Corneal transport of circulating glutathione in normal and galactosemic guinea pigs.
OBJECTIVE
To study synthesis and transport of glutathione (GSH) in guinea-pig cornea and to determine the effect of galactose feeding and growth on transport.


METHODS
Steady-state level and maximal rate of synthesis of GSH (GSH-SR) were determined in guinea pigs fed 50% galactose diet for 10 days and in controls. By using a model of in situ eye perfusion, corneal transport of [35S]GSH (4 nM) and [14C]sucrose was measured as a function of time (1-10 min) in normal guinea pigs under gamma-glutamyltranspeptidase (GGT)-inhibited and uninhibited conditions. The unidirectional constant of GSH uptake was determined as a function of GSH concentration in the perfusate. The effect of galactose feeding on corneal uptake of tracer GSH was determined in control and 10-day galactose-fed guinea pigs. Levels of GSH and uptake of GSH also were measured in corneas from three different age groups: 10 days, 4 weeks, and 9 months.


RESULTS
The mean GSH level (nmol/ mg protein) in corneas of control guinea pigs was 47.6, which decreased to 36.4 (p<0.05) in 10-day galactose-fed animals. The GSH-SR was not significantly different in the two groups (1.98 vs. 2.27 nmol/min/mg protein, respectively; p = NS). Corneal uptake of tracer [35S]GSH (4 nM) was linear up to 10 min and was several-fold higher than that of the impermeable marker [14C]sucrose. The unidirectional rate constant (corrected for sucrose) for GSH was 4.03+/-0.21x10(-3)/min. GSH uptake in normal guinea pigs occurred by a saturable process with a Km of 24+/-4 microM and Vmax of 92+/-14 pmol/min/g and was significantly inhibited by GSH and GSH monoethyl ester at 60 microM concentrations. Corneal uptake of tracer GSH in galactose-fed guinea pigs showed a dramatic decrease (almost to that of sucrose) as compared with control guinea pigs. GSH uptake was similar in corneas of 10-day and 4-week-old guinea pigs, whereas that in 9-month-old guinea pigs showed a significant (approximately 80%) decrease in uptake.


CONCLUSIONS
Cellular uptake of GSH by the cornea in the young, adult guinea pigs is carrier mediated via mechanism(s) that can be dissociated from the transpeptidation pathway. The reduced availability of circulating GSH may be an important factor in the development of corneal pathology associated with aging and corneal hydration due to relative GSH deficiency.
Kannan
R
R
Division of Gastrointestinal and Liver Diseases, University of Southern California School of Medicine, Los Angeles 90033, USA. kannan@hsc.usc.edu
Mackic
J B
JB
Zlokovic
B V
BV
D000375:Aging / Q000502:physiology; D000818:Animals; D001692:Biological Transport; D003315:Cornea / Q000378:metabolism*; D004040:Dietary Carbohydrates / Q000008:administration & dosage; D005260:Female; D005690:Galactose / Q000008:administration & dosage; D005693:Galactosemias / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism*; D006168:Guinea Pigs; D008297:Male
D004040:Dietary Carbohydrates; D005978:Glutathione; D005690:Galactose