HUMAN MU-CLASS GLUTATHIONE S-TRANSFERASES PRESENT IN LIVER, SKELETAL-MUSCLE AND TESTICULAR TISSUE

The major human Mu-class glutathione S-transferases (GST) have been pu rified to allow comparisons of their catalytic, physicochemical and im munochemical properties. GST isoenzymes, purified from hepatic, testic ular and skeletal muscle tissue were found to comprise three distinct subunits (M1, M2 and M3) which may combine to form both homodimeric an d heterodimeric proteins. Two distinct subunits, M1a and M1b, which re present allelic charge variants have been isolated but no polymorphic forms encoded at the GST M2 and M3 loci have been observed. Three GST isoenzymes (M1a-1a, M1a-1b and M1b-1b) have been purified from a singl e liver specimen. In addition, GST M1a-2, M1b-2, M2-2 and M2-3 have be en isolated from muscle, whilst the M3-3 homodimer has been purified f rom human testis. The homodimeric enzymes GST M1a-1a, M1b-1b, M2-2 and M3-3 have pI values of 6.1, 5.5, 5.3 and 5.0, whilst SDS-PAGE indicat ed that M1a, M1b, M2 and M3 have molecular masses of 26.7, 26.6, 26.0 and 26.3 kDa, respectively. The M1, M2 and M3 subunits isolated from e ither liver, skeletal muscle or testis, are catalytically distinct. Bo th M1-type subunits (M1a and M1b) possess a high activity for trans-4- phenyl-3-buten-2-one, whereas, the skeletal muscle subunit M2 has a hi gh activity towards 1,2-dichloro-4-nitrobenzene. By contrast, the test icular GST subunit M3 has no detectable activity towards either of the se substrates. However, all three Mu-class subunits are active towards the compounds 4-hydroxynonenal and 4-hydroxydecinal, possible endogen ous substrates which are produced by lipid peroxidation. The human Mu- class subunits can be distinguished immunochemically; antisera raised against the testicular GST M3-3 showed no reactivity towards either th e M1 or M2 subunits. The M3 subunit has a blocked N-terminus but autom ated amino-acid sequencing of a CNBr-derived peptide allowed 14 residu es of the M3 subunit to be identified. These data indicated that testi cular GST M3-3 is likely to correspond to the brain/testis Mu-class GS T cDNA described by Campbell et al. (Campbell E., Takahashi Y., Abramo vitz M., Peretz M., & Listowsky 1. (1990) J. Biol. Chem. 265, 9188-919 3).