An impact of tropical cyclone on meiobenthic fauna of Chennai coast, Tamil Nadu, India: A case study of cyclone Mandous

The meiofaunal diversity is used as an indicator to assess the complex and diverse impacts on the coastal environment during the natural calamities. The present study evaluates the effects of Mandous cyclone on ecologically two different stations, Pattinampakkam beach and Adyar estuary, which are located on Chennai coast, Tamil Nadu, India. The impact of the Mandous cyclone on physico-chemical parameters and meiobenthic faunal composition was investigated during, prior to, and after the cyclone. Thirty-nine species of meiofauna belonging to 15 taxa were recorded in both the stations. Nematoda, Oligochaeta and Harpacticoida taxa occurred with higher diversity and abundance than other meiofaunal taxa. Among these taxa, Polygastrophora sp. of Nematoda, Grania pusilla of Oligochaeta, and Arenosetella indica of Harpacticoida were the predominant species recorded during the study period. There was a prominent decline in the population density of meiofauna due to the Mandous cyclone, and thereafter, it took three weeks for recolonization and restoration to normalcy. Statistically, significant impact of the Mandous cyclone on the diversity, density, and evenness of the meiofaunal community with abiotic factors were observed through the Ecological indices and Canonical Correspondence Analysis. The Mandous cyclone assessment with special emphasis on meiofaunal communities allowed to fill the gap with knowledge regarding the diversity, abundance, composition, and distribution of meiofauna between pre- and post-Mandous cyclone, which helped in understanding the physico-chemical changes and response of meiofauna in a sandy beach and estuary.

Impact of anthropogenic accumulation on phytoplankton community and harmful algal bloom in temporarily open/closed estuary

Spatio-temporal variation in phytoplankton community dynamics in a temporarily open/closed Swarnamukhi river estuary (SRE), located on the South East coast of India was investigated and correlated to that of the adjacent coastal waters. Understanding the seasonal variability of the phytoplankton community and influencing factors are essential to predicting their impact on fisheries as the river and coastal region serve as the main source of income for the local fishing communities. Downstream before the river meets the sea, an arm of the Buckingham Canal (BC), carrying anthropogenic inputs empties into the Swarnamukhi River (SR1). The impact of anthropogenic effects on the phytoplankton community at BC was compared to other estuarine stations SR2 (upstream), SR1 (downstream), SRM (river mouth) and coastal station (CS). In BC station, harmful algal blooms (HABs) of Chaetoceros decipiens (2940 × 103 cells L-1) and Oscillatoria sp. (1619 × 103 cells L-1) were found during the southwest monsoon and winter monsoon, respectively. These HABs can be linked to the anthropogenic input of increased nutrients and trace metals. The HABs of Oscillatoria sp. were shown to be induced by elevated concentrations of nitrate (10.18 µM) and Ni (3.0 ppm) compared to ambient, while the HABs of C. decipiens were caused by elevated concentrations of silicate (50.35 µM), nitrite (2.1 µM), and phosphate (4.37 µM). Elevated nutrients and metal concentration from the aquaculture farms, and other anthropogenic inputs could be one of the prime reasons for the recorded bloom events at BC station. During this period, observed bloom species density was found low at other estuarine stations and absent at CS. The formation of bloom events during the closure of the river mouth could be a major threat to the coastal ecosystem when it opens. During the Osillatoria sp. bloom, both the Cu and Ni levels were higher at BC. The elevated concentration of nutrients and metals could potentially affect the coastal ecosystem and in turn fisheries sector in the tropical coastal ecosystem.

Seasonal variations influencing the abundance and diversity of plankton in the Swarnamukhi River Estuary, Nellore, India

An integrated approach was used to study the seasonal influence on the abundance and diversity of phytoplankton and zooplankton in the Swarnamukhi River Estuary (SRE) and the adjacent coast covering five stations by collecting monthly samples from the years 2014 to 2017. A total of 54 phytoplankton species conforming to four families and 58 zooplankton species conforming to nine families were recorded. Phytoplankton abundance and richness were high during pre-monsoon (PRM - 56410 cells/L) followed by monsoon (MON – 42210 cells/L). A similar trend was observed in the case of zooplankton, where abundance was recorded high during PRM (124261 ind./m3) followed by MON (111579 ind./m3). Moreover, phytoplankton and zooplankton were dominated by the diatoms and copepods, respectively. Both phytoplankton and zooplankton exhibited significant temporal (F= 26.4, p <0.05) and spatial (F= 32.1, p <0.05) variations. The higher density and abundance were recorded in the inner stations compared to the open sea. The present study reveals that the SRE have a rich diversity which could be attributed to a higher nutrient influx in the inner stations. The anthropogenic discharge from the surrounding aqua farms, agricultural land, and human settlement area could cause concerns for the local flora and fauna if a proper mitigation plan is not evolved through long-term monitoring study in this coastal region. 

