Substrate recognition principles for the PP2A-B55 protein phosphatase

The PP2A-B55 phosphatase regulates a plethora of signaling pathways throughout eukaryotes. How PP2A-B55 selects its substrates presents a severe knowledge gap. By integrating AlphaFold modelling with comprehensive high resolution mutational scanning, we show that α-helices in substrates bind B55 through an evolutionary conserved mechanism. Despite a large diversity in sequence and composition, these α-helices share key amino acid determinants that engage discrete hydrophobic and electrostatic patches. Using deep learning protein design, we generate a specific and potent competitive peptide inhibitor of PP2A-B55 substrate interactions. With this inhibitor, we uncover that PP2A-B55 regulates the nuclear exosome targeting complex by binding to an α-helical recruitment module in RBM7. Collectively, our findings provide a framework for the understanding and interrogation of PP2A-B55 in health and disease.

Structural basis for competitive binding of productive and degradative co-transcriptional effectors to the nuclear cap-binding complex

The nuclear cap-binding complex (CBC) coordinates co-transcriptional maturation, transport, or degradation of nascent RNA polymerase II (Pol II) transcripts. CBC with its partner ARS2 forms mutually exclusive complexes with diverse "effectors" that promote either productive or destructive outcomes. Combining AlphaFold predictions with structural and biochemical validation, we show how effectors NCBP3, NELF-E, ARS2, PHAX, and ZC3H18 form competing binary complexes with CBC and how PHAX, NCBP3, ZC3H18, and other effectors compete for binding to ARS2. In ternary CBC-ARS2 complexes with PHAX, NCBP3, or ZC3H18, ARS2 is responsible for the initial effector recruitment but inhibits their direct binding to the CBC. We show that in vivo ZC3H18 binding to both CBC and ARS2 is required for nuclear RNA degradation. We propose that recruitment of PHAX to CBC-ARS2 can lead, with appropriate cues, to competitive displacement of ARS2 and ZC3H18 from the CBC, thus promoting a productive rather than a degradative RNA fate.

Dual agonistic and antagonistic roles of ZC3H18 provide for co-activation of distinct nuclear RNA decay pathways

The RNA exosome is a versatile ribonuclease. In the nucleoplasm of mammalian cells, it is assisted by its adaptors the nuclear exosome targeting (NEXT) complex and the poly(A) exosome targeting (PAXT) connection. Via its association with the ARS2 and ZC3H18 proteins, NEXT/exosome is recruited to capped and short unadenylated transcripts. Conversely, PAXT/exosome is considered to target longer and adenylated substrates via their poly(A) tails. Here, mutational analysis of the core PAXT component ZFC3H1 uncovers a separate branch of the PAXT pathway, which targets short adenylated RNAs and relies on a direct ARS2-ZFC3H1 interaction. We further demonstrate that similar acidic-rich short linear motifs of ZFC3H1 and ZC3H18 compete for a common ARS2 epitope. Consequently, while promoting NEXT function, ZC3H18 antagonizes PAXT activity. We suggest that this organization of RNA decay complexes provides co-activation of NEXT and PAXT at loci with abundant production of short exosome substrates.

ARS2 instructs early transcription termination-coupled RNA decay by recruiting ZC3H4 to nascent transcripts

The RNA-binding ARS2 protein is centrally involved in both early RNA polymerase II (RNAPII) transcription termination and transcript decay. Despite its essential nature, the mechanisms by which ARS2 enacts these functions have remained unclear. Here, we show that a conserved basic domain of ARS2 binds a corresponding acidic-rich, short linear motif (SLiM) in the transcription restriction factor ZC3H4. This interaction recruits ZC3H4 to chromatin to elicit RNAPII termination, independent of other early termination pathways defined by the cleavage and polyadenylation (CPA) and Integrator (INT) complexes. We find that ZC3H4, in turn, forms a direct connection to the nuclear exosome targeting (NEXT) complex, hereby facilitating rapid degradation of the nascent RNA. Hence, ARS2 instructs the coupled transcription termination and degradation of the transcript onto which it is bound. This contrasts with ARS2 function at CPA-instructed termination sites where the protein exclusively partakes in RNA suppression via post-transcriptional decay.

