Metabolism of N-acetyl-L-cysteine. Some structural requirements for the deacetylation and consequences for the oral bioavailability.

Rat liver, lung and intestine homogenates deacetylated N-acetyl-L-cysteine. Nearly stoichiometric amounts of L-cysteine were recovered. In rat liver, the enzyme activity was associated with the cytosolic fraction. Liver cytosol was much less active. N-Acetyl-D-cysteine or the disulphide of N-acetyl-L-cysteine were not deacetylated or in other ways consumed in vitro. Isolated, perfused rat liver did not retain or metabolize N-acetyl-L-cysteine to any measurable extent during single-pass experiments. N-Acetyl-L-cysteine or N-acetyl-D-cysteine were injected into a ligated segment of rat intestine in situ. After 1 hr 2% of the L-isomer and 35% of the D-isomer remained in the intestinal lumen. Systemic plasma levels were less than 3 microM of the L-form and congruent to 40 microM of the D-form. We conclude that deacetylation in the intestinal mucosa and possibly in the intestinal lumen is the major factor determining the low oral bioavailability of N-acetyl-L-cysteine. The deacetylation is discussed on the basis of the subcellular localization and the structural requirement of the reaction.