Evaluation of factors influencing the trace metals in Puducherry and Diu coasts of India through multivariate techniques

In recent years, urban and industrial development initiatives at Puducherry and Diu such as tourism, shipping, and fisheries have led to sediment contamination by trace metals, and contributed to this investigation that extended from 2016 to 2017. Strong factor loadings of Cd (0.94), Ni (0.84), Al (0.84), Cr (0.83), Co (0.82), and Fe (0.78) illustrated the variability at Puducherry, whereas Cr (0.88), Cd (0.86), Ni (0.83), Co (0.77), Cu (0.77), and Fe (0.77) showed variability at Diu. The mean rank order distribution of the top three metals in sediment was Fe > Al > Mn, which exhibited higher variability. The highest contamination factor was observed for Cd at Diu, whereas the lowest was observed at Puducherry for Al. Similarly, the risk index also exhibited considerable risk which could be attributed to Cd contamination in the sediment at Diu compared with that at Puducherry. The results obtained are essential to establish a reference for better comparison and management of the tropical environments.

Assessment of ecological health of Swarnamukhi river estuary, southeast coast of India, through AMBI indices and multivariate tools

A combination of biotic indices, geo-accumulation (I_geo) index, and a multivariate approach were applied to assess the anthropogenic influence on the benthic community at five stations from 2018 to 2019 in the Swarnamukhi river estuary, Nellore, India. Non-metric multidimensional scaling and cluster analysis indicated that the Buckingham canal (BC) station showed azoic conditions and formed a separate cluster. Strong positive factor loadings of Cd (0.96), Al (0.93), Zn (0.91), Fe (0.90), Co (0.89), Cu (0.89), Ni (0.87), Pb (0.85), Cr (0.77), organic matter (0.94), Silt (0.92), and clay (0.93) and negative loading of sand (-0.90) showed the variability in sediment. AMBI results illustrated the disturbance status of each station and classified BC station as 'extremely disturbed' class, and M-AMBI assessed the ecological status as 'bad'. The I_geo index also revealed metal (Cd) contamination. The present study illustrated that the combined approach is effective for ecological assessment of coastal ecosystem.

Evaluation of trace metals in seawater, sediments, and bivalves of Nellore, southeast coast of India, by using multivariate and ecological tool

Urbanization in recent years has driven us to investigate metal contamination on Nellore coast by collecting seawater, sediment, and bivalve samples monthly at five stations from 2015 to 2017. Non-metric multidimensional scaling and cluster analysis indicated that open sea (OS) samples were markedly different from the samples collected at other stations. Strong factor loadings of Al (0.76), Mn (0.79), and Cd (0.78) showed variability in seawater, while those for Fe (0.76), Ni (0.77), Zn (0.85), and Pb (0.81) showed variability in sediment. The mean values of Fe (346 ppm) and Mn (21 ppm) were high in bivalves compared to the mean values of other metals. A higher contamination factor was observed for Cd at Buckingham Canal, while the lowest was observed for sediment in OS. The order of trace metals in sediments according to risk index was Cd > Pb > Cu > Cr > Zn. The results obtained are essential to establish a reference for better comparison of tropical environments.

Beta-lactamase-inhibitor combinations in the 21st century: current agents and new developments

Combinations of beta-lactams and beta-lactamase inhibitors have become one of the most successful antibacterial strategies in our global battle against bacterial infections. The success of these agents is particularly emphasized by the continued efficacy of Augmenting (amoxicillin and clavulanate) after nearly 20 years of clinical use. The clinical situation now dictates that second-generation beta-lactamase inhibitors capable of encompassing both class A and class C beta-lactamases would combat emerging resistance and provide a vital addition to our armory of hospital antibiotics. This realization has generated a renewed interest in beta-lactamase inhibitors and improved the prospects for the delivery of such agents in the future.