Structure and regulation of the nuclear exosome targeting complex guides RNA substrates to the exosome

In mammalian cells, spurious transcription results in a vast repertoire of unproductive non-coding RNAs, whose deleterious accumulation is prevented by rapid decay. The nuclear exosome targeting (NEXT) complex plays a central role in directing non-functional transcripts to exosome-mediated degradation, but the structural and molecular mechanisms remain enigmatic. Here, we elucidated the architecture of the human NEXT complex, showing that it exists as a dimer of MTR4-ZCCHC8-RBM7 heterotrimers. Dimerization preconfigures the major MTR4-binding region of ZCCHC8 and arranges the two MTR4 helicases opposite to each other, with each protomer able to function on many types of RNAs. In the inactive state of the complex, the 3′ end of an RNA substrate is enclosed in the MTR4 helicase channel by a ZCCHC8 C-terminal gatekeeping domain. The architecture of a NEXT-exosome assembly points to the molecular and regulatory mechanisms with which the NEXT complex guides RNA substrates to the exosome.

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NEXT homodimerizes through two intertwined ZCCHC8 subunits

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ZCCHC8 binds MTR4 with both constitutive and regulatory interactions

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Stable MTR4 arch interactions orient the two helicases in opposite directions

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Regulatory interactions at the MTR4 helicase domain guide RNA to the exosome

Chromatin modifier HUSH co-operates with RNA decay factor NEXT to restrict transposable element expression

Transposable elements (TEs) are widespread genetic parasites known to be kept under tight transcriptional control. Here, we describe a functional connection between the mouse-orthologous “nuclear exosome targeting” (NEXT) and “human silencing hub” (HUSH) complexes, involved in nuclear RNA decay and the epigenetic silencing of TEs, respectively. Knocking out the NEXT component ZCCHC8 in embryonic stem cells results in elevated TE RNA levels. We identify a physical interaction between ZCCHC8 and the MPP8 protein of HUSH and establish that HUSH recruits NEXT to chromatin at MPP8-bound TE loci. However, while NEXT and HUSH both dampen TE RNA expression, their activities predominantly affect shorter non-polyadenylated and full-length polyadenylated transcripts, respectively. Indeed, our data suggest that the repressive action of HUSH promotes a condition favoring NEXT RNA decay activity. In this way, transcriptional and post-transcriptional machineries synergize to suppress the genotoxic potential of TE RNAs.

Garland et al. report a physical and functional connection between the NEXT complex, involved in RNA decay, and the HUSH complex, involved in chromatin regulation. Together, NEXT and HUSH cooperate to control transposable element (TE) RNA expression in embryonic stem cells, suppressing pA⁻ and pA⁺ transcripts, respectively.

A Functional Link between Nuclear RNA Decay and Transcriptional Control Mediated by the Polycomb Repressive Complex 2

Pluripotent embryonic stem cells (ESCs) constitute an essential cellular niche sustained by epigenomic and transcriptional regulation. Any role of post-transcriptional processes remains less explored. Here, we identify a link between nuclear RNA levels, regulated by the poly(A) RNA exosome targeting (PAXT) connection, and transcriptional control by the polycomb repressive complex 2 (PRC2). Knockout of the PAXT component ZFC3H1 impairs mouse ESC differentiation. In addition to the upregulation of bona fide PAXT substrates, Zfc3h1^−/− cells abnormally express developmental genes usually repressed by PRC2. Such de-repression is paralleled by decreased PRC2 binding to chromatin and low PRC2-directed H3K27 methylation. PRC2 complex stability is compromised in Zfc3h1^−/− cells with elevated levels of unspecific RNA bound to PRC2 components. We propose that excess RNA hampers PRC2 function through its sequestration from DNA. Our results highlight the importance of balancing nuclear RNA levels and demonstrate the capacity of bulk RNA to regulate chromatin-associated proteins.

