Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

Combining acid-base, redox and substrate binding functionalities to give a complete model for the [FeFe]-hydrogenase

Some enzymes function by coupling substrate turnover with electron transfer from a redox cofactor such as ferredoxin. In the [FeFe]-hydrogenases, nature's fastest catalysts for the production and oxidation of H(2), the one-electron redox by a ferredoxin complements the one-electron redox by the diiron active site. In this Article, we replicate the function of the ferredoxins with the redox-active ligand Cp*Fe(C(5)Me(4)CH(2)PEt(2)) (FcP*). FcP* oxidizes at mild potentials, in contrast to most ferrocene-based ligands, which suggests that it might be a useful mimic of ferredoxin cofactors. The specific model is Fe(2)[(SCH(2))(2)NBn](CO)(3)(FcP*)(dppv) (1), which contains the three functional components of the active site: a reactive diiron centre, an amine as a proton relay and, for the first time, a one-electron redox module. By virtue of the synthetic redox cofactor, [1](2+) exhibits unique reactivity towards hydrogen and CO. In the presence of excess oxidant and base, H(2) oxidation by [1](2+) is catalytic.

Homocamptothecins: synthesis and antitumor activity of novel E-ring-modified camptothecin analogues

Homocamptothecin (hCPT), a camptothecin (CPT) analogue with a seven membered beta-hydroxylactone which combines enhanced plasma stability and potent topoisomerase I (Topo I)-mediated activity, is an attractive template for the elaboration of new anticancer agents. Like CPT, hCPT carries an asymmetric tertiary alcohol and displays stereoselective inhibition of Topo I. The preparation and biological screening of racemic hCPT analogues are described. The 10 hCPTs tested were better Topo I inhibitors than CPT. Fluorinated hCPTs 23c, d,f,g were found to have potent cytotoxic activity on A427 and PC-3 tumor cell lines. Their cytotoxicity remained high on the K562adr and MCF7mdr cell lines, which overexpress a functionally active P-glycoprotein. Fluorinated hCPTs were more efficacious in vivo than CPT on HT-29 xenografts. In this model, a tumor growth delay of 25 days was reached with hCPT 23g at a daily dose of 0.32 mg/kg, compared to 4 days with CPT at 0.625 mg/kg. Thus difluorinated hCPT 23g warrants further investigation as a novel Topo I inhibitor with high cytotoxicity toward tumor cells and promising in vivo efficacy.

Mild redox complementation enables H2 activation by [FeFe]-hydrogenase models

Mild oxidants such as [Fe(C(5)Me(5))(2)](+) accelerate the activation of H(2) by [Fe(2)[(SCH(2))(2)NBn](CO)(3)(dppv)(PMe(3))](+) ([1](+)), despite the fact that the ferrocenium cation is incapable of oxidizing [1](+). The reaction is first-order in [1](+) and [H(2)] but independent of the E(1/2) and concentration of the oxidant. The analogous reaction occurs with D(2) and proceeds with an inverse kinetic isotope effect of 0.75(8). The activation of H(2) is further enhanced with the tetracarbonyl [Fe(2)[(SCH(2))(2)NBn](CO)(4)(dppn)](+) ([2](+)), the first crystallographically characterized model for the H(ox) state of the active site containing an amine cofactor. These studies point to rate-determining binding of H(2) followed by proton-coupled electron transfer. Relative to that by [1](+), the rate of H(2) activation by [2](+)/Fc(+) is enhanced by a factor of 10(4) at 25 °C.

Topoisomerase I-mediated antiproliferative activity of enantiomerically pure fluorinated homocamptothecins

Homocamptothecin (hCPT) is an E-ring modified camptothecin (CPT) analogue bearing a methylene spacer between the alcohol and carboxyl functions of the CPT lactone. Combining pronounced inhibitory activity of topoisomerase I (Topo I) with enhanced plasma stability, hCPT constitutes an attractive template for the elaboration of new anticancer agents. Fluorinated hCPT analogues, prepared in enantiomerically pure form, were assayed by their stimulation of Topo I-mediated DNA cleavage. Translation into cytotoxicity against tumor cells was evaluated on HT29 human colon adenocarcinoma and on the multidrug resistant lung and bladder tumor cell lines, A549 and T24r. Good correlation is observed between the ability of the drugs to stimulate Topo I-mediated DNA cleavage and the respective 50% inhibitory concentrations (IC(50) values) of the HT29, A549, and T24r cell growth. Fluorine substitution in the A-ring of hCPT was found to have a pronounced influence on biological activity, providing several compounds which are up to 100-fold more potent than CPT in terms of IC(50). Among these, 10,11-difluoro-hCPT has been selected for further development.

