Purification and characterization of human kidney cytosolic cysteine conjugate beta-lyase activity.
The central role of cysteine conjugate beta-lyase (beta-lyase) in the bioactivation of nephrotoxic halogenated hydrocarbons and the possibility of human exposure to these chemicals has focused interest on the beta-lyase from human kidney. Human kidney tissue was collected as surgical waste material, and subcellular fractions were isolated by differential centrifugation. Human beta-lyase activity, determined with S-(2-benzothiazolyl)-L-cysteine (BTC) as the substrate, was present in the cytosolic, mitochondrial, and microsomal fractions, but was highest in the cytosolic fraction. Activity in human kidney cytosol was about 10% of that present in rat kidney cytosol. Human kidney cytosolic beta-lyase activity, with BTC as the substrate, was not stimulated by pyridoxal phosphate or by exogenous 2-keto acids. Cytosolic beta-lyase activity was purified 280-fold with a yield of 12% from human kidneys unsuitable for transplantation. The beta-lyase activity copurified with cytosolic glutamine transaminase K and exhibited a molecular mass of 85 kDa on a Sephacryl 5300 column and a subunit molecular mass of 45 kDa by gel electrophoresis. Both BTC and its homocysteine analogue S-(2-benzothiazolyl)-L-homocysteine were excellent substrates, exhibiting Km and kcat values of 0.97 mM and 2.78 mM and 9.35 min-1 and 6.90 min-1, respectively. beta-Lyase activity was inhibited by aminooxyacetic acid, indicating that the human cytosolic enzyme contains pyridoxal phosphate, and by the nephrotoxins S-(1,2-dichlorovinyl)-L-cysteine and S-(1,2-dichlorovinyl)-L-homocysteine, which serve as alternative substrates.(ABSTRACT TRUNCATED AT 250 WORDS)