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Depletion of ZFC3H1 in mouse ESCs results in differentiation defects

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PRC2 target genes are deregulated in Zfc3h1^−/− cells

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Chromatin binding of PRC2 and H3K27me3 is reduced in Zfc3h1^−/− cells

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Increased binding of RNA impairs PRC2 complex stability

The human ZC3H3 and RBM26/27 proteins are critical for PAXT-mediated nuclear RNA decay

Recruitment of the human ribonucleolytic RNA exosome to nuclear polyadenylated (pA+) RNA is facilitated by the Poly(A) Tail eXosome Targeting (PAXT) connection. Besides its core dimer, formed by the exosome co-factor MTR4 and the ZFC3H1 protein, the PAXT connection remains poorly defined. By characterizing nuclear pA+-RNA bound proteomes as well as MTR4-ZFC3H1 containing complexes in conditions favoring PAXT assembly, we here uncover three additional proteins required for PAXT function: ZC3H3, RBM26 and RBM27 along with the known PAXT-associated protein, PABPN1. The zinc-finger protein ZC3H3 interacts directly with MTR4-ZFC3H1 and loss of any of the newly identified PAXT components results in the accumulation of PAXT substrates. Collectively, our results establish new factors involved in the turnover of nuclear pA+ RNA and suggest that these are limiting for PAXT activity.

Abstract B08: ETC inhibitors alter oncogenic KRAS signal transduction

KRAS proteins regulate many cellular processes and gain-of-function KRAS mutations constitutively activate effector signal transduction pathways independent of mitogenic stimuli. Oncogenic KRAS-driven cancers also frequently have high glycolytic rates and may have increased sensitivity to metabolic inhibitors. In this study, we characterize a small molecule found to be active in a drug screen that uses transgenic Drosophila melanogaster expressing human KRASG12V in the wing to assess kRAS suppressing activity. The compound partially restored normal wing development in the KRASG12V flies and inhibited growth and signal transduction in multiple oncogenic KRAS-driven cell lines. It also inhibited Complex 1 of the electron transport chain (ETC). Increased glycolysis or addition of exogenous aspartate was sufficient to rescue KRAS function and cellular growth after ETC inhibition. These data indicate that oncogenic KRAS signal transduction requires high cellular ATP and amino acid biosynthesis and suggest a synthetic lethal interaction between KRAS and metabolic targets. We also describe co-mutations that sensitize KRAS-driven cell lines to OxPhos Inhibition and show how differential OxPhos inhibitor sensitivity in lung and pancreatic cancer cell lines can correlate with low and high glycolysis ssGSEA gene signatures.

Citation Format: Kanika Sharma, Ming Yi, Giovanna Grandinetti, Ashish B. Chougule, Philip Liaw, Stephen Yanofsky, Solomon Ungashe, Jeffery H. Thomas, William Garland, Matthew Holderfield. ETC inhibitors alter oncogenic KRAS signal transduction [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr B08.

Nuclear sorting of RNA

The majority of the mammalian genome is transcribed by RNA polymerase II, yielding a vast amount of noncoding RNA (ncRNA) in addition to the standard production of mRNA. The typical nuclear biogenesis of mRNA relies on the tightly controlled coupling of co- and post-transcriptional processing events, which ultimately results in the export of transcripts into the cytoplasm. These processes are subject to surveillance by nuclear RNA decay pathways to prevent the export of aberrant, or otherwise "non-optimal," transcripts. However, unlike mRNA, many long ncRNAs are nuclear retained and those that maintain enduring functions must employ precautions to evade decay. Proper sorting and localization of RNA is therefore an essential activity in eukaryotic cells and the formation of ribonucleoprotein complexes during early stages of RNA synthesis is central to deciding such transcript fate. This review details our current understanding of the pathways and factors that direct RNAs towards a particular destiny and how transcripts combat the adverse conditions of the nucleus. This article is categorized under: RNA Export and Localization > Nuclear Export/Import RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Rrp47 functions in RNA surveillance and stable RNA processing when divorced from the exoribonuclease and exosome-binding domains of Rrp6

