Dahm L J, Jones D P
Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia 30322.
Toxicol Appl Pharmacol. 1994 Dec;129(2):272-82. doi: 10.1006/taap.1994.1252.
Organs of the digestive tract, including pancreas, small intestine, and colon, have mechanisms to modulate plasma thiol-disulfide balance. Because plasma glutathione disulfide (GSSG) concentration may be elevated from < 1 microM in control rats to over 25 microM during oxidative stress, we examined whether GSSG was cleared from rat mesenteric vasculature. When 100 microM GSSG was perfused through the gut via the superior mesenteric artery, an average of 45% was lost in a single pass. Results showed that gamma-glutamyltransferase (gamma-GT)-dependent and -independent mechanisms were involved in GSSG loss. Acivicin (AT125) treatment inhibited gamma-GT activity in the mesenteric vasculature by 94% and attenuated the loss of GSSG equivalents by 44%. These results supported a role for gamma-GT in GSSG loss from the mesenteric vasculature but indicated that still other mechanisms were involved in GSSG clearance. Elevations of portal levels of glutathione (GSH) and the mixed disulfide of cysteine and GSH (CySSG) also occurred with vascular GSSG perfusion and could account for about 40% of GSSG equivalents lost. Because portal elevations of GSH and CySSG were not inhibited by AT125, they were formed by a gamma-GT-independent mechanism(s). Given that cysteine was present in the mesenteric vasculature, the most likely mechanism to explain GSH and CySSG formation was via nonenzymatic thiol-disulfide exchange between GSSG and cysteine. Uptake of vascular GSSG by pancreas, small intestine (jejunum and ileum), or colon apparently did not occur as tissue contents of GSSG or GSH were not elevated, except for a small elevation of GSH in pancreas when mesenteric gamma-GT was inhibited with AT125. Additionally, GSSG was not transported from mesenteric vasculature into the small intestinal lumen because luminal levels of GSSG or GSH were not elevated. Further, total cysteine equivalents in lumen were unchanged indicating that GSSG was not transported to lumen and degraded to cystine by gamma-GT and dipeptidases localized to the intestinal brush-border. These results indicate that GSSG present in mesenteric vasculature is metabolized in the vascular compartment by gamma-GT-dependent and -independent reactions; together, these account for over 80% of lost GSSG equivalents. They also suggest that organs of the mesentery may play a quantitatively important role in plasma GSSG clearance and modulation of vascular thiol-disulfide balance.
包括胰腺、小肠和结肠在内的消化道器官具有调节血浆硫醇 - 二硫键平衡的机制。由于在氧化应激期间，血浆谷胱甘肽二硫化物（GSSG）浓度可能从对照大鼠中的<1微摩尔/升升高至超过25微摩尔/升，我们研究了GSSG是否从大鼠肠系膜血管系统中清除。当通过肠系膜上动脉向肠道灌注100微摩尔/升的GSSG时，单次通过平均损失45%。结果表明，GSSG的损失涉及γ-谷氨酰转移酶（γ-GT）依赖性和非依赖性机制。阿西维辛（AT125）处理使肠系膜血管系统中的γ-GT活性抑制了94%，并使GSSG当量的损失减少了44%。这些结果支持了γ-GT在肠系膜血管系统中GSSG损失中的作用，但表明仍有其他机制参与GSSG的清除。随着血管GSSG灌注，门静脉中谷胱甘肽（GSH）和半胱氨酸与GSH的混合二硫化物（CySSG）水平也升高，并且可以解释约40%的GSSG当量损失。由于门静脉中GSH和CySSG的升高不受AT125抑制，它们是由γ-GT非依赖性机制形成的。鉴于肠系膜血管系统中存在半胱氨酸，解释GSH和CySSG形成的最可能机制是通过GSSG与半胱氨酸之间的非酶促硫醇 - 二硫键交换。胰腺、小肠（空肠和回肠）或结肠对血管GSSG的摄取显然未发生，因为GSSG或GSH的组织含量未升高，除了用AT125抑制肠系膜γ-GT时胰腺中GSH有小幅度升高。此外，GSSG未从肠系膜血管系统转运到小肠腔，因为腔内GSSG或GSH水平未升高。此外，肠腔内总半胱氨酸当量未改变，表明GSSG未转运到肠腔并被定位于肠刷状缘的γ-GT和二肽酶降解为胱氨酸。这些结果表明，肠系膜血管系统中存在的GSSG在血管腔内通过γ-GT依赖性和非依赖性反应进行代谢；这些反应共同占损失的GSSG当量的80%以上。它们还表明，肠系膜器官可能在血浆GSSG清除和血管硫醇 - 二硫键平衡调节中起重要的定量作用。
