LOW DE-NOVO GLUTATHIONE SYNTHESIS FROM CIRCULATING SULFUR AMINO-ACIDSIN THE LENS EPITHELIUM

Transport of circulating sulfur amino acids (SAA) into the lens epithe lium and de novo glutathione (GSH) synthesis were studied in the perfu sed guinea-pig eye. Plasma-to-aqueous transfer of SAA was in their int act form (greater than or equal to 98%) and comparable with sucrose (a n extracellular marker) within 30 min. The unidirectional transport ra tes (ml min(-1) g(-1)) of S-35-labeled cystine, cystine and methionine into the epithelium were: 0.0057, 0.0003 and 0.0073 from plasma, and 1.41, 0.005 and 1.69 from aqueous, respectively. The unidirectional ep ithelial uptake was limited to 1 min for all three [S-35]SAA, and the isotopic steady-state ratio was achieved between 1 and 30 min. Cortica l uptake was time-dependent and progressive between 1 and 30 min, but undetectable within 1 min. The high performance liquid chromatography (HPLC) analysis of the epithelium revealed that following 1 min of uni directional [S-35]cysteine transport, 3% of the label was incorporated into GSH and greater than or equal to 95% was as cysteine. An average incorporation of [S-35]cysteine into GSH within the 30 min period was 0.83% min(-1) and 1%/min for the epithelium and cortex, respectively Infusions of [S-35]cystine and methionine failed to demonstrate incorp oration into GSH. Maximal rates of de novo GSH epithelial synthesis we re similar to 3 and 12 pmol g(-1) from plasma and aqueous cysteine, re spectively. A t(1/2) of 5480 hr was estimated if epithelial GSH had to be replaced exclusively by synthesis from aqueous cysteine. Given the limited aqueous and epithelial cysteine pools, and low (from cysteine ) or undetectable (from cystine and methionine) incorporation of the l abel into GSH, we conclude that de novo GSH synthesis from circulating and aqueous SAA can be only a minor source of the millimolar concentr ation of GSH in the epithelium. (C) 1997 Academic Press Limited.