The eukaryotic exosome exoribonuclease Rrp6 forms a complex with Rrp47 that functions in nuclear RNA quality control mechanisms, the degradation of cryptic unstable transcripts (CUTs), and in the 3' end maturation of stable RNAs. Stable expression of Rrp47 is dependent upon its interaction with the N-terminal domain of Rrp6 (Rrp6NT). To address the function of Rrp47 independently of Rrp6, we developed a DECOID (decreased expression of complexes by overexpression of interacting domains) strategy to resolve the Rrp6/Rrp47 complex in vivo and employed mpp6Δ and rex1Δ mutants that are synthetic lethal with loss-of-function rrp47 mutants. Strikingly, Rrp47 was able to function in mpp6Δ and rex1Δ mutants when separated from the catalytic and exosome-binding domains of Rrp6, whereas a truncated Rrp47 protein lacking its C-terminal region caused a block in cell growth. Northern analyses of the conditional mutants revealed a specific block in the 3' maturation of box C/D snoRNAs in the rex1 rrp47 mutant and widespread inhibition of Rrp6-mediated RNA surveillance processes in the mpp6 rrp47 mutant. In contrast, growth analyses and RNA northern blot hybridization analyses showed no effect on the rrp47Δ mutant upon overexpression of the Rrp6NT domain. These findings demonstrate that Rrp47 and Rrp6 have resolvable functions in Rrp6-mediated RNA surveillance and processing pathways. In addition, this study reveals a redundant requirement for Rrp6 or Rex1 in snoRNA maturation and demonstrates the effective use of the DECOID strategy for the resolution and functional analysis of protein complexes.

The exosome cofactor Rrp47 is critical for the stability and normal expression of its associated exoribonuclease Rrp6 in Saccharomyces cerevisiae

Rrp6 is a conserved catalytic subunit of the eukaryotic nuclear exosome ribonuclease complex that functions in the productive 3' end maturation of stable RNAs, the degradation of transiently expressed noncoding transcripts and in discard pathways that eradicate the cell of incorrectly processed or assembled RNAs. The function of Rrp6 in these pathways is at least partially dependent upon its interaction with a small nuclear protein called Rrp47/Lrp1, but the underlying mechanism(s) by which Rrp47 functions in concert with Rrp6 are not established. Previous work on yeast grown in rich medium has suggested that Rrp6 expression is not markedly reduced in strains lacking Rrp47. Here we show that Rrp6 expression in rrp47∆ mutants is substantially reduced during growth in minimal medium through effects on both transcript levels and protein stability. Exogenous expression of Rrp6 enables normal levels to be attained in rrp47∆ mutants. Strikingly, exogenous expression of Rrp6 suppresses many, but not all, of the RNA processing and maturation defects observed in an rrp47∆ mutant and complements the synthetic lethality of rrp47∆ mpp6∆ and rrp47∆ rex1∆ double mutants. Increased Rrp6 expression in the resultant rrp47∆ rex1∆ double mutant suppresses the defect in the 3' maturation of box C/D snoRNAs. In contrast, increased Rrp6 expression in the rrp47∆ mpp6∆ double mutant diminishes the block in the turnover of CUTs and in the degradation of the substrates of RNA discard pathways. These results demonstrate that a principal function of Rrp47 is to facilitate appropriate expression levels of Rrp6 and support the conclusion that the Rrp6/Rrp47 complex and Rex1 provide redundant exonuclease activities for the 3' end maturation of box C/D snoRNAs.

