Properties of the microsomal and cytosolic glutathione transferases involved in hexachloro-1:3-butadiene conjugation.

Hexachloro-1,3-butadiene (HCBD) is a substrate for the hepatic microsomal glutathione transferases and is metabolised at higher rates by these enzymes than their cytosolic counterparts. Conjugation reactions catalysed by the microsomal and cytosolic transferases have been studied and characterized using this substrate and 1-chloro-2,4-dinitrobenzene (CDNB). In rat liver microsomes the Km values for HCBD and CDNB were 0.91 and 0.012 mM and in cytosol 0.51 and 0.10 mM respectively. Vmax values for HCBD were 1.39 and 0.35 nmol conjugate formed/min/mg protein for microsomes and cytosol respectively. In microsomal systems HCBD was a potent competitive inhibitor of the metabolism of CDNB with a Ki value of approximately 10 microM. However, CDNB did not inhibit HCBD metabolism significantly. These data suggest that more than one microsomal enzyme is involved in HCBD metabolism. The microsomal membrane could be solubilized without significant inhibition of HCBD activity; however, some detergents did inhibit the conjugation reaction. Activity was also lost on treatment of microsomal membranes with trypsin indicating the enzyme is localized on the cytoplasmic surface of the endoplasmic reticulum. Pretreatment of the rats with Aroclor 1254, 3-methylcholanthrene or phenobarbital did not change the microsomal conjugation of HCBD or CDNB with glutathione. Of seven species investigated, a human liver sample showed the highest ratio of microsomal to cytosolic glutathione transferase activity for HCBD (in microsomes 40-fold higher specific activity than in cytosol). Glutathione conjugation appears to play a critical role in the toxicity and carcinogenicity of some halogenated hydrocarbons. These data substantiate the potentially important role for the microsomal glutathione transferase in catalysing these reactions.