A HIGHLY-ACTIVE MICROSOMAL GLUTATHIONE TRANSFERASE FROM FROG (XENOPUS-LAEVIS) LIVER THAT IS NOT ACTIVATED BY N-ETHYLMALEIMIDE

Microsomal glutathione transferase has hitherto only been purified fro m mammalian species. N-ethylmaleimide and trypsin activation (discrimi nating features of this enzyme) has only been observed in microsomes f rom mammals. In this paper me describe the first isolation and charact erization of a non-mammalian microsomal glutathione transferase from f rog (Xenopus laevis) liver. This protein has a molecular weight simila r to that of the mammalian enzyme (approximate to 17 kDa), but cannot be activated by N-ethylmaleimide or trypsin. In fact the enzyme is rap idly inactivated by this sulfhydryl reagent and protease, It thus appe ars that N-ethylmaleimide activation is not an obligatory property of microsomal glutathione transferase. The frog liver microsomal glutathi one transferase has one of the highest specific activities towards the second substrate 1-chloro-2,4-dinitrobenzene (CDNB) (200 mu mol/min m g) obtained with any glutathione transferase and accounts for the high activity found in frog Liver microsomes, The k(cat)/K-m for glutathio ne and CDNB are 0.017 and 1.1 X 10(6) M-1 s(-1), respectively. The enz yme also functions as a glutathione peroxidase (dilinoleoyl phosphatid ylcholine hydroperoxide is reduced (5.2 mu mol/min mg)). It is now evi dent that a highly active microsomal glutathione transferase, with a m olecular weight similar to that of the mammalian enzymes also exists i n a non-mammal species. (C) 1998 Academic Press.