Abstract 758: Studies in cancer models with AGX51, a Direct Transcriptional RegulatorTM

Studies in cancer models with AGX51, a Direct Transcriptional RegulatorTM

The inhibitors of differentiation (Id) proteins (Id1, Id2, Id3, and Id4) are negative regulators of differentiation that act by sequestering basic helix loop helix (bHLH, e.g. E47) transcription factors. Ids are highly expressed during embryonic development but in adult tissues their expression is rare to absent. However, Id proteins are required for tumor angiogenesis, highly expressed in many cancers and as shown in many clinical studies, associated with an aggressive phenotype and a poor clinical outcome As a proof of concept, decreased angiogenesis and tumor load as a result of blocking Id expression by antisense and siRNA molecules demonstrated that Id is a compelling anti-cancer target. Despite these findings, the Ids are a heretofore unexplored target for the treatment of cancer. While no anti-Id agent has been studied clinically or has undergone preclinical development, AGX recently discovered and patented the first unique and potent anti-Id agent, AGX51. This small molecule is an inhibitor of the Id1-E47 interaction and was discovered through a systematic research effort using x-ray crystallography, gel shift (EMSA) assays, Matrigel evaluations and xenograft studies. AGX51 is the founding member of a new class of therapeutics: Direct Transcriptional RegulatorsTM.

In preclinical studies: (1) At a dose below 7 mg/kg, bid, AGX51 significantly blocked the growth of tumors in mice with implanted human breast cancer cells when treated briefly with a taxane like Taxol®, paclitaxel, or Taxotere®, docetaxel. This effect of AGX51 was expected based on the discovery that endothelial cell production in the bone marrow is responsible for the rapid repair to damaged vasculature after administration of paclitaxel or vascular disrupters such as ZD6126 or AVE8062, through mobilization of endothelial precursor cells (EPCs) followed by homing of the EPCs to the damaged vasculature; (2) At a dose below 20 nM, AGX51 restored cell cycle control in DU-145 human prostate cancer cells; (3) At low micromolar concentrations, AGX51 blocked migration of DU-145 cancer cells in “scratch” assays; (4) As a single agent, AGX51 completely blocked tumor associated angiogenesis in nude mice implanted with MDA-MB231 human breast cancer cells at a dose of 60 mg/kg, bid; (5) As expected, AGX51 restored levels of p21 and p27, two important mediators of cell cycle regulation in both PC3 and DU145 cancer cells; (6) AGX51 caused no toxicity in multiple day dosing based on weight, clinical chemistry or hematology measurements; (7) AGX51 has a terminal elimination half-life in mice of approximately 6 hours, which is suggestive of ultimate bid and perhaps even qd dosing in man. In summary, a specific inhibitor of Id-E47 interaction that has anti-tumor activity is described for the first time. This research was supported by AngioGenex, Inc.

Citation Format: William Garland, Richard Salvador, Jaideep Chaudhary. Studies in cancer models with AGX51, a Direct Transcriptional RegulatorTM. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 758. doi:10.1158/1538-7445.AM2013-758

Anti-sphingosine-1-phosphate monoclonal antibodies inhibit angiogenesis and sub-retinal fibrosis in a murine model of laser-induced choroidal neovascularization

