Jean-Pierre Dufour

Functional complementation of a null mutation of the yeast Saccharomyces cerevisiae plasma membrane H(+)-ATPase by a plant H(+)-ATPase gene

In plants, the proton pump-ATPase (H+-ATPase) of the plasma membrane is encoded by a multigene family. The presence within an organ of several isoforms prevents a detailed enzymatic characterization of individual H+-ATPases. We therefore used the yeast Saccharomyces cerevisiae as a heterologous host for the expression of PMA2, an H+-ATPase isoform of Nicotiana plumbaginifolia. Yeast transformed by the plant pma2 was still able to grow under conditions where the yeast ATPase gene (PMA1) was either repressed or deleted. The transformed yeast strain was resistant to hygromycin, and its growth was prevented when the medium pH was lowered to 5.0. The N. plumbaginifolia PMA2 expressed in S. cerevisiae has unusual low Km for ATP (23 μM) and high pH optimum (6.8). Electron microscopic examination revealed PMA2 in internal structures of the karmellae type which proliferated when cell growth was arrested, either at a nonpermissive pH or at the stationary phase in a minimal medium. Under the latter conditions, subcellular fractionation on sucrose gradients revealed, in addition to the expected plant PMA2 peak linked to the plasma membrane fraction, a low density peak containing PMA2 and KAR2, an endoplasmic reticulum marker. These observations suggest that the partial internal accumulation of PMA2 occurs in membranes derived from the endoplasmic reticulum and largely depends on growth conditions.

Characteristic aroma profiles of unifloral honeys obtained with a dynamic headspace GC-MS system

The volatile compounds of 84 unifloral honeys (from 14 unifloral sources in 10 countries) were studied with a dynamic headspace gas chromatograph coupled directly to a mass spectrometer system. The average concentrations of the 47 compounds identified are tabulated, together with relevant chromatographic data. Some compounds appeared to be characteristic of the floral source, particularly in lavender (caproaldehyde (hexanal) and heptanal), fir (acetone), eucalyptus (diketones, sulphur compounds, alkanes) and dandelion and rape (three unidentified compounds) honeys. It is concluded that further studies on less volatile compounds are needed in order to further characterize aromas of unifloral honeys and to differentiate honeys derived from floral sources such as chestnut, orange, lime and robinia. Some compounds, such as alcohols, branched aldehydes and furan derivatives, reflected the microbiological purity and processing and storage conditions of the honeys, rather than their floral origins.

Decrease of the plasma membrane H+-ATPase activity during late exponential growth of Saccharomyces cerevisiae

During the last cell division of exponential growth, the H+-ATPase activity from the yeast plasma membrane decreases by a factor of two to three. This arrest growth control of ATPase activity is not accompanied by modification of the sensitivity to vanadate.

Relationships between the chemical composition and sensory evaluation of lager beers

Abstract This paper examines how several typical components of lager beers affect appreciation of the product. Sensory analyses clearly show a preference in bitterness for isohumulone (17·5–25 EBU), as compared to isocohumulone. As for esters, isoamyl acetate was found to be the most appreciated compound (2·4 ppm). In terms of flavour units, only ethyl caproate can reach a value of 4 without exhibiting an unpleasant aroma. The optimal dimethylsulphide concentration is in the range of 50 ppb.

Inhibition of Yeast Plasma-membrane H+-atpase By Fluoroaluminates

The mechanism of inhibition of the Schizosaccharomyces pombe plasma membrane H+-ATPase by fluoroaluminates has been investigated. A biphasic inhibitory process was observed at pH 7.5, with a preference for the AlF4- species. The dissociation constant found for AlF4- is 8.5 mu M, Mg2+ being an essential cofactor for the inhibition. The rate constant of the rapid inhibition phase is decreased at pH values lower than 7.0 which may reveal a preferential action of AlF4- on the E(2) conformation of the enzyme. The slow phase of inhibition was found to be quasi-irreversible and highly dependent on the water activity. This dependence was studied by adding Me(2)SO in the solvent and can be explained by the release of five water molecules upon fluoroaluminate binding. It is proposed that inhibition of the H+-ATPase is due to the formation of a stable E(2)-Mg-AlF4 complex analog to the phosphorylated intermediate of the H+-ATPase.