Hydrogen Production Catalyzed by Bidirectional, Biomimetic Models of the [FeFe]-Hydrogenase Active Site

Active site mimics of [FeFe]-hydrogenase are shown to be bidirectional catalysts, producing H₂ upon treatment with protons and reducing equivalents. This reactivity complements the previously reported oxidation of H₂ by these same catalysts in the presence of oxidants. The complex Fe₂(adt^Bn)(CO)₃(dppv)(PFc*^Et₂ ) ([1]⁰; adt^Bn = (SCH₂)₂NBn, dppv = cis-1,2-bis(diphenylphosphino)ethylene, PFc*^Et₂ = Et₂PCH₂C₅Me₄FeCp*) reacts with excess [H(OEt₂)₂]BAr^F₄ (BAr^F₄^- = B(C₆H₃-3,5-(CF₃)₂)₄^-) to give ∼0.5 equiv of H₂ and [Fe₂(adt^BnH)(CO)₃(dppv)(PFc*^Et₂ )]²⁺ ([1H]²⁺). The species [1H]²⁺ consists of a ferrocenium ligand, an N-protonated amine, and an Fe^IFe^I core. In the presence of additional reducing equivalents in the form of decamethylferrocene (Fc*), hydrogen evolution is catalytic, albeit slow. The related catalyst Fe₂(adt^Bn)(CO)₃(dppv)(PMe₃) (3) behaves similarly in the presence of Fc*, except that in the absence of excess reducing agent it converts to the catalytically inactive μ-hydride derivative [μ-H3]⁺. Replacement of the adt in [1]⁰ with propanedithiolate (pdt) results in a catalytically inactive complex. In the course of synthesizing [FeFe]-hydrogenase mimics, new routes to ferrocenylphosphine ligands and nonamethylferrocene were developed.

Stimulus-secretion coupling of arginine-induced insulin release. Metabolism of L-arginine and L-ornithine in pancreatic islets

Exogenous L-arginine and L-ornithine rapidly accumulate in rat pancreatic islets. L-Arginine is converted to L-ornithine and urea. Endogenous or exogenous L-ornithine generates di- and polyamines, the putrescine turnover being faster than that of spermidine and spermine. However, the major pathway for L-ornithine metabolism consists of its transamination to L-glutamaldehyde and further conversion to L-glutamate. The amines and L-glutamate derived from exogenous L-ornithine are incorporated into islet proteins at the intervention of transglutaminase and cycloheximide-sensitive biosynthetic processes, respectively. These findings suggest the hypothesis that the insulinotropic action of L-arginine and L-ornithine could somehow be related to the metabolism of these cationic amino acids in islet cells.

Amputation neuroma of the orbit. Report of two cases and review of the literature

Two patients developed proptosis and a slowly enlarging mass with increasing discomfort in the orbital socket 17 and 25 years following enucleation. Preoperative CT scans revealed a single cystic structure within the orbits of each case with distinct soft tissue tumors adjacent to the cysts. The cystic structures and the adjacent solid masses were removed en bloc necessitating reconstruction of the orbits with a dermal fat pad (case 1) and a mucous membrane graft (case 2). Postoperatively the patients were free of complaints. Microscopically, the cystic structures were identified as conjunctival inclusion cysts while the soft tissue masses were traumatic neuromas with irregular tangles and whorls composed of proliferated axons, Schwann cells and connective tissue. Only seven amputation neuromas of the orbit have been reported. Pain related to the neuroma is rarely encountered and is probably caused by mechanical irritation of the amputation neuroma, by retracting scar tissue, or compression from an adjacent cystic mass as in one of our cases.

The homocamptothecin BN 80915 is a highly potent orally active topoisomerase I poison

BN 80915, a lead compound of the homocamptothecin (hCPT) family, has entered clinical trials. BN 80915 is a difluoro-hCPT where the six-membered alpha-hydroxylactone ring of camptothecin (CPT) is replaced by a seven-membered beta-hydroxylactone ring. Preclinical data reported here show that in spite of the modification to the crucial E-ring of CPTs, BN 80915 retains topoisomerase I poisoning activity as shown in living HT29 cells as well as in cell-free assays, where BN 80915 always performs better than SN-38 or TPT. In antiproliferative assays BN 80915 is also very potent as evidenced by IC50s values consistently lower than those of SN38 in sensitive cell lines as well as in their related multidrug-resistant lines overexpressing P-glycoprotein or multidrug resistance-associated protein. Furthermore, in human plasma, in contrast to CPT analogs, the hydrolysis of BN 80915 is slow, leading to improved plasma stability, and irreversible, thus avoiding toxicity related to the accumulation of active principle during excretion in the urinary tract. These findings may account for the good in vivo efficacy observed in PC3 xenograft experiments where BN 80915 administered orally at very low doses doubled the tumor growth delay in comparison to CPT-11 administered i.p. Altogether, these results strongly support further development of BN 80915.

