Equilibrium and Kinetics Studies of Transnitrosation Between S-Nitrosothiols and Thiols Kun Wang, a,b Zhong Wen, a Wei Zhang, a Ming Xian, a,b Jin-Pei Cheng b and Peng George Wang a, * a Department of Chemistry, Wayne State University, Detroit, MI 48202, USA b Department of Chemistry, Nankai University, Tianjin 300071, China Received 26 September 2000; accepted 30 November 2000 AbstractÐUsing UV±vis spectrometrical measurements, equilibrium constants for NO transfer between S-nitroso-N-acetyl-peni- cillamine (SNAP) and dierent thiols as well as kinetic data for NO transfer from S-nitroso bovine serum albumin (BSANO) to thiols have been obtained. NO transfer from SNAP to other primary/secondary thiols are thermodynamically favorable, whereas other S-nitrosothiols exhibit similar NO transfer potential. The obtained Gibbs free energy, enthalpy and entropy data indicated that NO transfer reactions from SNAP to four thiols are exothermic with entropy loss. The kinetic behavior of BSANO/RSH transfer can be related to both the acidity of sulfhydryl group and the electronic structure in thiol. # 2001 Elsevier Science Ltd. All rights reserved. In recent years, tremendous attention has been focused on the chemistry and biology of nitric oxide (NO) due to the discovery that NO plays a key role in a wide variety of human physiological processes. 1 It has been suggested that S-nitrosothiols (RSNOs) may be directly involved in many of the biological functions of nitric oxide. 2 4 RSNOs have been detected in human airway lining ¯uid plasma, platelets and neutrophils, 5 therefore they can act as an NO carrier in the form of free thiol or cysteine containing protein in biological systems. 6 These endogenous S-nitrosothiols may play an important role in NO storage, transport, and delivery. Some RSNO compounds, such as S-nitroso-N-acetyl-penicillamine (SNAP) 7 and S-nitrosocaptopril, 8 can indeed be used as therapeutic drugs for treatment of angina and other circulation problems, their functions also rely on their ability to generate NO in vivo. 9 Fully understanding the biological roles of NO requires fundamental knowledge of the kinetics and thermo- dynamics of the NO-related reactions. NO transfer from one NO donor, especially S-nitrosothiol, to another thiol has been a focus of interest due to their role in NO transfer in vivo. Mechanistic and kinetics studies on transnitrosation from RSNOs to thiols have been reported previously by Williams, 10 Meyer, 11 Rossi, 12 and Means, 13 using UV±vis spectroscopic methods. Recently using HPLC method, Hogg 14 reported equili- brium and kinetics study on transnitrosation. As an eort to oer a thermodynamic view of NO transfer both in vitro and in vivo, we reported herein our inves- tigation on the equilibrium of the transnitrosation between SNAP and thiols as well as kinetic study on the transnitrosation between S-nitroso bovine serum albumin (BSANO) and thiols. Equilibrium Study of NO Exchange Between SNAP and Thiols In the present study, SNAP was selected as the NO donor since it has a maximum absorption at 590 nm while l max for primary S-nitrosothiols is around 540 nm, transnitrosation between SNAP and primary thiols can be readily analyzed using UV±vis spectrometric measurement (Fig. 1). UV±vis spectra of reaction mix- tures were obtained by adding an equimolar amount of thiol to a cuvette containing RSNO. After the equili- brium was reached (Fig. 2), the concentrations of the reactants and products can be calculated from the change of the absorption of SNAP at 590 nm according to the Beer±Bouger±Lambert law. Equilibrium constant (K eq ) of NO transfer between SNAP and dierent thiols can be determined from the calculated concentrations as 0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0960-894X(00)00688-0 Bioorganic & Medicinal Chemistry Letters 11 (2001) 433±436 *Corresponding author. Tel.: +1-313-577-6759; fax: +1-313-577- 2554; e-mail: pwang@chem.wayne.edu