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Biomedical subjects

M Choquette

Publications and source records attributed to M Choquette.

3 recordsLinked to original sources
PubMed2004

Geochemical characterization of acid mine drainage from a waste rock pile, Mine Doyon, Québec, Canada.

Water quality in the unsaturated and saturated zones of a waste rock pile containing sulphides was investigated. The main objectives of the project were (1) the evaluation of geochemical trends including the acid mine drainage (AMD)-buffering mechanism and the role of secondary minerals, and (2) the investigation of the use of stable isotopes for the interpretation of physical and geochemical processes in waste rock. Pore water in unsaturated zone was sampled from suction lysimeters and with piezometers in underlying saturated rocks. The investigation revealed strong temporal (dry period vs. recharge period), and spatial (slope vs. central region of pile) variability in the formation of acid mine drainage. The main secondary minerals observed were gypsum and jarosite. There was a higher concentration of gypsum in solid phase at Site TBT than at Site 6, suggesting that part of the gypsum formed at Site 6 in the early stage of AMD has been already dissolved. Formation of secondary minerals contributed to the formation of AMD by opening of foliation planes in waste rock, thus increasing the access of oxidants like O2 and Fe3+ to previously encapsulated pyrite. The behavior of several dissolved species such as Mg, Al, and Fe2+ can be considered as conservative in the leachate. Stable isotopes, deuterium and 18O, indicated internal evaporation within the pile, and were used to trace recharge pulses from snowmelt. Isotope trends for 34S and 18O(SO4) indicated a lack of sulfate reduction and zones of active oxidation of pyrite, respectively. Results of numerical modeling of pyrite oxidation and gas and water transport were consistent with geochemical and isotopic trends and confirmed zones of high evaporation rate within the rock pile close to the slope. The results indicate that physical and chemical processes within the pile are strongly coupled and cannot be considered separately when oxidation rates are high and influence gas transport as a result of heat generation.

Environmental Monitoring↗
PubMed2001

Retinoic acid receptor alpha1 variants, RARalpha1DeltaB and RARalpha1DeltaBC, define a new class of nuclear receptor isoforms.

Retinoic acid (RA) binds and activates retinoid X receptor (RXR)/retinoic acid receptor (RAR) heterodimers, which regulate the transcription of genes that have retinoic acid response elements (RARE). The RAR isotypes (alpha, beta and gamma) are comprised of six regions designated A-F. Two isoforms of RARalpha, 1 and 2, have been identified in humans, which have different A regions generated by differential promoter usage and alternative splicing. We have isolated two new splice variants of RARalpha1 from human B lymphocytes. In one of these variants, exon 2 is juxtaposed to exon 5, resulting in an altered reading frame and a stop codon. This variant, designated RARalpha1DeltaB, does not code for a functional receptor. In the second variant, exon 2 is juxtaposed to exon 6, maintaining the reading frame. This isoform, designated RARalpha1DeltaBC, retains most of the functional domains of RARalpha1, but omits the transactivation domain AF-1 and the DNA-binding domain. Consequently, it does not bind nor transactivate RARE on its own. Nevertheless, RARalpha1DeltaBC interacts with RXRalpha and, as an RXRalpha/RARalpha1DeltaBC heterodimer, transactivates the DR5 RARE upon all-trans-RA binding. The use of RAR- and RXR-specific ligands shows that, whereas transactivation of the DR5 RARE through the RXRalpha/RARalpha1 heterodimer is mediated only by RAR ligands, transactivation through the RXRalpha/RARalpha1DeltaBC heterodimer is mediated by RAR and RXR ligands. Whilst RARalpha1 has a broad tissue distribution, RARalpha1DeltaBC has a more heterogeneous distribution, but with significant expression in myeloid cells. RARalpha1DeltaBC is an infrequent example of a functional nuclear receptor which deletes the DNA-binding domain.

Alternative Splicing↗