Mucronine J a 14-membered cyclopeptide alkaloid from Zizyphus mucronata

From the CH2Cl2 extract of the root bark of Z. mucronata (Rhamnaceae), a new cyclopeptide alkaloid, named mucronine J, was isolated together with previously known alkaloids abyssenine A and mucronine D. The structure of mucronine J was elucided by mass spectrometry and 1D and 2D NMR. A solution conformation is proposed on the basis of NOE experiments in combination with MM2 calculations.

Bilateral and unilateral vertical ground reaction forces and leg asymmetries in soccer players

The purposes of this study were to assess unilateral and bilateral vertical jump performance characteristics, and to compare the vertical ground reaction force characteristics of the impulse and landing phase of a vertical jump between the dominant and non-dominant leg in soccer players. The sample consisted of 20 male soccer players (22.80 ± 2.71 years, 1.88 ± 0.06 m, 76.47 ± 8.80 kg) who competed in the third division of the Spanish football league. Vertical jump performance was determined by testing the impulse and landing phase of a bilateral vertical jump, dominant leg vertical jump and non-dominant leg vertical jump. Significant differences (p < 0.05) between dominant and non-dominant legs were found in counter movement jump (CMJ) flight time (LA = -2.38%, d = 0.33), CMJ flight height (LA = -4.55%, d = 0.33) and CMJ speed take-off (LA = -2.91%, d = 0.42). No significant differences were found between the dominant and non-dominant leg in the F1 and F2 magnitudes during the landing phase, the time from the first contact of the foot with the ground to the production of F1, the time from the second contact of the foot with the ground to the production of F2, and the time to stabilization of the landing phase. Although differences were found between the dominant and non-dominant leg in the impulse phase of the jump, no significant differences were found between dominant and non-dominant legs in the landing phase of vertical jump variables.

Femoral deformity planning: intentional placement of the apex of deformity

Traditionally, correction of femoral deformity has been performed with osteotomies through the center of rotation of angulation (CORA), but the CORA location is not always practical. If the osteotomy is created at a site adjacent to the CORA, an additional translation must be performed to accurately correct the deformity. However, at times, the ideal osteotomy site may require an unfeasible amount of translation. Multiple osteotomies may also be problematic, and when overcorrection of the mechanical axis is planned, the CORA method is not practical.This article describes a novel method by which the surgeon may choose the location of the osteotomy regardless of the location of the CORA and may consolidate a multiapical deformity into a single corrective osteotomy. Furthermore, intentional mechanical axis overcorrection may be performed to unload knee joint arthritis. Simple, complex, and multiapical deformities may now be corrected via a single familiar surgical procedure, such as a distal femoral osteotomy, and the need for translation is eliminated.

Ketone bodies and islet function: 45Ca handling, insulin synthesis, and release

D-(-)-beta-Hydroxybutyrate and acetoacetate cause a rapid, sustained, and rapidly reversible stimulation of insulin release from rat pancreatic islets incubated in the presence, but not absence, of D-glucose. This coincides with stimulation of both proinsulin biosynthesis and 45Ca net uptake. The ketone bodies also decrease 45Ca outflow from prelabeled islets perifused in the absence of Ca2+ and, in contrast, enhance effluent radioactivity in the presence of Ca2+. In the presence of D-glucose, the secretory response to D-(-)-beta-hydroxybutyrate is concentration related in the 2.5-20 mM range, abolished in the absence of Ca2+ or presence of KCN, and enhanced by theophylline and forskolin. It corresponds grossly to a shift to the left of the sigmoidal curve relating insulin output to the ambient concentration of D-glucose. The secretory, biosynthetic, and cationic response to acetoacetate is less marked than that evoked by an equimolar concentration of D-(-)-beta-hydroxybutyrate. These features are compatible with the view that the insulinotropic action of ketone bodies would be causally linked to their metabolism in islet cells.

