Stoichiometry of the reaction between horseradish peroxidase and p-cresol.

Over a wide range of pH horseradish peroxidase compound I can be reduced quantitatively via compound II to the native enzyme by only 1 molar equivalent of p-cresol. Since 2 molar equivalents of electrons are required for the single turnover of the enzymatic cycle, p-cresol behaves as a 2-electron reductant. With p-cresol and compound I in a 1:1 ratio compound II and p-methylphenoxy radicals are obtained in the transient state. Compound II is then reduced to the native enzyme. A possible explanation for the facile reduction of compound II involves reaction with the dimerization product of these radicals, 1/2 molar equivalent of 2,2'-dihydroxy-5,5'-dimethylbiphenyl. If only 1/2 molar equivalent of p-cresol is present, than at high pH the reduction stops at compound II. The major steady state peroxidase oxidation product of p-cresol (with p-cresol in large excess compared to the enzyme concentration) is Pummerer's ketone. Pummerer's ketone is only reactive at pH values greater than about 9 where significant amounts of the enol can be formed via the enolate anion. Therefore, in alkaline solution it is reactive with compound I, but not with compound II, which is converted into an unreactive basic form. These results indicate that Pummerer's ketone cannot be the intermediate free radical product responsible for reducing compound II in the single turnover experiments. It is postulated that Pummerer's ketone is formed only in the steady state by the reaction of the p-methylphenoxy radical with excess p-cresol.     

Paracrine effects of oocyte secreted factors and stem cell factor on porcine granulosa and theca cells <Emphasis Type="Italic">in vitro</Emphasis>

Oocyte control of granulosa and theca cell function may be mediated by several growth factors via a local feedback loop(s) between these cell types. This study examined both the role of oocyte-secreted factors on granulosa and thecal cells, cultured independently and in co-culture, and the effect of stem cell factor (SCF); a granulosa cell derived peptide that appears to have multiple roles in follicle development. Granulosa and theca cells were isolated from 2–6 mm healthy follicles of mature porcine ovaries and cultured under serum-free conditions, supplemented with: 100 ng/ml LR3 IGF-1, 10 ng/ml insulin, 100 ng/ml testosterone, 0–10 ng/ml SCF, 1 ng/ml FSH (granulosa), 0.01 ng/ml LH (theca) or 1 ng/ml FSH and 0.01 ng/ml LH (co-culture) and with/without oocyte conditioned medium (OCM) or 5 oocytes. Cells were cultured in 96 well plates for 144 h, after which viable cell numbers were determined. Medium was replaced every 48 h and spent medium analysed for steroids.     

Tricyclic aminopyrimidine histamine H4 receptor antagonists

This report discloses the development of a series of tricyclic histamine H(4) receptor antagonists. Starting with a low nanomolar benzofuranopyrimidine HTS hit devoid of pharmaceutically acceptable properties, we navigated issues with metabolism and solubility to furnish a potent, stable and water soluble tricyclic histamine H(4) receptor antagonist with desirable physiochemical parameters which demonstrated efficacy a mouse ova model.     

Analysis of the oxidation of short chain alkynes by flavocytochrome P450 BM3

Bacillus megaterium flavocytochrome P450 BM3 (BM3) is a high activity fatty acid hydroxylase, formed by the fusion of soluble cytochrome P450 and cytochrome P450 reductase modules. Short chain (C6, C8) alkynes were shown to be substrates for BM3, with productive outcomes (i.e. alkyne hydroxylation) dependent on position of the carbon-carbon triple bond in the molecule. Wild-type P450 BM3 catalyses ω-3 hydroxylation of both 1-hexyne and 1-octyne, but is suicidally inactivated in NADPH-dependent turnover with non-terminal alkynes. A F87G mutant of P450 BM3 also undergoes turnover-dependent heme destruction with the terminal alkynes, pointing to a key role for Phe87 in controlling regioselectivity of alkyne oxidation. The terminal alkynes access the BM3 heme active site led by the acetylene functional group, since hydroxylated products are not observed near the opposite end of the molecules. For both 1-hexyne and 1-octyne, the predominant enantiomeric product formed (up to ～90%) is the (S)-(-)-1-alkyn-3-ol form. Wild-type P450 BM3 is shown to be an effective oxidase catalyst of terminal alkynes, with strict regioselectivity of oxidation and potential biotechnological applications. The absence of measurable octanoic or hexanoic acid products from oxidation of the relevant 1-alkynes is also consistent with previous studies suggesting that removal of the phenyl group in the F87G mutant does not lead to significant levels of ω-oxidation of alkyl chain substrates.     