Identification and characterization of UDP-N-acetylenolpyruvylglucosamine reductase (MurB) from the Gram-positive pathogen Streptococcus pneumoniae

The UDP-N-acetylenolpyruvylglucosamine reductase (MurB) from a Gram-positive pathogen, Streptococcus pneumoniae, was identified and characterized. The enzyme from S. pneumoniae shows 31% identity with the MurB protein from Escherichia coli, and contains the catalytic residues, substrate-binding residues and FAD-binding motif identified previously in the E. coli protein. The gene was cloned into the pET28a+ expression vector, and the 34.5 kDa protein that it encodes was overexpressed in E. coli strain BL21(DE3) to 30% of total cell protein. The majority of the protein was found to be insoluble. A variety of methods were used to increase the amount of soluble protein to 10%. This was then purified to near homogeneity in a two-step process. The absorption spectrum of the purified protein indicated it to be a flavoprotein, like its E. coli homologue, with a characteristic absorption at 463 nm. The enzyme was shown to be active, reducing UDP-N-acetylglucosamine enolpyruvate with the concomitant oxidation of NADPH, and was characterized kinetically with respect to its two substrates. The enzyme showed properties similar to those of its E. coli counterpart, being activated by univalent cations and being subject to substrate inhibition. The characterization of an important cell wall biosynthesis enzyme from a Gram-positive pathogen provides a good starting point for the discovery of antibacterial agents against MurB.

Structure-function aspects and inhibitor design of type 5 17beta-hydroxysteroid dehydrogenase (AKR1C3)

17beta-Hydroxysteroid dehydrogenase (17beta-HSD) type 5 has been cloned from human prostate and is identical to type 2 3alpha-HSD and is a member of the aldo-keto reductase (AKR) superfamily; it is formally AKR1C3. In vitro the homogeneous recombinant enzyme expressed in Escherichia coli functions as a 3-keto-, 17-keto- and 20-ketosteroid reductase and as a 3alpha-, 17beta- and 20alpha-hydroxysteroid oxidase. The enzyme will reduce 5alpha-DHT, Delta(4)-androstene-3,17-dione, estrone and progesterone to produce 3alpha-androstanediol, testosterone, 17beta-estradiol and 20alpha-hydroxprogesterone, respectively. It will also oxidize 3alpha-androstanediol, testosterone, 17beta-estradiol and 20alpha-hydroxyprogesterone to produce 5alpha-androstane-3,17-dione, Delta(4)-androstene-3,17-dione, and progesterone, respectively. Many of these properties are shared by the related AKR1C1, AKR1C2 and AKR1C4 isoforms. RT-PCR shows that AKR1C3 is dominantly expressed in the human prostate and mammary gland. Examination of k(cat)/K(m) for these reactions indicates that as a reductase it prefers 5alpha-dihydrotestosterone and 5alpha-androstane-3,17-dione as substrates to Delta(4)-androstene-3,17-dione, suggesting that in the prostate it favors the formation of inactive androgens. Its concerted reductase activity may, however, lead to a pro-estrogenic state in the breast since it will convert estrone to 17beta-estradiol; convert Delta(4)-androstene-3,17-dione to testosterone (which can be aromatized to 17beta-estradiol); and it will reduce progesterone to its inactive metabolite 20alpha-hydroxyprogesterone. Drawing on detailed structure-function analysis of the related rat 3alpha-HSD (AKR1C9), which shares 69% sequence identity with AKR1C3, it is predicted that AKR1C3 catalyzes an ordered bi bi mechanism, that the rate determining step is k(chem), and that an oxyanion prevails in the transition state. Based on these relationships steroidal-based inhibitors that compete with the steroid product would be desirable since they would act as uncompetitive inhibitors. With regards to transition state analogs steroid carboxylates and pyrazoles may be preferred while 3alpha, 17beta or 20alpha-spiro-oxiranes may act as mechanism-based inactivators.