Stimulus-secretion coupling of arginine-induced insulin release: metabolism of L-arginine and L-ornithine in tumoral islet cells

The metabolism of L-arginine and L-ornithine was examined in tumoral islet cells of the RINm5F line and compared to the situation previously characterized in normal rat islets. The maximal velocity of arginase in cell homogenates, as well as either the production of 14C-urea or the steady-state content of 14C-labelled ornithine in intact cells exposed to L-[U-14C]arginine were about one order of magnitude lower in tumoral than normal islet cells. The activity of ornithine-glutamate transaminase was similar in both cell types, and this coincided with a comparable rate of 14C-labelled L-glutamate generation by intact cells exposed to L-[1-14C]ornithine. Despite a comparable cell content in 14C-labelled ornithine of normal and tumoral cells exposed to exogenous ornithine, the rate of di- and polyamine generation was about one order of magnitude higher in tumoral than normal islet cells, this coinciding with a much higher activity of ornithine decarboxylase in RINm5F cell than islet homogenates.

Different media and methodologies for the detection of aflatoxin production by Aspergillus flavus strains isolated from trout feed

We have studied the aflatoxin producing capacity of 41 Aspergillus flavus strains isolated from the mycoflora present of natural media (wheat, rice and mixed feed) synthetic medium (Aflatoxin Producing Ability Medium) and semisynthetic media (Coconut Agar Medium and Glucose Yeast Extract Agar) were compared. Aflatoxins were analysed on days 4 and 8 post-inoculation under an incubation temperature of 28 degrees C. A total of 30 strains (75.7%) were producers on natural media as detected by Thin Layer Chromatography: 23 strains on wheat, 27 on rice and 12 on mixed feed. The results by qualitative fluorescence tests on synthetic and semisynthetic media were: 3 strains positive on Coconut Agar Medium (CAM) 1 on Glucose Yeast Extract Agar (GY + Agar) and none on Aflatoxin Producing Ability Medium (APA).

Monitoring premature infants in car seats: implementing the American Academy of Pediatrics policy in a community hospital

OBJECTIVES

In 1990, the American Academy of Pediatrics (AAP) Committee on Injury and Poison Prevention issued a policy statement which recommended that all infants born at less than 37 weeks gestational age be monitored in their car seat before discharge to identify infants at risk for apnea associated with car seat use. The objective of this report is to summarize the experience of a community hospital in implementing this recommendation, including: education of health professionals about the problem and its management, development of an effective hospital policy and procedure for testing at-risk infants, overcoming logistical difficulties for nursing staff who implement the policy, overcoming liability concerns, and summarizing the clinical results which we observed during a 15-month period.

METHODS

MetroWest Medical Center, Framingham Union Campus is a community hospital with a Level II Neonatal Unit with approximately 2200 deliveries annually. Hospital procedures and policies were developed to implement the AAP policy for safe transportation of premature infants. Consequently, infants less than 37 weeks gestation at birth were monitored for a 90-minute observation period for oxygen saturation, apnea, or bradycardia in the car seat that they would be going home in from the nursery.

RESULTS

Policies and procedures for car seat monitoring were instituted successfully during 15-month period. Sixteen of 87 (18.4%) monitored infants 26 to 36 weeks gestation had abnormal results. In addition, one 37-week gestational newborn who was monitored because of duskiness during feeds also became apneic and bradycardic in the car seat.

CONCLUSIONS

A car seat monitoring program can be incorporated effectively into a Level II community hospital nursery discharge plan provided that appropriate medical staff policies, nursing procedures, and administrative support to deal with the logistics of the program is established. Our monitoring results are consistent with previous reports that premature infants are at risk of desaturation in a car seat. In addition, we have demonstrated that a full-term neonate may also be at risk if other evidence of desaturation (eg, duskiness during feedings) is observed.

A comparison of the efficacy, tolerance and residual effects of zopiclone, flurazepam and placebo in insomniac outpatients

A double-blind randomized parallel group design was used to compare the tolerance and efficacy of zopiclone 7.5 mg, flurazepam 30 mg and placebo taken each night for 3 weeks in a group of 24 out-patients complaining of sleep disturbance. Analogue rating scales were used to assess the efficacy of the treatments while tolerance and residual effects were measured weekly using a battery of performance tests (Critical Flicker Fusion Threshold, Choice Reaction Time, Letter Cancellation and Digit Span). Physical and clinical examinations, including ECG and EEG, were conducted before and after the experimental period and spontaneously reported side-effects were recorded. Analysis of variance on 24 completed cases (8 patients in each treatment group) showed both active treatments to be significantly better than placebo in shortening sleep onset latency. Flurazepam increased the duration of sleep and produced a "hangover" of impaired psychomotor function. Zopiclone had no observable effect on early morning performance and was free from residual sedative activity. The lack of residual effects with zopiclone 7.5 mg, at a dose shown to be clinically effective, suggests its use in ambulant or out-patient populations where daytime sedation could interfere with every day activities.

