannaschii are similar to the amino and carboxyl halves of phosphonopyruvate decarboxylase, the enzyme that catalyzes the second step of fosfomycin biosynthesis in Streptomyces wedmorensis. These two M. jannaschii genes were recombinantly expressed in Escherichia coli, and their gene products were tested for the ability to catalyze the decarboxylation of a series of alpha-ketoacids. Both subunits are required to form an alpha(6)beta(6) dodecamer that specifically catalyzes the decarboxylation of sulfopyruvic acid to sulfoacetaldehyde. This transformation is the fourth step in the biosynthesis of coenzyme M, a crucial cofactor in methanogenesis and aliphatic alkene metabolism. The M. jannaschii sulfopyruvate decarboxylase was found to be inactivated by oxygen and reactivated by reduction with dithionite. The two subunits, designated ComD and ComE, comprise the first enzyme for the biosynthesis of coenzyme M to be described.

Amino Acid Sequence↗
PubMed2000

Identification of enzymes homologous to isocitrate dehydrogenase that are involved in coenzyme B and leucine biosynthesis in methanoarchaea.

Two putative Methanococcus jannaschii isocitrate dehydrogenase genes, MJ1596 and MJ0720, were cloned and overexpressed in Escherichia coli, and their gene products were tested for the ability to catalyze the NAD- and NADP-dependent oxidative decarboxylation of DL-threo-3-isopropylmalic acid, threo-isocitrate, erythro-isocitrate, and homologs of threo-isocitrate. Neither enzyme was found to use any of the isomers of isocitrate as a substrate. The protein product of the MJ1596 gene, designated AksF, catalyzed the NAD-dependent decarboxylation of intermediates in the biosynthesis of 7-mercaptoheptanoic acid, a moiety of methanoarchaeal coenzyme B (7-mercaptoheptanylthreonine phosphate). These intermediates included (-)-threo-isohomocitrate [(-)-threo-1-hydroxy-1,2, 4-butanetricarboxylic acid], (-)-threo-iso(homo)(2)citrate [(-)-threo-1-hydroxy-1,2,5-pentanetricarboxylic acid], and (-)-threo-iso(homo)(3)citrate [(-)-threo-1-hydroxy-1,2, 6-hexanetricarboxylic acid]. The protein product of MJ0720 was found to be alpha-isopropylmalate dehydrogenase (LeuB) and was found to catalyze the NAD-dependent decarboxylation of one isomer of DL-threo-isopropylmalate to 2-ketoisocaproate; thus, it is involved in the biosynthesis of leucine. The AksF enzyme proved to be thermostable, losing only 10% of its enzymatic activity after heating at 100 degrees C for 10 min, whereas the LeuB enzyme lost 50% of its enzymatic activity after heating at 80 degrees C for 10 min.

3-Isopropylmalate Dehydrogenase↗
PubMed2000

Optimization of inpatient warfarin therapy: impact of daily consultation by a pharmacist-managed anticoagulation service.

OBJECTIVE: To determine the effect of daily consultation by a team of hospital pharmacists on the accuracy and rapidity of optimizing warfarin therapy. DESIGN: Comparison of a historical control cohort with a prospective cohort matched for treatment indication. SETTING: A 400-bed university teaching hospital. PATIENTS: Sixty consecutive patients hospitalized in 1992 and starting warfarin for the first time, with anticoagulation therapy managed by physicians, were compared with 60 patients matched for warfarin indication hospitalized in 1995, but with anticoagulation therapy managed with pharmacy consultation. RESULTS: Pharmacist management of initial warfarin therapy resulted in a significant reduction in the length of hospitalization compared with physician dosing, from 9.5 +/- 5.6 days to 6.8 +/- 4.4 days (p = 0.009). The number of patients and patient-days with international normalized ratio (INR) values >3.5 were reduced by pharmacist dosing from 37 patients and 142 days to 16 patients and 29 days, respectively (p < 0.001). Similarly, the number of patients and patient-days with INR >6.0 were reduced from 20 patients and 50 days to two patients and six days, respectively (p < 0.001). There were six documented bleeding complications in 1992 compared with one in 1995 (p = 0.11). The mean INR at discharge was significantly lower in the pharmacy surveillance group, 2.6 +/- 0.58, compared with the physician cohort, 3.3 +/- 2.1 (p = 0.07). Readmissions after discharge due to bleeding or recurrent thrombosis were reduced from five (at 1 mo) and 10 (at 3 mo) to two and five readmissions, respectively, by pharmacist intervention (p = 0.43). The number of patients with concurrently prescribed drugs known to significantly interact with warfarin was significantly lower (6 vs. 13; p = 0.02) in the pharmacy surveillance group. CONCLUSIONS: Among patients starting warfarin for the first time, daily consultation by a pharmacist significantly decreased the length of hospital stay and the number of patients who received excessive anticoagulation therapy. These findings translate into improved quality of care and potentially significant cost savings.

Anticoagulants↗