Developmental changes in plasma thiol-disulfide turnover in rats: a multicompartmental approach

Immune system stimulation increases the plasma cysteine flux and whole-body glutathione synthesis rate in starter pigs1

Glutathione (GSH) is the major intracellular thiol that plays a role in numerous detoxification, bio-reduction, and conjugation reactions. The availability of Cys is thought to be the rate-limiting factor for the synthesis of GSH. The effects of immune system stimulation (ISS) on GSH levels and the GSH synthesis rate in various tissues, as well as the plasma flux of Cys, were measured in starter pigs fed a sulfur AA (SAA; Met + Cys) limiting diet. Ten feed-restricted gilts with initial body weight (BW) of 7.0 ± 0.12 kg were injected i.m. twice at 48-h intervals with either sterile saline (n = 4; ISS-) or increasing amounts of Escherichia coli lipopolysaccharide (n = 6; ISS+). The day after the second injection, pigs received a primed constant infusion of 35S-Cys (9,300 kBq/pig/h) for 5 h via a jugular catheter. Blood and tissue free Cys and reduced GSH were isolated and quantified as the monobromobimane derivatives by HPLC. The rate of GSH synthesis was determined by measurement of the specific radioactivity of GSH and tissue free Cys at the end of the infusion period. Plasma Cys and total SAA levels were reduced (16% and 21%, respectively), but plasma Cys flux was increased (26%) by ISS (P < 0.05). Immune system stimulation increased GSH levels in the plasma (48%; P < 0.05), but had no effect on GSH levels in the liver, small and large intestines, heart, muscle, spleen, kidney, lung, and erythrocytes. The fractional synthesis rate (FSR) of GSH was higher (P < 0.05) in the liver (34%), small intestine (78%), large intestine (72%), heart (129%), muscle (37%), and erythrocytes (47%) of ISS+ pigs compared to ISS- pigs. The FSR of GSH tended (P = 0.08) to be higher in the lungs (45%) of ISS+ pigs than in ISS- pigs. The absolute rate of GSH synthesis was increased by ISS (mmol/kg wet tissue/d ± SE, ISS- vs. ISS+; P < 0.05) in the liver (5.22 ± 0.22 vs. 7.20 ± 0.59), small intestine (2.54 ± 0.25 vs. 4.52 ± 0.56), large intestine (0.61 ± 0.06 vs. 1.06 ± 0.16), heart (0.21 ± 0.03 vs. 0.48 ± 0.08), lungs (1.50 ± 0.10 vs. 2.90 ± 0.21), and muscle (0.21 ± 0.03 vs. 0.34 ± 0.04), but it remained unchanged in erythrocytes, the kidney, and the spleen (P > 0.80). The current findings suggest that GSH synthesis is increased during ISS, contributing to enhanced maintenance sulfur amino acid requirements in starter pigs during ISS.

Expression cloning of the cDNA for a polypeptide associated with rat hepatic sinusoidal reduced glutathione transport: characteristics and comparison with the canalicular transporter

Using the Xenopus oocyte expression system, we previously identified an approximately 4-kb fraction of mRNA from rat liver that expresses sulfobromophthalein reduced glutathione S-conjugate (BSP-GSH)-insensitive and an approximately 2.5-kb fraction expressing BSP-GSH-sensitive reduced glutathione (GSH) transport. From the former, a 4.05-kb cDNA was cloned and characterized as the putative rat canalicular GSH transporter. Starting with a cDNA library constructed from the approximately 2.5-kb fraction, we have now isolated a single clone that leads to expression of a BSP-GSH- and cystathionine-inhibitable GSH transporter activity with Km approximately 3 mM characteristic of the sinusoidal GSH transporter. The cDNA for the rat sinusoidal GSH transporter-associated polypeptide (RsGshT) is 2733 bases with an open reading frame of 1059 nucleotides encoding a polypeptide of 353 amino acids (39,968 Da) with two putative membrane-spanning domains. No identifiable homologies were found in searching various data bases. An approximately 40-kDa protein is generated in in vitro translation of cRNA for RsGshT. Northern blot analysis revealed a single approximately 2.8-kb transcript in rat and human liver with negligible hybridization signal in other organs. The abundance of mRNA for RsGshT did not increase with phenobarbital treatment. Cis-inhibition by BSP-GSH and trans-inhibition by cystathionine and lack of induction by phenobarbital are characteristic of sinusoidal GSH secretion and thus indicate that RsGshT either encodes the sinusoidal GSH transporter itself or a regulatory subunit of the transporter that determines its liver-specific activity.