Jc. Fernandezcheca et al., INHIBITION OF RAT SINUSOIDAL GSH TRANSPORTER BY THIOETHERS - SPECIFICITY, SIDEDNESS, AND KINETICS, American journal of physiology: Gastrointestinal and liver physiology, 33(6), 1996, pp. 969-975
In isolated hepatocytes, cystathionine, methionine, and thioether anal
ogues of methionine, cysteine, and homocysteine, including S-adenosyl
derivatives, inhibited reduced glutathione (GSH) efflux. The potency o
f inhibition by thioethers with different S-alkyl moieties was methyl
< ethyl < butyryl < aminoethyl < alpha-aminopropionyl. Inhibition of G
SH efflux by cystathionine from hepatocytes that were allowed to resyn
thesize GSH resulted in greater repletion (30-40%) of GSH levels compa
red with absence of cystathionine. To address unequivocally the sided
ness of inhibition, i.e., cis vs. trans, we examined the effect of cys
tathionine on the activity of GSH transport in Xenopus oocytes express
ing 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. Converse
ly, when oocytes expressing RsGshT were loaded with cystathionine, no
inhibition of uptake or efflux was observed. The same structural requi
rement of a thioether bond to exert an inhibitory effect on GSH transp
ort was observed in oocytes expressing RsGshT. Oocytes expressing RsGs
hT do not transport methionine, whereas oocytes expressing total rat l
iver mRNA express methionine transport. Inhibition of both GSH efflux
from and uptake by oocytes expressing RsGshT exhibited a competitive t
ype of kinetics: cystathionine increased the Michaelis constant for GS
H transport (4.5 +/- 0.9 vs. 10 +/- 2.5 mM and 7.5 +/- 0.6 vs. 12.9 +/
- 1.5 mM for uptake and efflux, respectively) without affecting the ma
ximal velocity for transport. Thus thioethers such as methionine and c
ystathionine inhibit the transport of GSH by interacting in a competit
ive and specific fashion with the sinusoidal GSH transporter without t
hemselves being transported by this carrier.