Structure-activity relationships of cysteine esters and their effects on thiol levels in rat lung in vitro.

Pretreatment with cysteine esters increases cysteine (CySH) levels in rat lung and protects against the lethal effects of inhaled perfluoroisobutene in vivo. There are marked differences in the duration of protection achieved with different cysteine esters. In this study we have compared the uptake and metabolism of CySH, N-acetyl cysteine (NAc), cysteine esters and cystine esters in vitro using rat lung and liver homogenates and lung slices. Liver homogenates metabolized CySH and cysteine esters faster than lung homogenates. The half life (T1/2) of CySH in lung was 58.8 +/- 17.3 min and in liver was 14.0 +/- 1.6 min (mean +/- SEM). T1/2 of the esters in lung ranged between 6.5 and 12.1 min and in liver between 1.9 and 5.3 min. Cysteine tertiary butyl ester, which does not protect in vivo, was not hydrolysed to CySH by lung or liver homogenates. All esters increased and prolonged intracellular CySH concentrations in lung slices to a much greater extent than CySH itself. NAc did not raise intracellular CySH above that of the controls and no NAc appeared within the slice. After CySH incubation intracellular CySH was 0.9 +/- 0.1 nmol/mg wet wt at 10 min whereas after incubation with the esters it ranged between 2.60 and 3.65 nmol/mg wet wt. Cysteine cyclohexyl ester prolonged the increase of CySH the longest and cysteine methyl ester the shortest. CySH levels with cysteine cyclohexyl ester were 2.74 +/- 0.15 and 4.13 +/- 0.37 nmol/mg wet wt at 10 and 60 min, respectively, whereas with cysteine methyl ester, CySH levels were 2.60 +/- 0.5 and 1.25 +/- 0.08 nmol/mg wet wt at similar times. Cystine esters increased intracellular concentrations of both cystine and CySH. CySH concentrations ranged between 2.92 and 3.19 nmol/mg wet wt and cystine between 1.39 and 1.47 nmol/mg wet wt at 60 min. The elevation and duration of CySH in lung slices is well correlated with the duration of protection against perfluoroisobutene achieved in vivo.