Euclein: A new binaphthaquinone from Euclea pseudebenus

7-MethyljugIone, 8,8′-dihydroxy-4,4′-dimethoxy-6,6′-dimethyl-2,2′-binaphthyl-1,1′-quinone, 2-methylnaphthazarin, mamegakinone and euclein have been isolated from Euclea pseudebenus. Euclein is the 3,6′-dimer of 7-methyljuglone.     

Rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Identification of essential sulfhydryl residues in the primary sequence of the enzyme

The kinase and sugar phosphate exchange reactions of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were inactivated by treatment with 5'-p-fluorosulfonylbenzoyladenosine or 8-azido-ATP, but activity could be restored by the addition of dithiothreitol. This inactivation was accompanied by incorporation of 5'-p-sulfonylbenzoyl[8-14C]adenosine into the enzyme that was not released by the addition of dithiothreitol. The lack of effect of ATP analogs on the ATP/ADP exchange or on bisphosphatase activity and reversal of their effects on the kinase and sugar phosphate reactions by dithiothreitol suggest that 1) they reacted with sulfhydryl groups important for sugar phosphate binding in the kinase reaction, and 2) the inactivation of the kinase by these analogs involves a specific reaction that is not related to their general mechanism of attacking nucleotide-binding sites. In addition, alkylation of the enzymes' sulfhydryls with iodoacetamide prevented inactivation by 5'-p-fluorosulfonylbenzoyladenosine, suggesting that the same thiols were involved. o-Iodosobenzoate inactivated the kinase and sugar phosphate exchange; the inactivation was reversed by dithiothreitol; but there was no effect on the bisphosphatase or nucleotide exchange, indicating that oxidation occurred at the same sulfhydryl that are associated with sugar phosphate binding. ATP or ADP, but not fructose 6-phosphate, protected these groups from modification by 5'-p-fluorosulfonylbenzoyladenosine, 8-azido-ATP, and o-iodosobenzoate. ATP also induced dramatic changes in the circular dichroism spectrum of the enzyme, suggesting that adenine nucleotide protection of thiol groups resulted from changes in enzyme secondary structure. Analysis of cyanogen bromide fragments of 14C-carboxamidomethylated enzyme showed that all radioactivity was associated with cysteinyl residues in a single cyanogen bromide fragment. Three of these cysteinyl residues are clustered in a 38-residue region, which probably plays a role in maintaining the conformation of the kinase sugar phosphate-binding site.     

Oxidative metabolism of spironolactone: evidence for the involvement of electrophilic thiosteroid species in drug-mediated destruction of rat hepatic cytochrome P450

In a preliminary paper [Decker et al. (1986) Biochem. Biophys. Res. Commun. 136, 1162] we have shown that the antimineralocorticoid spironolactone (SPL) preferentially inactivates dexamethasone (DEX) inducible rat hepatic cytochrome P450p isozymes in a suicidal manner. These findings are now confirmed, and the kinetic characteristics of such a process are detailed. In an effort to elucidate the mechanism of SPL-mediated inactivation of cytochrome P450, we have examined the metabolism of SPL in vitro. Incubation of [14C]SPL and NADPH with liver microsomes prepared from DEX-pretreated rats results in the formation of several polar metabolites separable by HPLC with UV detection. This process is found to be dependent on NADPH, O2, SPL, and enzyme concentration, as well as temperature. Furthermore, metabolite formation was significantly attenuated by P450 inhibitors CO and n-octylamine. Mass spectral analysis (thermospray LC/MS, FAB/MS, and FAB/MS/MS) of the two most prominent polar metabolites indicated that these compounds had molecular weights that corresponded to the sulfinic and sulfonic acid derivatives of deacetyl-SPL (SPL-SH). These findings document the formation of previously unreported polar metabolites of SPL by rat liver microsomes enriched in cytochrome P450p and implicate a role for this isozyme in the oxidation of the thiol moiety of deacetyl-SPL. The detection of such metabolites also implicates a catalytic trajectory that includes the thiyl radical and/or sulfenic acid species as a plausible protagonist in drug-mediated inactivation of cytochrome P450p.     

Role of gastrin/pentagastrin in regulation of intestinal cytochrome P-450

1. In the absence of intraluminal inducers, low "basal" levels of cytochrome P-450 and its dependent MFO activities are detected in the rat intestinal mucosa, and may be regulated by endogenous hormones. 2. Rats were nutritionally maintained by either short term (48 hr) intravenous glucose infusion or chronic (8 days) intravenous hyperalimentation, and were treated with various doses of pentagastrin in the infusate. 3. Regardless of the dose (6-90 micrograms/kg/hr) or duration of infusion (2-8 days), pentagastrin had no effect on small intestinal cytochrome P-450, its dependent MFO activity, or the activity of delta-aminolevulinic acid synthetase. 4. The intestinal trophic peptide hormone, gastrin, apparently does not regulate the cytochrome P-450-dependent MFO system of the small intestine.     

