Corneal transport of circulating glutathione in normal and galactosemic guinea pigs

Purpose, To study synthesis and transport of glutathione (GSH) in guinea-pi g cornea and to determine the effect of galactose feeding and growth on tra nsport. Methods. Steady-state level and maximal rate of synthesis of GSH (G SH-SR) were determined in guinea pigs fed 50% galactose diet for 10 days an d in controls. By using a model of in situ eye perfusion, corneal transport of [S-35]GSH (4 nM) and [C-14]sucrose was measured as a function of time ( 1-10 min) in normal guinea pigs under gamma-glutamyltranspeptidase (GGT)-in hibited and uninhibited conditions. The unidirectional constant of GSH upta ke 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 o f GSH also were measured in corneas from three different age groups: 10 day s, 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 differen t in the two groups (1.98 vs. 2.27 nmol/min/mg protein, respectively; p = N S). Corneal uptake of tracer [35S]GSH (4 nM) was linear up to 10 min and wa s several-fold higher than that of the impermeable marker [C-14]sucrose. Th e unidirectional rate constant (corrected for sucrose) for GSH was 4.03 +/- 0.21 x 10(-3)/min. GSH uptake in normal guinea pigs occurred by a saturabl e process with a K-m of 24 +/- 3 mu M and V-max of 92 +/- 14 pmol/min/g and was significantly inhibited by GSH and GSH monoethyl ester at 60 mu M conc entrations. Corneal uptake of tracer GSH in galactose-fed guinea pigs showe d 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 gui nea pigs, whereas that in 9-month-old guinea pigs showed a significant (sim ilar to 80%) decrease in uptake. Conclusion. 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 ava ilability of circulating GSH may be an important factor in the development of corneal pathology associated with aging and corneal hydration due to rel ative GSH deficiency.