Measurement of Cysteine S‐Conjugate β‐Lyase Activity

Abstract

AbstractCysteine S‐conjugate β‐lyases are pyridoxal 5′‐phosphate (PLP)–containing enzymes that catalyze the conversion of cysteine S‐conjugates [RSCH2CH(NH3+)CO2−] and selenium Se‐conjugates [RSeCH2CH(NH3+)CO2−] that contain a leaving group in the β position to pyruvate, ammonium and a sulfur‐containing fragment (RSH) or selenium‐containing fragment (RSeH), respectively. In mammals, at least ten PLP enzymes catalyze β‐elimination reactions with such cysteine S‐conjugates. All are enzymes involved in amino acid metabolism that do not normally catalyze a β‐lyase reaction, but catalyze a non‐physiological β‐lyase side‐reaction that depends on the electron‐withdrawing properties of the –SR or –SeR moiety. In the case of cysteine S‐conjugates, if the eliminated RSH is stable, the compound may be S‐thiomethylated and excreted (thiomethyl shunt) or S‐glucuronidated and harmlessly excreted. However, if RSH is chemically reactive, the cysteine S‐conjugate may be toxic as a result of the β‐lyase reaction. The cysteine S‐conjugate β‐lyase pathway is of particular interest to toxicologists because it is involved in the bioactivation (toxification) of halogenated alkenes and certain drugs. This unit provides protocols for the analysis of cysteine S‐conjugate β‐lyase activity. Curr. Protoc. Toxicol. 44:4.36.1‐4.36.18. © 2010 by John Wiley & Sons, Inc.