Differential haemin-mediated restoration of cytochrome P-450 N-demethylases after inactivation by allylisopropylacetamide.

Administration of allylisopropylacetamide (AIA) to phenobarbital-pretreated rats results in the destruction of several phenobarbital-inducible cytochrome P-450 isoenzymes and a correspondingly marked loss of benzphetamine N-demethylase and ethylmorphine N-demethylase activities. Accordingly, the ion-exchange h.p.l.c. or DEAE-cellulose-chromatographic profile of solubilized microsomal preparations from such rats revealed a marked decrease in the cytochrome P-450 content of several eluted fractions compared with that of microsomes from corresponding non-AIA-treated controls. Incubation of liver homogenates from such rats with haemin restores not only cytochrome P-450 content from 35 to 62% of original values, but also benzphetamine N-demethylase and ethylmorphine N-demethylase activities, from 23 to 67%, and from 12 to 36% of original values respectively. Moreover, the chromatographic profiles of microsomes prepared from such homogenates indicated increases of cytochrome P-450 content only in some fractions. Reconstitution of mixed-function oxidase activity of cytochrome P-450 by addition of NADPH: cytochrome P-450 reductase to these fractions indicated that incubation with haemin restored benzphetamine N-demethylase activity predominantly, but ethylmorphine N-demethylase activity only minimally. After injection of [14C]AIA, a significant amount of radiolabel was found covalently bound to protein in chromatographic fraction III, and this binding was unaffected by incubation with haemin. Furthermore, the extent of this binding is apparently equimolar to the amount of cytochrome P-450 refractory to haemin reconstitution in that particular fraction. Whether such refractoriness reflects structural inactivation of the apo-cytochrome remains to be determined. Nevertheless, the evidence presented very strongly argues for AIA-mediated inactivation of multiple phenobarbital-induced isoenzymes, only a few of which are structurally and functionally reparable by haemin.     

γ-Aminobutyric Acid Turnover in Rat Striatum: Effects of Glutamate and Kainic Acid

The turnover rate of gamma-aminobutyric acid (GABA) in the rat striatum was estimated by measuring its accumulation after inhibition of GABA-transaminase (GABA-T) with gabaculine. Intrastriatal injections of 100 micrograms gabaculine induced a rapid and complete inhibition of GABA-T. GABA accumulation was linear with time for at least 60 min (estimated turnover rate = 25 nmol/mg protein/h). The accumulation of GABA after gabaculine administration in animals that had been treated with kainic acid (5 nmol intrastriatally, 7 days) was only 40% of the control value, indicating that a major fraction of the net increase in GABA content induced by gabaculine originates in kainic acid-sensitive neurons. Intrastriatal injection of a mixture of kainic acid (5 nmol) and gabaculine caused a net increase in striatal GABA content significantly greater than that observed in controls, suggesting that neuronal death induced by kainic acid is preceded by a period of increased neuronal activity. Glutamic acid, the putative neurotransmitter for the excitatory corticostriatal pathway, also produced a significant increase in striatal GABA accumulation when injected together with gabaculine. This effect was blocked by the administration of the glutamate receptor antagonist glutamic acid diethyl ester. The interactions between GABAergic neurons and other neurotransmitters present in the striatum were also analyzed.     

Substrate and metal specificity in the enzymic synthesis of cyclic monoterpenes from geranyl and neryl pyrophosphate

A partially purified enzyme (carbocyclase) from the flavedo of Citrus limonum formed α-pinene, β-pinene, limonene, and γ-terpinene from geranyl pyrophosphate (GPP) and neryl pyrophosphate. The maximum specific activities obtained were 7.0 and 3.6 nmol/ min/mg, respectively. Cross-inhibition by the two substrates were observed and the ability to utilize neryl pyrophosphate was almost completely lost with aging. Citronellyl pyrophosphate and dimethylallyl pyrophosphate were the most effective inhibitors of carbocyclase. Isopentenyl pyrophosphate, the monophosphate esters of nerol and geraniol, as well as inorganic pyrophosphate were much less effective inhibitors. The enzyme had an absolute requirement for Mn²⁺. It could be replaced with about 2% effectiveness by Mg²⁺ and Co²⁺. Kinetic studies showed that the observed reaction rate correlates with the calculated concentration of the GPP (Mn²⁺)₂ species. Previous evidence with nonenzymatic reactions and the results presented support the view that the mechanism of carbocyclase may be the intramolecular analog of prenyltransferase.     

Enzymic aspects of the biosynthesis monoterpenes in plants

The biosynthesis of monoterpenes in higher plants is reviewed, with particular emphasis on recent studies of the enzymology of biosynthesis.