title: Interaction of N-acetylcysteine and cysteine in human plasma.
abstract: N-acetyl-L-cysteine (NAC), a well-known antioxidant, has been successfully used as adjuvant therapy for late-stage childhood cerebral adrenoleukodystrophy (c-ALD); however, the mechanisms of NAC action are poorly understood. Previous research indicates that NAC serves as a precursor to L-cysteine (Cys), the rate-limiting substrate in the biosynthesis of glutathione (GSH), a potent, endogenous antioxidant. We hypothesized that NAC acts by liberating protein-bound Cys in plasma in an NAC concentration-dependent manner, which increases unbound Cys available for GSH biosynthesis. Human plasma was incubated for 1 h with varying, clinically relevant concentrations of NAC (0-1000 µg/mL). The effect of this interaction over time was evaluated by incubating plasma for 5-90 min with 100 µg/mL NAC. Unbound and bound Cys and NAC were separated by ultrafiltration, and concentrations were measured using high-performance liquid chromatography-mass spectrometry. Significant increases in unbound Cys were observed with increasing NAC concentrations. Also, Cys plasma protein binding decreased from 85% (10 µg/mL NAC) to approximately 0% (1000 µg/mL). Total endogenous Cys was 66% unbound at 5 min after incubation. These results demonstrate that NAC liberates endogenous, protein-bound Cys in human plasma at clinically relevant NAC concentrations. A greater understanding of NAC actions will aid in the optimization of NAC therapy including its use in c-ALD.
authors.affiliation: Center for Orphan Drug Research, Department of Experimental and Clinical Pharmacology, College of Pharmacy, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: Kendra K
authors.initials: KK
authors.lastname: Radtke
authors.forename: Lisa D
authors.initials: LD
authors.lastname: Coles
authors.forename: Usha
authors.initials: U
authors.lastname: Mishra
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Orchard
authors.forename: Mary
authors.initials: M
authors.lastname: Holmay
authors.forename: James C
authors.initials: JC
authors.lastname: Cloyd
mesh_terms: D000111:Acetylcysteine / Q000097:blood / Q000494:pharmacology*; D000975:Antioxidants / Q000494:pharmacology*; D001798:Blood Proteins / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003545:Cysteine / Q000097:blood / Q000378:metabolism*; D006801:Humans; D013058:Mass Spectrometry; D011485:Protein Binding / Q000187:drug effects
chemical_list: D000975:Antioxidants; D001798:Blood Proteins; D003545:Cysteine; D000111:Acetylcysteine