Characterization of sterol uptake in leaf tissues of sugar beet.
The uptake of cholesterol has been characterized in leaf discs from mature leaves of sugar beet ( Beta vulgaris L.). This transport system exhibited a simple saturable phase with an apparent Michaelis constant ranging from 30 to 190 microM depending on the sample. When present at 10 M excess, other sterols were able to inhibit cholesterol uptake. Moreover, binding assays demonstrated the presence of high-affinity binding sites for cholesterol in purified plasma membrane vesicles. In the range 1-60 microM, cholesterol uptake showed an active component evidenced by action of the protonophore carbonyl cyanide m-chlorophenylhydrazone. Energy was required as shown by the inhibition of uptake induced by respiration inhibitors (NaN(3)), darkness and photosynthesis inhibitors [3-(3,4-dichlorophenyl)-1,1-dimethylurea, methyl viologen]. Moreover, the process was strongly dependent on the experimental temperature. Uptake was optimal at acidic pH (4.0), sensitive to ATPase modulators, inhibited by thiol reagents (N-ethylmaleimide, p-chloromercuribenzenesulfonic acid, Mersalyl) and by the histidyl-group reagent diethyl pyrocarbonate. The addition of cholesterol did not modify H(+) flux from tissues, indicating that H(+)-co-transport was unlikely to be involved. MgATP did not increase the uptake, arguing against involvement of an ABC cassette-type transporter. By contrast, cryptogein, a sterol carrier protein from the Oomycete Phytophtora cryptogea, greatly increased absorption. Taken together, the results reported in this work suggest that plant cells contain a specific plasma membrane transport system for sterols.
Rossard
Stéphanie
S
Laboratoire de Physiologie, Biochimie et Biologie Moléculaire Végétales, University of Poitiers, UMR CNRS 6161, 40 avenue du Recteur Pineau, 86022 Poitiers, France.
Bonmort
Janine
J
Guinet
Frédéric
F
Ponchet
Michel
M
Roblin
Gabriel
G
D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors; D027461:Beta vulgaris / Q000187:drug effects / Q000378:metabolism*; D001665:Binding Sites / Q000187:drug effects; D001667:Binding, Competitive / Q000187:drug effects; D002784:Cholesterol / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004336:Drug Antagonism; D004875:Ergosterol / Q000494:pharmacology; D006027:Glycosides / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D018515:Plant Leaves / Q000187:drug effects / Q000378:metabolism*; D012855:Sitosterols / Q000494:pharmacology; D013265:Stigmasterol / Q000494:pharmacology; D013438:Sulfhydryl Compounds / Q000494:pharmacology; D013696:Temperature; D013997:Time Factors
D006027:Glycosides; D012855:Sitosterols; D013438:Sulfhydryl Compounds; C007808:fusicoccin; C025473:gamma-sitosterol; D002784:Cholesterol; D013265:Stigmasterol; D000251:Adenosine Triphosphatases; D004875:Ergosterol