Differential expression of α, μ and π classes of isozymes of glutathione S-transferase in bovine lens, cornea, and retina

Abstract

Isozyme characterization of glutathione S-transferase (GST) isolated from bovine ocular tissue was undertaken. Two isozymes of lens, GST 7.4 and GST 5.6, were isolated and found to be homodimers of a M r 23,500 subunit. Amino acid sequence analysis of a 20-residue region of the amino terminus was identical for both isozymes and was identical to GST ψ and GST μ of human liver. Antibodies raised against GST ψ cross-reacted with both lens isozymes. Although lens GST 5.6 and GST 7.4 demonstrated chemical and immunological relatedness, they were distinctly different as evidenced by their p I and comparative peptide fingerprint. A corneal isozyme, GST 7.2, was also isolated and established to be a homodimer of M r 24,500 subunits. Sequence analysis of the amino-terminal region indicated it to be about 67% identical with the GST π isozyme of human placenta. Antibodies raised against GST π cross-reacted with cornea GST 7.2. Another corneal isozyme, GST 8.7, was found to be a homodimer of M r 27,000 subunits. Sequence analysis revealed it to have a blocked amino-terminus. GST 8.7 immunologically cross-reacted with the antibodies raised against cationic isozymes of human liver indicating it to be of the α class. Two isozymes of retina, GST 6.8 and GST 6.3, were isolated and identified to be heterodimers of subunits of M r 23,500 and 24,500. Amino-terminal sequence analysis gave identical results for both retina GST 6.8 and GST 6.3. The sequence analysis of the M r 23,500 subunit was identical to that obtained for lens GSTs. Similarly, sequence analysis of the M r 24,500 subunit was identical to that obtained for the cornea GST 7.2 isozyme. Both the retina isozymes cross-reacted with antibodies raised against human GST ψ as well as GST π. The results of these studies indicated that all three major classes of GST isozymes were expressed in bovine eye but the GST genes were differentially expressed in lens, cornea, and retina. In lens only the μ class of GST was expressed, whereas cornea expressed α and π classes and retina expressed μ and π classes of GST isozymes.