The efficacy of novel monoclonal antibodies that neutralize the pro-angiogenic mediator, sphingosine-1-phosphate (S1P), were tested using in vitro and in vivo angiogenesis models, including choroidal neovascularization (CNV) induced by laser disruption of Bruch's membrane. S1P receptor levels in human brain choroid plexus endothelial cells (CPEC), human lung microvascular endothelial cells, human retinal vascular endothelial cells, and circulating endothelial progenitor cells were examined by semi-quantitative PCR. The ability of murine or humanized anti-S1P monoclonal antibodies (mAbs) to inhibit S1P-mediated microvessel tube formation by CPEC on Matrigel was evaluated and capillary density in subcutaneous growth factor-loaded Matrigel plugs was determined following anti-S1P treatment. S1P promoted in vitro capillary tube formation in CPEC consistent with the presence of cognate S1P(1-5) receptor expression by these cells and the S1P antibody induced a dose-dependent reduction in microvessel tube formation. In a murine model of laser-induced rupture of Bruch's membrane, S1P was detected in posterior cups of mice receiving laser injury, but not in uninjured controls. Intravitreous injection of anti-S1P mAbs dramatically inhibited CNV formation and sub-retinal collagen deposition in all treatment groups (p<0.05 compared to controls), thereby identifying S1P as a previously unrecognized mediator of angiogenesis and subretinal fibrosis in this model. These findings suggest that neutralizing S1P with anti-S1P mAbs may be a novel method of treating patients with exudative age-related macular degeneration by reducing angiogenesis and sub-retinal fibrosis, which are responsible for visual acuity loss in this disease.

Distribution of 1-aminobenzotriazole in male rats after administration of an oral dose

1. 14C-labelled 1-aminobenzotriazole (ABT), a suicide inactivator of cytochrome P450, was synthesized and administered orally to male rats. The rats were killed at 1, 6, 24, 48 or 72 h after dosing and the concentration of total radioactivity in various tissues and organs measured. 2. The compound appears to be absorbed slowly with 50% of the radioactivity remaining in the stomach at 6 h after dosing and maximum plasma and tissue concentrations were observed at 24 h. 3. Approximately 71% of the dose of 14C was excreted in the urine and 12% in the faeces over 72 h, indicating oral absorption of at least 71%. Tissue-to-plasma ratios of 14C were highest in the liver, adrenals and kidneys, which all contain significant amounts of cytochrome P450; the half-lives of elimination for total 14C in liver, adrenals and kidneys were approximately 24, 16 and 12 h, respectively, while the half-life in plasma was approximately 9 h. 4. ABT was metabolized by N-acetylation, with the acetylated product attaining concentrations equal to ABT in the plasma; two other major metabolites were also excreted in the urine namely, the N-glucuronide of 1-aminobenzotriazole and the N-glucuronide of benzotriazole.

The metabolism of acitretin and isoacitretin in the in situ isolated perfused rat liver

1. The metabolism of acitretin and its 13-cis isomer, isoacitretin, has been investigated in the in situ isolated perfused rat liver in order to differentiate the action of the liver from that of the gut on the metabolism of these isomers. 2. Acitretin undergoes alpha-oxidation, chain shortening O-demethylation, and glucuronidation in the perfused rat liver. 3. Isoacitretin undergoes glucuronidation as the major, almost exclusive, route of metabolism in the perfused rat liver. 4. The difference in the hepatic metabolism of the cis and trans isomers of this retinoid may explain the differences in their pharmacokinetics, and may help in understanding the pharmacokinetics of related retinoids.

Purification and characterization of naturally occurring benzodiazepine receptor ligands in rat and human brain

Chemicals that are active at the benzodiazepine receptor (endozepines) are naturally present in the CNS. These substances are present in tissue from humans and animals and in plants and fungi. Using selective extraction protocols, HPLC purification, receptor binding displacement studies, and selective anti-benzodiazepine antibodies, we have identified six or seven peaks of endozepines in rat and human brain. All material could competitively displace [3H]flunitrazepam binding to cerebellar benzodiazepine binding sites. Two peaks also competitively displaced Ro 5-4864 binding to the mitochondrial benzodiazepine binding site. Total amounts of brain endozepines were estimated to be present in potentially physiological concentrations, based on their ability to displace [3H]flunitrazepam binding. Although endozepine peaks 1 and 2 had HPLC retention profiles similar to those of nordiazepam and diazepam, respectively, gas chromatography-mass spectrometry as well as high-performance TLC revealed biologically insignificant amounts of diazepam (less than 0.02 pg/g) and nordiazepam (less than 0.02 pg/g) in the purified material. Electrophysiologically, some purified endozepines positively modulated gamma-aminobutyric acid (GABA) action on Cl- conductance, monitored in patch-clamped cultured cortical neurons or in mammalian cells transfected with cDNA encoding various GABAA receptor subunits. These studies demonstrate that mammalian brains contain endozepines that could serve as potent endogenous positive allosteric modulators of GABAA receptors.

