Gamma-glutamyl transpeptidase-dependent iron reduction and LDL oxidation--a potential mechanism in atherosclerosis.
BACKGROUND
gamma-Glutamyl transpeptidase (gamma-GT) is found in serum and in the plasma membranes of virtually all cell types. Its physiologic role is to initiate the hydrolysis of extracellular glutathione (GSH), a tripeptide in which cysteine lies between alpha-glycine and gamma-glutamate residues. Cysteine and other thiol compounds are known to promote LDL oxidation by reducing Fe(III) to redox active Fe(II); therefore, we sought to determine whether similar reactions can be sustained by GSH and influenced by gamma-GT.


METHODS
Fe(III) reduction and LDL oxidation were studied by monitoring the formation bathophenanthroline-chelatable Fe(II) and the accumulation of thiobarbituric acid-reactive substances, respectively. Human atheromatous tissues were examined by histochemical techniques for the presence of oxidized LDL and their colocalization with cells expressing gamma-GT activity.


RESULTS
A series of experiments showed that the gamma-glutamate residue of GSH affected interactions of the juxtaposed cysteine thiol with iron, precluding Fe(III) reduction and hence LDL oxidation. Both processes increased remarkably after addition of purified gamma-GT, which acts by removing the gamma-glutamate residue. GSH-dependent LDL oxidation was similarly promoted by gamma-GT associated with the plasma membrane of human monoblastoid cells, and this process required iron traces that can be found in advanced or late stage atheromas. Collectively, these findings suggested a possible role for gamma-GT in the cellular processes of LDL oxidation and atherogenesis. Histochemical analyses confirmed that this may be the case, showing that gamma-GT activity is expressed by macrophage-derived foam cells within human atheromas, and that these cells colocalize with oxidized LDL.


CONCLUSIONS
Biochemical and histochemical correlates indicate that gamma-GT can promote LDL oxidation by hydrolyzing GSH into more potent iron reductants. These findings may provide mechanistic clues to the epidemiologic evidence for a possible correlation between persistent elevation of gamma-GT and the risk of fatal reinfarction in patients with ischemic heart disease.
Paolicchi
A
A
Department of Experimental Pathology, University of Pisa School of Medicine, Italy.
Minotti
G
G
Tonarelli
P
P
Tongiani
R
R
De Cesare
D
D
Mezzetti
A
A
Dominici
S
S
Comporti
M
M
Pompella
A
A
D000328:Adult; D000368:Aged; D001161:Arteriosclerosis / Q000378:metabolism*; D002374:Catalase / Q000494:pharmacology; D002614:Chelating Agents / Q000378:metabolism; D005487:Foam Cells / Q000201:enzymology / Q000473:pathology; D005978:Glutathione / Q000494:pharmacology; D006801:Humans; D007124:Immunoenzyme Techniques; D007501:Iron / Q000737:chemistry / Q000378:metabolism*; D015227:Lipid Peroxidation / Q000502:physiology; D008077:Lipoproteins, LDL / Q000737:chemistry / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D010618:Phenanthrolines / Q000378:metabolism; D013482:Superoxide Dismutase / Q000378:metabolism; D017392:Thiobarbituric Acid Reactive Substances / Q000378:metabolism; D020298:U937 Cells; D005723:gamma-Glutamyltransferase / Q000378:metabolism*
D002614:Chelating Agents; D008077:Lipoproteins, LDL; D010618:Phenanthrolines; D017392:Thiobarbituric Acid Reactive Substances; C017049:bathophenanthroline disulfonic acid; D007501:Iron; D002374:Catalase; D013482:Superoxide Dismutase; D005723:gamma-Glutamyltransferase; D005978:Glutathione