Transport and metabolism of glutathione in the lens

Abstract

Measurements were made of influx and efflux of reduced and oxidized glutathione (GSH and GSSG) in rabbit lens. Three times as much radioactivity was found in the lens when the anterior surface was exposed to [ 35S]-GSH compared with the posterior side. Nonlabeled GSH added to the medium decreased the penetration of labeled GSH, primarily across the anterior surface. The entry of [ 35S]-GSSG into the lens was approximately the same across both surfaces. In cultured lenses, the accumulation of [ 35S]-GSH decreased significantly in the absence of glucose and at reduced temperature, but was unaffected by ouabain. The entry process for GSH was also shown to be a saturable mechanism. Of the three constituent amino acids in the tripeptide only cysteine was found to compete for the transport system of GSH, suggesting that GSH and cysteine may share a common carrier. The rate of entry of 35S-GSSG in cultured lenses was unaffected by the presence of either nonlabeled cysteine or GSH. No significant efflux of GSH or GSSG occurs from intact lenses. The efflux is extremely rapid when both capsule and epithelium are removed from the lens. When capsule alone was removed with collagenase, leaving the epithelum intact, no detectable amount of GSH or GSSG was found to leave the lens. A significant amount of [ 35S]-GSH that entered the lens was degraded, giving rise to labeled cysteine. Lenses cultured in the presence of 14C-labeled cysteine rapidly incorporated the amino acid into GSH, suggesting that GSH is totally degraded and resynthesized in the lens. From the time course of decrease in the specific activity of glutathione introduced into the lens, the coefficient of turnover of GSH was calculated to be 0·014 hr −1, a value found to be in reasonable agreement with the turnover rate of GSH calculated previously from the rates of incorporation of constituent amino acids.