Syntéza

Mammalian GCS - gamma glutamylcysteine synthetase

• Heterodimer consisting of a
• Catalytically active heavy subunit (73 kDa)
• A light (regulatory) subunit (31 kDa)
• Heavy subunit contains all substrate binding sites
• Light subunit modulates the affinity of the heavy subunit for substrates and inhibitors

Km values of mammalian GCS for

• Glutamate 1.7 mmol/L
• Cysteine 0.15 mmol/L
• Similar to the intracellular concentrations
• Glutamate (2–4 mmol/L)
• Cysteine (0.15–0.25 mmol/L) in rat liver
• Cooperative binding for gamma-glutamyl substrate
• Km values of mammalian GSH synthetase for
• ATP 0.04 mmol/L
• Glycine 0.9 mmol/L
• Lower than intracellular concentrations
• ATP (2–4 mmol/L)
• Glycine (1.5–2 mmol/L) in rat liver

• Rate-controlling enzyme in de novo synthesis of GSH

Jaterní GSH syntéza

• Liver GSH synthesis occurs predominantly in perivenous hepatocytes and, to a lesser extent, in periportal cells
• Changes in plasma GSH levels may not necessarily reflect changes in GSH synthesis in specific cell types
• The kidney, lung, and intestine
• Are major consumers of the liver-derived GSH
• Interorgan metabolism of GSH functions to transport cysteine in a nontoxic form between tissues
• Also helps to maintain intracellular GSH concentrations and redox state [1]

Plasmatický glutathion

• Glutathione can be transported out of cells via a carrier-dependent facilitated mechanism
• Plasma GSH originates
• Primarily from the liver
• Some of the dietary and intestinally derived GSH can enter the portal venous plasma
• Glutathione molecules leave the liver
• Intact
• Gamma-Glu-(Cys)2 owing to gamma-glutamyl transpeptidase activity on the outer plasma membrane [1]

Nitrobuněčný GSH

• Transport of extracellular GSH or GSSG into cells is thermodynamically unfavorable
• Gamma-Glu-(Cys)2 is readily taken up by extrahepatic cells for GSH synthesis [1]

GSH Synthesis

• GSH from glutamate, cysteine, and glycine
• Catalyzed sequentially by two cytosolic enzymes
• Gamma-glutamylcysteine synthetase (GCS)
• GSH synthetase
• This pathway occurs in virtually all cell types
• Liver being the major producer and exporter of GSH [1]

• Gamma-carboxyl group of glutamate
• Reacts with the amino group of cysteine to form a peptidic gamma-linkage
• Protects GSH from hydrolysis by intracellular peptidases
• Although gamma-glutamyl-cysteine
• Can be a substrate for gamma-glutamylcyclotransferase
• GSH synthesis is favored in animal cells
• Because of the much higher affinity and activity of GSH synthetase [1]