Inhibition of rat sinusoidal gsh transporter by thioethers:
specificity, sidedness and kinetics.
Fern[acute]andez-Checa, Jos[acute]e C., Carmen Garc[acute]ia-Ruiz,
Anna Colell, Jian-Ru Yi, and Neil Kaplowitz.
Liver Unit and Servicio de Bioqu[acute]imica, Department of
Medicine, Hospital Clinic i Provincial, Universidad de Barcelona, and
Instituto Mixto de Investigaciones Biom[acute]edicas August Pi i
Su[tilde]ner, CSIC-UB, Barcelona, Spain, and # Division of GI and
Liver Diseases, Department of Medicine, USC School of Medicine and
the VA Outpatient Clinic, Los Angeles, California
APStracts 3:0002G, 1996.
In isolated hepatocytes cystathionine, methionine and thioether
analogues of methionine, cysteine and homocysteine, including S
-adenosyl-derivatives, inhibited GSH efflux. The potency of inhibition
by thioethers with different S-alkyl moieties was
methyl<ethyl<butyryl<aminoethyl<[alpha]
-aminopropionyl. The inhibition of GSH efflux by cystathionine from
hepatocytes that were allowed to resynthesize GSH resulted in greater
repletion (30-40%) of GSH levels compared to absence of
cystathionine. To address unequivocally the sidedness of inhibition,
i.e. cis versus trans, we examined the effect of cystathionine on the
activity of GSH transport in Xenopus oocytes expressing the cRNA of
cloned rat liver sinusoidal (RsGshT) and canalicular (RcGshT) GSH
transporters. Cystathionine trans-inhibited efflux of GSH and cis
-inhibited uptake of GSH by oocytes expressing RsGshT. Conversely,
when oocytes expressing RsGshT were loaded with cystathionine no
inhibition of uptake or efflux was observed. The same structural
requirement of thioether bond to exert an inhibitory effect on GSH
transport was observed in oocytes expressing RsGshT. Oocytes
expressing RsGshT do not transport methionine whereas oocytes
expressing total rat liver mRNA express methionine transport.
Inhibition of both GSH efflux from and uptake by oocytes expressing
RsGshT exhibited competitive type of kinetics: cystathionine
increased the Km for GSH transport (4.5+/-0.9 vs. 10+/-2.5 mM and
7.5+/-0.6 vs. 12.9+/-1.5mM for uptake and efflux, respectively)
without affecting the Vmax for transport. Thus, thioethers such as
methionine and cystathionine inhibit the transport of GSH by
interacting in a competitive and specific fashion with the sinusoidal
GSH transporter without themselves being transported by this carrier.
Received 7 August 1995; accepted in final form 15 December 1995.
APS Manuscript Number G339-5.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 22 January 96