Elevation of Endogenous Nucleophiles in Rat Lung by Cysteine and Glutathione Esters In Vitro

Abstract

In this study, we have compared the uptake of l-cysteine ( l-CySH), d-cysteine ( d-CySH), l-cysteine isopropyl ester ( l-CIPE) and d-cysteine isopropyl ester ( d-CIPE) in rat lung slices and tracheal sections and determined the effectiveness of glutathione (GSH), GSH isopropyl monoester, GSH isopropyl diester, γ-glutamylcysteine (γ-glu-cys) isopropyl monoester and γ-glu-cys isopropyl diester to elevate and prolong intracellular GSH concentrations in rat lung slices. Lung slices were incubated with 1.0 mM of thiol and the concentrations determined intracellularly and extracellularly with time. Slices incubated with GSH, GSH isopropyl diester and γ-glu-cys isopropyl diester had cellular GSH concentrations increased by up to 60%, 95% and 58%, respectively, whereas GSH isopropyl monoester and γ-glu-cys isopropyl monoester did not increase the intracellular GSH concentration. Extracellularly, the GSH concentration had decreased by 15%, GSH isopropyl diester by 27%, γ-glu-cys isopropyl diester by 66% and both isopropyl monoesters by over 90% at 120 min. Lung slices and tracheal sections incubated with l- or d-CySH at 37° had increased cellular concentrations of l- and d-CySH which ranged between 0.88–1.25 nmol mg −1 and 1.35–2.25 nmol mg −1, respectively. Reducing the incubation temperature to 4° had little effect on the accumulation of d-CySH; however, l-CySH concentrations increased progressively in the trachea and lung to reach 2.73 and 2.63 nmol mg −1 at 90 min, respectively. Lung slices incubated with l- or d-CIPE had increased l- or d-CySH concentrations up to a max of 13.7 and 11.1 nmol mg −1 and tracheal sections up to a max of 5.56 and 11.09 nmol mg −1. In the lung slice medium, l- and d-CIPE levels had decreased by 75.2% and 74.0% at 90 min, respectively, and from the tracheal section medium, l- and d-CIPE concentrations had decreased by 66.7% and 32.7%, respectively. Preincubation of lung slices and tracheal sections with the carboxylesterase inhibitor, bis ( p-nitrophenyl) phosphate (BNPP), almost completely prevented the disappearance of l- and d-CIPE extracellularly and greatly reduced the appearance of cellular l- and d-CySH. GSH, GSH isopropyl diester and γ-glu-cys isopropyl diester elevated and prolonged GSH concentrations in rat lung slices, but GSH isopropyl monoester and γ-glu-cys isopropyl monoester did not increase GSH levels. The uptake of l-CySH, but not d-CySH, is temperature sensitive in rat lung slices and tracheal sections and carboxylesterases appear to have a major influence on the uptake and metabolism of l- and d-CIPE by rat lung slices and tracheal sections.