The effect of the cytochrome P-450 suicide inactivator, 1-aminobenzotriazole, on the in vivo metabolism and pharmacologic activity of flurazepam

Flurazepam (F) is an extensively prescribed hypnotic (Dalmane) whose in vivo activity has been suggested to be due to its primary metabolites, hydroxyethyl flurazepam (HEF) and N-desalkylflurazepam (DAF). In order to determine the intrinsic pharmacologic activity of F, mice were administered various doses of the cytochrome P-450 suicide inactivator, 1-aminobenzotriazole (ABT), 1 hr before the ip administration of 1 mg/kg 14C-F. One hr after 14C-F, 70 mg/kg pentylenetetrazole was administered iv and the mice were observed for convulsions. F alone offered no protection from convulsion (mean brain concentrations were 3.9, 32, and 11 ng/g for F, DAF, and HEF, respectively). F with 25 mg/kg ABT also offered no protection despite a 6-fold increase in brain concentrations of F. F with 100 mg/kg ABT offered a 57% protection from convulsions (mean brain concentrations were 99, 62, and 41 ng/g for F, DAF, and HEF, respectively). One mg/kg F with 400 mg/kg ABT offered 100% protection from convulsions (brain concentrations were 190, 47, and 18 ng/g for F, DAF, and HEF, respectively). These data indicate that F has intrinsic pharmacologic activity which must be considered when evaluating the pharmacodynamics of F.

Quantitation of ceftetrame in human plasma and urine by high-performance liquid chromatography

A method is described for quantifying ceftetrame, the acid metabolite of methylene (6R,7R)-7-[(Z)-2-(2-amino-4-thiazolyl)-2-(methoxyimino)acetamido]-3- [(5-methyl-2H-tetrazol-2-yl)-methyl]-8-oxo-5-thia-1-azabicyclo[4.2 .0]oct-2-ene-2-carboxylate pivalate, an orally active cephalosporin. Sodium benzylpenicillin is added to the plasma as the reference standard. The compounds are extracted from plasma or urine using a Bond Elut phenyl column. An aliquot of the methanol eluate is analyzed by high-performance liquid chromatography using a Waters Nova-Pak phenyl column and a UV detector set to 225 nm. The ratios of the peak heights for ceftetrame and sodium benzylpenicillin are calculated and converted to concentrations of analyte with calibration curves that are generated from the analysis of analyte-free plasma or urine fortified with various amounts of ceftetrame and a fixed amount of sodium benzylpenicillin. For plasma, the limit of quantitation for the assay is 0.48 microgram/ml and the inter-assay precision (relative standard deviation) is 9.3%. For urine, the limit of quantitation for the assay is 19.1 micrograms/ml, and the inter-assay precision is 4.9%.

Respiratory pathways and fat synthesis in the developing castor oil seed

During castor oil seed development, changes occur in the activities of enzymes involved in fatty acid biosynthesis, glycolysis, and the pentose phosphate pathways. The activities of acetyl-CoA carboxylase, phosphofructokinase, pyruvate kinase, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase per seed increase during the phase of rapid oil synthesis in the endosperm. As the seed matures and the rate of fatty acid synthesis decreases, there is a corresponding diminution in the activities of these enzymes. An indication of the metabolic capacity of the plastids was determined by monitoring the ribulose-1,5-bisphosphate carboxylase activity in the endosperm.