Variation in the expression of Mu-class glutathione S-transferase isoenzymes from human skeletal muscle. Evidence for the existence of heterodimers

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Abstract
The cytosolic glutathione S-transferases (GST) from human skeletal muscle were purified by a combination of affinity chromatography and anion-exchange chromatography followed by either chromatofocusing or hydroxyapatite chromatography. Pi-class and Mu-class GST, but not Alpha-class GST, were isolated from muscle. In addition to a Pi-class GST subunit, which exists as a homodimer, this tissue also contains a total of three distinct neutral-type Mu-class GST subunits, which hybridize to form homodimers or heterodimers. The neutral-type subunits are referred to as N1-N3 and are defined by the decreasing isoelectric points of the homodimers; GST N1N1, N2N2 and N3N3 have estimated pI values of 6.1, 5.3 and less than 5.0 respectively. SDS/PAGE showed that N1, N2 and N3 have Mr values of 26,700, 26,000 and 26,300 respectively. The N1, N2 and N3 subunits are catalytically distinct, with N1 possessing a high activity for trans-4-phenylbut-3-en-2-one and N2 having high activity with 1,2-dichloro-4-nitrobenzene. In skeletal muscle the expression of the N1 subunit, but not of N2 and N3 subunits, was found to differ from specimen to specimen. The N1 subunit was absent from about 50% of samples examined, and the purification results from two different specimens are presented to illustrate this inter-individual variation. Skeletal muscle from one individual (M1), which did not express N1, contained only GST N2N2, N2N3 and pi, whereas the second sample examined (M2) contained GST N1N2, N2N2 and N2N3 as well as GST pi. N-Terminal amino acid sequence analysis supported the electrophoretic evidence that the N2 subunit in GST N1N2, N2N2 and N2N3 represents the same polypeptide. The peptides obtained from CNBr digests of N2 were subjected separately to automated amino acid sequencing, and the results indicate that N2 is distinct but closely related to the protein encoded by the human Mu-class cDNA clone GTH4 [DeJong, Chang, Whang-Peng, Knutsen & Tu (1988) Nucleic Acids Res. 16, 8541-8554]. GST N2N2 is probably identical with GST 4 [Board, Suzuki & Shaw (1988) Biochim. Biophys. Acta 953, 214-217], as over the 24 N-terminal residues of GST 4 there is complete identity between the two enzymes. Our data suggest that the GST 1 and GST 4 loci are part of the same multi-gene family.