The subunit structure of a major glutathione S‐transferase form, M_T, in rat testis

<jats:p>A major glutathione <jats:italic>S</jats:italic>‐transferase form (pI 5.7) in rat testis (M<jats:sub>T</jats:sub>) purified by <jats:italic>S</jats:italic>‐hexyl‐glutathione affinity chromatography, followed by chromatofocusing, showed two polypeptide of pI 6.7 (Yn<jats:sub>1</jats:sub>) and 6.0 (Yn<jats:sub>2</jats:sub>), having apparently the same molecular mass of 26 kDa on two‐dimensional gel electrophoresis. Rechromatofocusing o the M<jats:sub>T</jats:sub> preparation after 4 M guanidine hydrochloride treatment revealed two additional protein peaks (pI 6.2 and 5.4). These were identified as the two homodimers consisting of the subunits of M<jats:sub>T</jats:sub>, Yn<jats:sub>1</jats:sub> Yn<jats:sub>1</jats:sub> and Yn<jats:sub>2</jats:sub> Yn<jats:sub>2</jats:sub> respectively. Furthermore, M<jats:sub>T</jats:sub> could be reconstituted from Yn<jats:sub>1</jats:sub> Yn<jats:sub>1</jats:sub> and Yn<jats:sub>2</jats:sub> Yn<jats:sub>2</jats:sub>. These results indicate that M<jats:sub>T</jats:sub> is a heterodimer, Yn<jats:sub>1</jats:sub> Yn<jats:sub>2</jats:sub>, consisting of subunits with very similar molecular masses but different isoeleetric points. The Yn<jats:sub>1</jats:sub> Yn<jats:sub>1</jats:sub> form had glutathione <jats:italic>S</jats:italic>‐transferase activities towards l‐chloro‐2,4‐dinitrobenzene and 1,2 dichloro‐4‐nitrobenzene. However, the Yn<jats:sub>2</jats:sub> Yn<jats:sub>2</jats:sub> form had no activity towards any of the substrates examined N‐terminal amino acid sequences of subunits Yn<jats:sub>1</jats:sub> and Yn<jats:sub>2</jats:sub> revealed differences at two positions in the first 20 residues; the amino acid compositions of these subunits were also similar but not identical, indicating that these two subunits are different in the primary structure. Subunits Yn<jats:sub>1</jats:sub> and Yn<jats:sub>2</jats:sub> are immunologically related to each other and also to subunits 3 (Yb<jats:sub>1</jats:sub> and 4 (Yb<jats:sub>2</jats:sub>) but they are not identical. These four subunits also showed a high degree of similarity in N‐terminal amino acid sequences. Subunits Yn<jats:sub>l</jats:sub> and Yn<jats:sub>2</jats:sub> seem to belong to the rat GST 3‐4 family or class mu. Subunits Yn<jats:sub>1</jats:sub> and 4 can make a heterodimer, which is detectable not only in rat testis, but also in the heart, kidney and lung. The Yn<jats:sub>1</jats:sub> Yn<jats:sub>1</jats:sub> form was not detected in the testis, but is present in rat brain [Tsuchida et al. (1987) <jats:italic>Eur. J. Biochem. 170</jats:italic>, 159‐164]. The Yn<jats:sub>2</jats:sub> Yn<jats:sub>2</jats:sub> form seemed to differ from GST 5‐5 and may be a new form of rat glutathione <jats:italic>S</jats:italic>‐transferase.</jats:p>