Human 3alpha-hydroxysteroid dehydrogenase isoforms (AKR1C1-AKR1C4) of the aldo-keto reductase superfamily: functional plasticity and tissue distribution reveals roles in the inactivation and formation of male and female sex hormones

The kinetic parameters, steroid substrate specificity and identities of reaction products were determined for four homogeneous recombinant human 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) isoforms of the aldo-keto reductase (AKR) superfamily. The enzymes correspond to type 1 3alpha-HSD (AKR1C4), type 2 3alpha(17beta)-HSD (AKR1C3), type 3 3alpha-HSD (AKR1C2) and 20alpha(3alpha)-HSD (AKR1C1), and share at least 84% amino acid sequence identity. All enzymes acted as NAD(P)(H)-dependent 3-, 17- and 20-ketosteroid reductases and as 3alpha-, 17beta- and 20alpha-hydroxysteroid oxidases. The functional plasticity of these isoforms highlights their ability to modulate the levels of active androgens, oestrogens and progestins. Salient features were that AKR1C4 was the most catalytically efficient, with k(cat)/K(m) values for substrates that exceeded those obtained with other isoforms by 10-30-fold. In the reduction direction, all isoforms inactivated 5alpha-dihydrotestosterone (17beta-hydroxy-5alpha-androstan-3-one; 5alpha-DHT) to yield 5alpha-androstane-3alpha,17beta-diol (3alpha-androstanediol). However, only AKR1C3 reduced Delta(4)-androstene-3,17-dione to produce significant amounts of testosterone. All isoforms reduced oestrone to 17beta-oestradiol, and progesterone to 20alpha-hydroxy-pregn-4-ene-3,20-dione (20alpha-hydroxyprogesterone). In the oxidation direction, only AKR1C2 converted 3alpha-androstanediol to the active hormone 5alpha-DHT. AKR1C3 and AKR1C4 oxidized testosterone to Delta(4)-androstene-3,17-dione. All isoforms oxidized 17beta-oestradiol to oestrone, and 20alpha-hydroxyprogesterone to progesterone. Discrete tissue distribution of these AKR1C enzymes was observed using isoform-specific reverse transcriptase-PCR. AKR1C4 was virtually liver-specific and its high k(cat)/K(m) allows this enzyme to form 5alpha/5beta-tetrahydrosteroids robustly. AKR1C3 was most prominent in the prostate and mammary glands. The ability of AKR1C3 to interconvert testosterone with Delta(4)-androstene-3,17-dione, but to inactivate 5alpha-DHT, is consistent with this enzyme eliminating active androgens from the prostate. In the mammary gland, AKR1C3 will convert Delta(4)-androstene-3,17-dione to testosterone (a substrate aromatizable to 17beta-oestradiol), oestrone to 17beta-oestradiol, and progesterone to 20alpha-hydroxyprogesterone, and this concerted reductive activity may yield a pro-oesterogenic state. AKR1C3 is also the dominant form in the uterus and is responsible for the synthesis of 3alpha-androstanediol which has been implicated as a parturition hormone. The major isoforms in the brain, capable of synthesizing anxiolytic steroids, are AKR1C1 and AKR1C2. These studies are in stark contrast with those in rat where only a single AKR with positional- and stereo-specificity for 3alpha-hydroxysteroids exists.

Mutation of nicotinamide pocket residues in rat liver 3 alpha-hydroxysteroid dehydrogenase reveals different modes of cofactor binding

Rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD), an aldo-keto reductase, binds NADP(+) in an extended anti-conformation across an (alpha/beta)(8)-barrel. The orientation of the nicotinamide ring, which permits stereospecific transfer of the 4-pro-R hydride from NAD(P)H to substrate, is achieved by hydrogen bonds formed between the C3-carboxamide of the nicotinamide ring and Ser 166, Asn 167, and Gln 190 and by pi-stacking between this ring and Tyr 216. These residues were mutated to yield S166A, N167A, Q190A, and Y216S. In these mutants, K(d)(NADP(H)) increased by 2-11-fold but without a significant change in K(d)(NAD(H)). Steady-state kinetic parameters showed that K(m)(NADP)()+ increased 13-151-fold, and this was accompanied by comparable decreases in k(cat)/K(m)(NADP)()+. By contrast, K(m)(NAD)()+ increased 4-8-fold, but changes in k(cat)/K(m)(NAD)()+ were more dramatic and ranged from 23- to 930-fold. Corresponding changes in binding energies indicated that each residue contributed equally to the binding of NADP(H) in the ground and transition states. However, the same residues stabilized the binding of NAD(H) only in the transition state. These observations suggest that different modes of binding exist for NADP(H) and NAD(H). Importantly, these modes were revealed by mutating residues in the nicotinamide pocket indicating that direct interactions with the 2'-phosphate in the adenine mononucleotide is not the sole determinant of cofactor preference. The single mutations were unable to invert or racemize the stereochemistry of hydride transfer even though the nicotinamide pocket can accommodate both anti- and syn-conformers once the necessary hydrogen bonds are eliminated. When 4-pro-R-[(3)H]NADH was used to monitor incorporation into [(14)C]-5alpha-dihydrotestosterone, a decrease in the (3)H:(14)C ratio was observed in the mutants relative to wild-type enzyme reflecting a pronounced primary kinetic isotope effect. This observation coupled with the change in the binding energy for NAD(P)(H) in the transition state suggests that these mutants have altered the reaction trajectory for hydride transfer.

