Radiation inactivation studies of hepatic sinusoidal reduced glutathione transport system

Sinusoidal transport of reduced glutathione (GSH) is a carrier-mediated process. Perfused liver and isolated hepatocyte models revealed a low-affinity transporter with sigmoidal kinetics (K(m) approximately 3.2-12 mM), while studies with sinusoidal membrane vesicles (SMV) revealed a high-affinity unit (K(m) approximately 0.34 mM) besides a low-affinity one (K(m) approximately 3.5-7 mM). However, in SMV, both the high- and low-affinity units manifested Michaelis-Menten kinetics of GSH transport. We have now established the sigmoidicity of the low-affinity unit (K(m) approximately 9) in SMV, consistent with other models, while the high-affinity unit has been retained intact with Michaelis-Menten kinetics (K(m) approximately 0.13 mM). We capitalized on the negligible cross-contributions of the two units to total transport at the low and high ends of GSH concentrations and investigated their characteristics separately, using radiation inactivation, as we did in canalicular GSH transport (Am. J. Physiol. 274 (1998) G923-G930). We studied the functional sizes of the proteins that mediate high- and low-affinity GSH transport in SMV by inactivation of transport at low (trace and 0.02 mM) and high (25 and 50 mM) concentrations of GSH. The low-affinity unit in SMV was much less affected by radiation than in canalicular membrane vesicles (CMV). The target size of the low-affinity sinusoidal GSH transporter appeared to be considerably smaller than both the canalicular low- and high-affinity transporters. The high-affinity unit in SMV was markedly inactivated upon irradiation, revealing a single protein structure with a functional size of approximately 70 kDa. This size is indistinguishable from that of the high-affinity GSH transporter in CMV reported earlier.