GGT is an enzyme normally present in the serum and on the outer surface of numerous cell types ( 3). It is a vital step in maintaining in vivo homeostasis of GSH and cysteine. GGT can provide cysteine, the rate-limiting amino acid, for GSH de novo synthesis by breaking down extracellular GSH into its constitutive amino acids. GGT catalyzes the transfer of the gamma-glutamyl moiety from GSH or GSH conjugated to acceptors such as amino acids, dipeptides and molecules with similar traits. Comparatively, GGT activity was decided primarily by the free-GGT and small-GGT fractions. In that study, four GGT fractions: big-GGT, medium-GGT, small-GGT and free-GGT fractions of different molecular weight (molecular masses >2000 kDa, 940 kDa, 140 kDa and 70 kDa, respectively) were detected by a procedure based on gel filtration chromatography, followed by postcolumn injection of a fluorescent GGT substrate. Franzini et al ( 2) performed quantitative analysis of serum GGT fractions. GGT is a glycosylated protein that is partially embedded in the outer surface of the plasma membrane at the N-terminal transmembrane domain. Therefore, the transcriptional process of GGT was presumed to be controlled by multiple promoters in a linear arrangement similar to the TRE, AP-2 combining site and SP-1 cis elements that exist in the proximal region of the GGT gene. These GGT complementary DNA transcripts share the same coding sequence, but their 5′-untranslated regions are different. Several GGT complementary DNA segments have been obtained from hepatoma cells, placenta, lung, pancreas and other tissues. To date, the exact protein structure, gene-expression patterns and regulatory mechanisms of GGT have not been elucidated. Human GGT is a multigene family of proteins composed of seven GGT genes and pseudogenes.
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