The arginine 276 anchor for NADP(H) dictates fluorescence kinetic transients in 3 alpha-hydroxysteroid dehydrogenase, a representative aldo-keto reductase

Fluorescence stopped-flow studies were conducted with recombinant rat liver 3 alpha-HSD, an aldo-keto reductase (AKR) that plays critical roles in steroid hormone inactivation, to characterize the binding of nicotinamide cofactor, the first step in the kinetic mechanism. Binding of NADP(H) involved two events: the fast formation of a loose complex (E.NADP(H)), followed by a conformational change in enzyme structure leading to a tightly bound complex (E.NADP(H)), which was observed as a fluorescence kinetic transient. Binding of NAD(H) was not characterized by a similar kinetic transient, implying a difference in the mode of binding of the two cofactors. Unlike previously characterized AKRs, the rates associated with the formation and decay of E.NADP(H) and E.NADP(H) were much faster than kcat for the oxidoreduction of various substrates, indicating that binding and release of cofactor is not rate-limiting overall in 3 alpha-HSD. Mutation of Arg 276, a highly conserved residue in AKRs that forms a salt bridge with the adenosine 2'-phosphate of NADP(H), resulted in large changes in Km and Kd for NADP(H) that were not observed with NAD(H). The loss in free energy associated with the increase in Kd for NADP(H) is consistent with the elimination of an electrostatic link. Importantly, this mutation abolished the kinetic transient associated with NADPH binding. Thus, anchoring of the adenosine 2'-phosphate of NADPH by Arg 276 appears to be obligatory for the fluorescence kinetic transients to be observed. The removal of Trp 86, a residue involved in fluorescence energy transfer with NAD(P)H, also abolished the kinetic transient, but mutation of Trp 227, a residue on a mobile loop associated with cofactor binding, did not. It is concluded that in 3 alpha-HSD, the time dependence of the change in Trp 86 fluorescence is due to cofactor anchoring, and thus, Trp 86 is a distal reporter of this event. Further, the loop movement that accompanies cofactor binding is spectrally silent.

Retention of NADPH-linked quinone reductase activity in an aldo-keto reductase following mutation of the catalytic tyrosine

Aldo-keto reductases (AKR) are monomeric oxidoreductases that retain a conserved catalytic tetrad (Tyr, Lys, Asp, and His) at their active sites in which the Tyr acts as a general acid-base catalyst. In rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD, AKR1C9), a well-characterized AKR, the catalytic tyrosine is Tyr 55. This enzyme displays a high catalytic efficiency for a common AKR substrate 9,10-phenanthrenequinone (9,10-PQ). Surprisingly, Y55F and Y55S mutants of 3alpha-HSD reduced 9,10-PQ with high kcat values. This is the first report whereby the invariant catalytic tyrosine of an AKR has been mutated with retention of kcat values similar to wild-type enzyme. The Y55F and Y55S mutants displayed narrow substrate specificity and reduced select aromatic quinones and alpha-dicarbonyls. kcat versus pH profiles for steroid oxidoreduction catalyzed by wild-type 3alpha-HSD exhibited a single ionizable group with a pK= 7.0-7.5, which has been assigned to Tyr 55. This group was not evident in the kcat versus pH profiles for 9, 10-PQ reduction catalyzed by either wild-type or the Tyr 55 mutant enzymes, indicating that the protonation state of Tyr 55 is unimportant for 9,10-PQ turnover. Instead, wild-type and the active-site mutants Y55F, Y55S, H117A, D50N, K84R, and K84M showed the presence of a new titratable group with a pKb = 8.3-9.9. Thus, the group being titrated is not part of the tetrad. All the mutants decreased kcat/Km considerably more than they decreased kcat. Thus, the K84R mutant demonstrated a 30-fold decrease in the pH-independent value of kcat but 2200-fold decrease in the pH-independent value of kcat/Km. This suggests that all the tetrad residues influence quinone binding and that Lys 84 plays a dominant role in maintaining proper substrate orientation. Using wild-type enzyme, the energy of activation (Ea) for 9,10-PQ reduction was approximately 11 kcal/mol less than steroid oxidoreduction. The Ea for 9,10-PQ reduction was unchanged in the Tyr 55 mutants, suggesting that the reaction proceeds through the same low-energy barrier in the wild-type enzyme and these mutants. The retention of quinone reductase activity in this AKR in the absence of Tyr 55 with kcat versus pH rate profiles and activation energies identical to wild-type enzyme suggests that quinone reduction occurs via a mechanism that differs from 3-ketosteroid reduction. In this mechanism, the electron donor (NADPH) and acceptor (o-quinone) are bound in close proximity, which permits hydride transfer without formal protonation of the acceptor carbonyl by Tyr 55. This represents a rare example where one enzyme can catalyze the same chemical reaction (carbonyl reduction) by either acid catalysis or by a propinquity effect and where these two mechanisms can be discriminated by site-directed mutagenesis.

