Browsing by Author "ISRAEL, Y"

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    GAMMA-GLUTAMYL TRANSFERASE ECTOACTIVITY IN THE INTACT RAT-LIVER - EFFECT OF CHRONIC ALCOHOL-CONSUMPTION
    (1990) SPEISKY, H; ISRAEL, Y
    The localization of gamma-glutamyl transferase (GGT) in the intact rat liver was studied by a new approach in which the chromogenic gamma-glutamyl donor substrate of GGT gamma-glutamyl-p-nitroanilide is perfused through the portal vein to yield p-nitroaniline, which is monitored spectrophotometrically. GGT activity was markedly increased by the gamma-glutamyl acceptors glycyl-glycine, cystine and methionine, following Michaelis-Menten kinetics. Infusion of glutathione (GSH), the natural substrate of GGT, was shown to markedly reduce or to abolish the formation of p-nitroaniline without entering the liver cells, indicating the existence of a GGT ectoactivity acesssible to the sinusoidal circulation. This ectoenzyme was shown to remove significant amounts of GSH from the circulation, amounting, in the naive rat, to 20-25% of the net rate at which GSH is contributed by the liver into the circulation. Chronic alcohol consumption is known to increase hepatic GGT activity, although the biological significance of such an effect unknown remains unknown. Present studies show that chronic administration of alcohol to rats leads to a significant (40-75%) increase in hepatic GGT ectoactivity. Livers of alcohol-fed rats showed an increased (80-110%) capacity to remove circulating GSH which strongly correlated with total liver GGT (r = .96; p < 0.0001). These studies demonstrate: a) the occurrence of a significant sinusoidal GGT ectoactivity in the intact rat liver, capable of catalyzing the removal of circulating GSH, and b) the increase in liver GGT associated with alcohol consumption are associated with a markedly enhanced removal of GSH from the circulation.
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    ROLE OF HEPATIC GAMMA-GLUTAMYL-TRANSFERASE IN THE DEGRADATION OF CIRCULATING GLUTATHIONE - STUDIES IN THE INTACT GUINEA-PIG PERFUSED LIVER
    (1990) SPEISKY, H; SHACKEL, N; VARGHESE, G; WADE, D; ISRAEL, Y
    The role of hepatic .gamma.-glutamyltransferase in the breakdown of circulating glutathione was studied in the perfused guinea pig liver. Hepatic .gamma.-glutamyltransferase activity in the guinea pig is seven-fold higher than in the rat and is comparable to its activity in man. Guinea pig livers were found to remove, in a single pass, 50% to 90% of glutathione (10 to 50 .mu.mol/L) added to the portal perfusate. Removal of portal glutathione was totally dependent on the activity of .gamma.-glutamyltransferase and led to the near quantitative appearance of cysteinyl-glycine and cysteine in the caval perfusate. Glutathione removal by the intact liver followed saturation with a Michaelis constant (Km) of 59 .mu.mol/L for glutathione and a maximum velocity of 235 nmol glutathione/min/gm of liver weight. The capacity of the guinea pig liver to remove circulating glutathione was estimated to be sevenfold to 10-fold higher than its net rate of output of glutathione into the circulation. Inhibition of .gamma.-glutamyltransferase activity in the perfused liver led to threefold to sixfold increases in the hepatic output of glutathione into the circulation, indicating that more than two thirds of glutathione transported extracellularly is broken down. Data obtained demonstrate a major role of hepatic .gamma.-glutamyltransferase, both in the removal of portally carried glutathione and in the degradation of glutathione molecules released by the liver itself into the sinusoids. These findings suggest the existence of an intraorgan transport of glutathione in the liver, whereby periportal cells could provide glutathione precursors to pericentral cells.