Inhibition of proinsulin biosynthesis and conversion by a putative insulin mediator

The phospho-oligosaccharide (POS), presumed to act at the postreceptor level as the insulin second messenger, was recently reported to inhibit glucose-stimulated insulin release from rat pancreatic islets. In the present study, POS was also found to inhibit glucose-stimulated proinsulin biosynthesis and conversion in rat islets. By comparison with prior findings on the effects of both exogenous insulin and anti-insulin serum upon proinsulin synthesis, these results argue against the view that insulin would normally exert a negative feedback control upon the biosynthetic and secretory activities of islet B-cells.

D-glucose metabolism in tumoral pancreatic islet cells with rapid or decreased mitotic activity

The metabolism of D-glucose was investigated in tumoral islet cells of the RINm5F line with either unaltered or decreased mitotic activity, resulting from the incorporation of D, L-alpha-difluoromethylornithine (DFMO) in the culture medium. In DFMO-treated cells, the oxidation of D-[3,4-14C]glucose was less severely affected than the utilization of D-[5-3H] glucose, at least at hexose concentrations in the 1.0 to 16.7 mM range. This coincided with restoration of the process of glucose-induced stimulation of 3HOH generation from [2-3H]glycerol. Pretreatment with DFMO also resulted in a decreased circulation in the pentose phosphate pathway and the restoration of a preferential stimulation of D-[2-14C]glucose or D-[6-14C]glucose oxidation relative to D-[5-3H]glucose utilization when raising the hexose concentration from 0.2 to 1.0 mM. It is proposed that the interference of cell growth with the metabolic and functional responses of RINm5F cells to D-glucose is relevant to the difference between such responses otherwise found when comparing the behaviour of foetal or neonatal to adult normal islets.

Relationship between D-glucose oxidation and glycolysis in tumoral pancreatic islet cells with either rapid or decreased cell growth

The relationship between glucose oxidation and glycolysis was examined in tumoral pancreatic islet cells with either rapid or decreased cell growth. In the control RINm5F cells, the utilization of D-[5-3H]glucose progressively augmented at increasing concentrations of D-glucose (0.2 to 16.7 mM), whereas D-[6-14C]glucose oxidation reached its maximal value at about 3.0 mM D-glucose. The 14CO2/3HOH ratio progressively decreased at increasing hexose concentrations, being about 5-times higher at 0.2 than 10.0-16.7 mM. These experiments were repeated in cells cultured in the presence of D,L-alpha-difluoromethylornithine, which inhibits RINm5F cell proliferation. At 0.2 mM D-glucose, 14CO2 output was severely decreased but 3HOH production little affected, resulting in a marked fall of the 14CO2/3HOH ratio. On the contrary, at 16.7 mM D-glucose, 3HOH formation was more markedly decreased than 14CO2 output, resulting in an increased 14CO2/3HOH ratio. These findings provide a model for the interference of cell growth with D-glucose metabolism in islet cells. Rapid growth causes a preferential stimulation of the Krebs cycle relative to glycolysis at a low glucose concentration, whilst increasing glycolysis more than oxidation at high concentrations of the hexose.

Homocamptothecin, an E-ring modified camptothecin with enhanced lactone stability, retains topoisomerase I-targeted activity and antitumor properties

Homocamptothecin (hCPT) is a semisynthetic analogue of camptothecin (CPT) with a seven-membered beta-hydroxylactone resulting from the insertion of a methylene spacer between the alcohol moiety and the carboxyl function of the naturally occurring six-membered alpha-hydroxylactone of CPT. This E-ring modification provides a less reactive lactone with enhanced stability and decreased protein binding in human plasma. Biological testing against CPT revealed that, instead of being detrimental, the modified lactone of hCPT has a positive impact on topoisomerase I (Topo I) poisoning properties. In vitro tests showed hCPT to fully conserve the ability to stabilize Topo I-DNA cleavage complexes and to stimulate a higher level of DNA cleavage than CPT. A similar trend toward improvement was also observed in antiproliferative assays with human tumor cell lines (including cells overexpressing P-glycoprotein). In two distinct in vivo models, using L1210 murine leukemia or human colon carcinoma HT29, hCPT was found to be more efficacious than CPT. The slow, but irreversible, hydrolysis of hCPT, instead of the fast equilibrium of CPT, may account for its good in vivo activity. Overall, these results provide evidence that a highly reactive lactone is not a requisite for the Topo I-mediated antitumor activity of CPT analogues, and that hCPT is an interesting pharmacological tool with improved solution behavior as well as a promising new template for the preparation of more efficacious Topo I poisons.