Analyses of the NRAMP1 and IFN-gammaR1 genes in women with Mycobacterium avium-intracellulare pulmonary disease

Mycobacterium avium-intracellulare (MAI) pulmonary disease causes substantial morbidity in a population of older, HIV-negative women without preexisting lung disease. The cause for disease susceptibility in these patients is unknown, although their relative phenotypic homogeneity suggests the existence of a common, subtle immune deficiency. An investigation was undertaken to determine if these patients have a defect in their natural resistance-associated macrophage protein (NRAMP1) or interferon gamma receptor 1 (IFN-gammaR1) genes. A point mutation in murine nramp, an autosomal recessive gene controlling resistance to intracellular organisms, correlates with overwhelming Mycobacterium bovis infection in mice. The corresponding region in human NRAMP1, two coding polymorphisms and one promoter NRAMP1 polymorphism, as well as two IFN-gammaR1 polymorphisms, were analyzed to determine if an allele was present to correlate with disease. Genomic DNA was purified from eight women with MAI pulmonary disease and four controls. Regions of interest were amplified by PCR; three sites were analyzed by restriction fragment length polymorphisms, and three were analyzed using denaturing high-performance liquid chromatography. The NRAMP1 promoter polymorphism of 18 additional random controls was analyzed by microsatellite sizing. No allelism was found in NRAMP1 corresponding to the murine mutation, or in the two coding regions. In the NRAMP1 promoter microsatellite, 3 of 8 patients were heterozygous for a dinucleotide sequence insertion, as were 10 of 22 controls. None of the patients had either of the two known IFN-gammaR1 mutations. In conclusion, in women with MAI pulmonary disease, there is no evidence for a genetic defect in NRAMP1 or IFN-gammaR1 to correlate with disease.

Spectroscopic and kinetic characterization of the recombinant cytochrome c reductase fragment of nitrate reductase. Identification of the rate-limiting catalytic step

The recombinant NADH-cytochrome c reductase fragment of spinach NADH-nitrate reductase (EC 1.6.6.1), consisting of the contiguous heme-containing cytochrome b domain and flavin-containing NADH-cytochrome b reductase fragment, has been characterized spectroscopically and kinetically. Reductive titration with sodium dithionite indicates heme reduction takes place prior to flavin reduction, which correlates well with the reduction potentials for enzyme-bound heme (15 mV) and FAD (-280 mV). Reductive titration with NADH also indicates that the reduced enzyme forms a charge-transfer complex with NAD+. The circular dichroism spectrum of the oxidized fragment is primarily due to the flavin, whereas the ferrous heme dominates the circular dichroism spectrum of reduced enzyme. Three kinetic phases are observed in the course of the reaction of the enzyme with NADH, each with a distinct spectral signature. The fast phase represents flavin reduction, concomitant with the formation of a charge-transfer complex between reduced flavin and NAD+, and exhibits hyperbolic dependence on NADH concentration with a Kd of 3 microM and a limiting rate constant of 560 s-1. Electron transfer from reduced flavin to heme with a rate constant of 12 s-1 is the intermediate phase, which is rate-limited by breakdown of the charge-transfer complex between NAD+ and reduced flavin. The slow phase is dismutation of a pair of molecules of two-electron reduced enzyme (generated at the end of the second phase of the reaction) to give one molecule each of one- and three- electron reduced enzyme, with a second order rate constant of 2 x 10(6) M-1 s-1. In the presence of excess NADH, this dismutation reaction is followed by the rapid reaction of the one-electron reduced enzyme with a second equivalent of NADH to generate fully reduced enzyme. On the basis of this work, it appears that dissociation of NAD+ from the reduced flavin site rate limits electron transfer to the cytochrome and likely represents the overall rate-limiting step of catalysis.