The mechanism of Compound I formation revisited

The most recently proposed mechanisms for the formation of the Compound I intermediates of the peroxidases and catalases have been based on the crystallographic elucidation of the enzyme structures. It has been assumed that these mechanisms are compatible with an earlier proposal of the formation of a reversible enzyme-substrate intermediate called Compound 0, which was based on data that pre-dated the availability of the enzyme structures. However, it is argued here that this is not the case and some modifications of the existing mechanism are proposed which reconcile the structural, kinetic and energetic data for the reactions. This paper focuses attention on horseradish peroxidase isoenzyme C and particularly on the acid-base properties of the imidazole side chain of distal histidine 42. This imidazole group has an exceptionally low pK(a) value in the resting enzyme, which is higher in Compound I and higher still in Compound II. The pK(a) value must also be greatly increased following Compound 0 formation so that the imidazole can become an effective proton acceptor. An explanation is offered in a dielectric insertion (DI) model, in which the peroxide substrate, or fragments thereof, screens the influence of the positively charged heme iron on the pK(a) value of the imidazole group. It is proposed that Compound 0 is converted to a second intermediate, Compound 0*, by intramolecular proton transfer along a pre-existing hydrogen bond, a process which reduces the energy requirements of charge separation in the deprotonation of hydrogen peroxide.     

Phylogenetic analysis of the tenascin gene family: evidence of origin early in the chordate lineage

Background  

Tenascins are a family of glycoproteins found primarily in the extracellular matrix of embryos where they help to regulate cell proliferation, adhesion and migration. In order to learn more about their origins and relationships to each other, as well as to clarify the nomenclature used to describe them, the tenascin genes of the urochordate Ciona intestinalis, the pufferfish Tetraodon nigroviridis and Takifugu rubripes and the frog Xenopus tropicalis were identified and their gene organization and predicted protein products compared with the previously characterized tenascins of amniotes.     

Kinetics of oxidation of serotonin by myeloperoxidase compounds I and II.

The oxidation of serotonin (5-hydroxytryptamine) by the myeloperoxidase intermediates compounds I and II was investigated by using transient-state spectral and kinetic measurements at 25.0 +/- 0.1 degrees C. Rapid scan spectra demonstrated that both compound I and compound II oxidize serotonin via one-electron processes. Rate constants for these reactions were determined using both sequential-mixing and single-mixing stopped-flow techniques. The second order rate constant obtained for the one-electron reduction of compound I to compound II by serotonin is (1.7 +/- 0.1) x 10(7) M(-1) x s(-1), and that for compound II reduction to native enzyme is (1.4 +/- 0.1) x 10(6) M(-1) x s(-1) at pH 7.0. The maximum pH of the compound I reaction with serotonin occurs in the pH range 7.0-7.5. At neutral pH, the rate constant for myeloperoxidase compound I reacting with serotonin is an order of magnitude larger than for its reaction with chloride, (2.2 +/- 0.2) x 10(6) M(-1) x s(-1). A direct competition of serotonin with chloride for myeloperoxidase compound I oxidation was observed. Our results suggest that serotonin may have a role to protect lipoproteins from oxidation and to prevent enzymes from inactivation caused by the potent oxidants HOCl and active oxygen species.     

Molecular characterization of a novel geranylgeranyl pyrophosphate synthase from Plasmodium parasites

We present here a study of a eukaryotic trans-prenylsynthase from the malaria pathogen Plasmodium vivax. Based on the results of biochemical assays and contrary to previous indications, this enzyme catalyzes the production of geranylgeranyl pyrophosphate (GGPP) rather than farnesyl pyrophosphate (FPP). Structural analysis shows that the product length is constrained by a hydrophobic cavity formed primarily by a set of residues from the same subunit as the product as well as at least one other from the dimeric partner. Furthermore, Plasmodium GGPP synthase (GGPPS) can bind nitrogen-containing bisphosphonates (N-BPs) strongly with the energetically favorable cooperation of three Mg(2+), resulting in inhibition by this class of compounds at IC(50) concentrations below 100 nM. In contrast, human and yeast GGPPSs do not accommodate a third magnesium atom in the same manner, resulting in their insusceptibility to N-BPs. This differentiation is in part attributable to a deviation in a conserved motif known as the second aspartate-rich motif: whereas the aspartates at the start and end of the five-residue motif in FFPP synthases and P. vivax GGPPSs both participate in the coordination of the third Mg(2+), an asparagine is featured as the last residue in human and yeast GGPPSs, resulting in a different manner of interaction with nitrogen-containing ligands.     

Lactate metabolism in potato tubers deficient in lactate dehydrogenase activity

The aim of this work was to investigate the effect of decreased activity of lactate dehydrogenase (EC 1.1.1.27; LDH) on lactate metabolism in potato tubers. By expressing a cDNA‐encoding potato tuber LDH in the antisense orientation, we generated transgenic potato plants with a preferential decrease in two of the five isozymes of LDH. Surprisingly, transgenic tubers grown under normoxic conditions did not contain less lactate, but rather instead contained approximately two‐fold more lactate than control tubers. This result is explicable if the decreased isozymes of LDH are responsible for the oxidation of lactate to pyruvate in vivo. This was confirmed by measurements of the rate of metabolism of lactate supplied to tuber discs: the rate in transgenic tubers was approximately half that of control tubers. The decrease in LDH activity had no measurable effect on the accumulation of lactate in cold‐stored tubers under anoxia, nor during the subsequent utilization of this lactate upon return to normoxia. In both control and transgenic tubers, the accumulation of lactate during anoxia was not accompanied by an induction of LDH activity or a change in isozyme distribution. In contrast, the metabolism of lactate after a period of anoxia was accompanied by a two‐fold increase in LDH activity and the induction of two isozymes that were distinct from those which had been decreased in the transgenic plants.