Cytosolic rat liver glutathione transferase 4-4. Primary structure of the protein reveals extensive differences between homologous glutathione transferases of classes Alpha and Mu

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Abstract
The primary structure of the class Mu glutathione transferase 4-4 from rat liver was determined. The structural data characterize a class Mu protein within an enzyme family for which three classes have been distinguished (Alpha, Mu, Pi). The structures was determined by analysis of peptides obtained after treatment with trypsin, Glu-specific protease and CNBr. The protein is composed of two identical subunits, each with 217 amino acid residues. No evidence for microheterogeneity or for the presence of modified residues was encountered. The primary structure was found to be strictly homologous with corresponding parts in known regions of other class Mu enzymes of rat, mouse, human and bovine origin. Relationships to the cytosolic enzyme of other classes (Pi and Alpha) are considerably more distant. A comparison with the entire chain of the class Alpha subunit 1 from rat liver was carried out by three methods, alignment of amino acid sequences, correlation of hydrophilicity plots and predictions of secondary structures. All methods reveal weak similarities but also large differences. The overall positional identity is only 26%. Combined, the results establish the first complete class Mu structure, show distant inter-class relationships and relate subunit 4 (class Mu) and subunit 1 (class Alpha) in a family of enzymes rather than in a group of isoenzymes.