Purification of xanthine dehydrogenase and sulfite oxidase from chicken liver

Xanthine dehydrogenase and sulfite oxidase from chicken liver are oxomolybdenum enzymes which catalyze the oxidation of xanthine to uric acid and sulfite to sulfate, respectively. Independent purification protocols have been previously described for both enzymes. Here we describe a procedure by which xanthine dehydrogenase and sulfite oxidase are purified simultaneously from the same batch of fresh chicken liver. Also, unlike the protocols described earlier, this procedure avoids the use of acetone extraction as well as a heat step, thus minimizing damage to the molybdenum centers of the enzymes.

Spectroscopic and kinetic characterization of the recombinant wild-type and C242S mutant of the cytochrome b reductase fragment of nitrate reductase

Spectroscopic and kinetic studies comparing the behavior of the recombinant cytochrome b reductase fragment of corn leaf nitrate reductase and a mutant in which cysteine 242 is replaced with a serine residue (C242S) have been carried out. The visible and circular dichroism spectra of the wild-type and mutant protein are virtually identical and compare well with those reported for nitrate reductases from other sources. The reduced wild-type protein forms a charge-transfer complex with NAD+ that has an absorption envelope that extends into the near infrared, with a maximum around 800 nm. The C242S mutant forms a similar charge-transfer complex with NAD+ but to a lesser extent than the wild-type. The reduction potential of the flavin for the wild-type protein is -287 mV, and that for the mutant is -279 mV. The rate of reduction by NADH of the C242S mutant is 7-fold slower than that for the wild-type protein, and the Kd is larger by a factor of 2. These results indicate that the cysteine 242 residue plays a role principally in facilitating electron transfer from NADH to the flavin rather than in binding of NADH to the enzyme.

The molybdenum centers of xanthine oxidase and xanthine dehydrogenase. Determination of the spectral change associated with reduction from the Mo(VI) to the Mo(IV) state

The UV-visible absorbance change associated with reduction of the molybdenum centers of xanthine oxidase and xanthine dehydrogenase has been determined using a double-difference technique. At pH 8.5, the Mo(VI) minus Mo(IV) difference spectrum seen with xanthine oxidase exhibits a positive feature at 420 nm, having an extinction change of approximately 3,000 M-1 cm-1 as well as evidence for a negative feature below 340 nm. In xanthine oxidase this change is found to exhibit a marked pH dependence, implicating protonation/deprotonation events associated with changes in the molybdenum oxidation state. Application of the double-difference protocol to the respective circular dichroism spectra of xanthine oxidase and xanthine dehydrogenase reveals appreciable CD changes at 420 and 580 nm associated with the reduction of the molybdenum center. The present results demonstrate a direct spectroscopic handle on the molybdenum centers of both xanthine oxidase and xanthine dehydrogenase.

Paradoxical stabilization of the neutral flavin semiquinone of xanthine dehydrogenase at high pH

The pH dependence of the behavior of chicken liver xanthine dehydrogenase in the course of reductive titrations with sodium dithionite has been examined. Below pH 8.5, the behavior of xanthine dehydrogenase is similar to that of the much better understood milk xanthine oxidase, with the amount of neutral semiquinone accumulating transiently in the course of the titration increasing somewhat as the pH decreases. At pH 10, however, an anomalously large accumulation of the neutral semiquinone is observed by both UV/visible and EPR spectroscopy. Treatment of xanthine dehydrogenase with the thiol reagent iodoacetamide significantly diminishes the ability of the enzyme to stabilize the neutral flavin semiquinone at high pH. These data are consistent with the presence of a protein thiol in the immediate vicinity of the flavin, whose ionization above pH 8.5 results in thermodynamic stabilization of the neutral flavin semiquinone over the anionic form.