Stabilization of insulin receptor subunit structure by glutathione-insulin transhydrogenase

The occurrence of glutathione-insulin transhydrogenase (protein-disulfide interchange enzyme) in the lens

Glutathione-insulin transhydrogenase (GIT, thiol:protein-disulfide isomerase/oxidoreductase, E.C. 5.3.4.1/1.8.4.2) catalyzes via sulfhydryldisulfide interchange, the scission as well as formation of disulfide bonds in many diverse proteins. Using insulin as a substrate, the lens epithelial layer of cows, rats and rabbits was found to contain GIT activity. The enzyme's activity is activated by GSH and inhibited by N-ethylmaleimide. Subcellular distribution of bovine lens epithelial homogenates showed that the majority of GIT activity is located in the insoluble fraction (10,000 g pellet) and in the high molecular weight fraction (60,000 g pellet). Lens epithelial extracts were subjected to SDS-PAGE followed by Western blot, and probed with a polyclonal antibody to rat liver GIT, or with either of two monoclonal antibodies directed against different epitopes of the enzyme. Lens epithelium was found to contain two forms of GIT, one with the same molecular weight as the purified enzyme (Mr 56Kd), and a second having an Mr of 67Kd. Immunoblots using polyclonal antibodies revealed an additional major immunoreactive band of 32Kd in the cow lens epithelial layer as well as in the isolated cortical and nuclear portions. Rat lenses showed no immunoreactive 32Kd band. Using a bovine cortical/nuclear fraction the 32Kd reactivity was found to be associated with the beta H-crystallin fraction, but the extract failed to show GIT activity with the insulin substrate. This suggests that beta H-crystallin may share a common epitope with GIT.

Characterization of a cDNA for human glutathione-insulin transhydrogenase (protein-disulfide isomerase/oxidoreductase)

A human liver cDNA expression library in lambda-phage gt11 was screened with monoclonal antibodies to rat liver protein-disulfide isomerase/oxidoreductase (EC 5.3.4.1/1.8.4.2), also known as glutathione-insulin transhydrogenase (GIT). The nucleotide sequence of the largest cDNA insert (hgit-1) was determined. It contained approx. 1500 basepairs, representing an estimated 65% of the glutathione-insulin transhydrogenase message. The amino-acid sequence deduced from this cDNA insert contains a 7-amino-acid long polypeptide determined by sequencing the active-site fragment isolated from the rat GIT protein. A comparison of the nucleotide sequence of hgit-1 and a previously reported nucleotide sequence of rat glutathione-insulin transhydrogenase cDNA shows that the human hgit-1 clone corresponds to the middle of the transhydrogenase message at amino-acid residue number 275 of the rat protein, and codes for 206 amino-acid residues, including one of the two active-site regions of glutathione-insulin transhydrogenase, a stop codon (TAA), a long 3'-noncoding region of over 800 bases, a polyadenylation signal (AATAA), and a 29 base poly(A) tail. There exists high homology between the human and rat enzymes (94% in the overall amino-acid sequence, with 100% in the active site region and 81% in the nucleotide sequence within the coding portion of hgit-1). As with the rat enzyme, the human enzyme shows some identity with another dithiol-disulfide-exchange protein, Escherichia coli thioredoxin. Like rat cDNA, the human hgit-1 cDNA hybridized to rat mRNA of 2500 bases on a Northern blot. The relative quantitative abundance of GIT mRNA in nine rat tissues studied using hgit-1 as a hybridization probe was found to be in the same order as previously found with the rat cDNA. Thus, the above studies indicate that glutathione-insulin transhydrogenase is a highly conserved protein and that the human hgit-1 cDNA is suitable for use as a probe for further studies on gene regulation of this enzyme.

Sequence of membrane-associated thyroid hormone binding protein from bovine liver: its identity with protein disulphide isomerase

The complementary DNAs of the bovine liver membrane-associated 3,5,3'-triiodo-L-thyronine binding protein with 55 k-dalton (T3BP) were cloned and the nucleotide sequences were determined. Monospecific antibodies against T3BP were used to screen a bovine liver cDNA library in lambda gtll. We analyzed the sequences of two cloned T3BP cDNAs. The clones encoded a polypeptide of 510 amino acid residues, including a signal peptide of 20 amino acid. Northern blot analysis of bovine and human RNA showed that the mRNAs encoding T3BP are 2.7 kilobase in length. Amino acid sequence of N-terminal and other three peptides isolated from purified T3BP were found in the predicted amino acid sequence from the cDNA sequence. The predicted amino acid sequence is closely homologous (93%) with that of rat protein disulphide isomerase (EC 5.3.4.1), which catalyzes the isomerization of the protein disulphide bonds and has been shown to play an important role in post-translational regulation. The results suggest that T3BP and protein disulphide isomerase should be the same protein.

Characterization and application of monoclonal antibodies directed to separate epitopes of glutathione-insulin transhydrogenase

Five monoclonal antibodies specific for glutathione-insulin transhydrogenase were characterized. None of the monoclonal antibodies cross-reacted with another insulin-degrading enzyme, neutral thiopeptidase. The isotype of four antibodies was IgG1 and of the fifth IgG2b. Affinity studies, competitive binding studies and immunoblot analysis of CNBr and trypsin cleavage products of glutathione-insulin transhydrogenase demonstrated that the four IgG1 antibodies were directed to an epitope of the enzyme which was distinct from the epitope recognized by the IgG2b antibody. Inhibition studies indicated that each monoclonal antibody, when added singly to glutathione-insulin transhydrogenase, was unable to inhibit the insulin-degrading activity of the enzyme. However, when monoclonal antibodies directed against separate epitopes of glutathione-insulin transhydrogenase were presented together (i.e., the IgG2b with any one of the four IgG1 antibodies), a loss in enzymatic activity was noted. Immunoblot analysis of rat organ extracts with the IgG1 antibodies demonstrated one immunoreactive protein band of Mr 56,000 in all tissues examined (liver, fat, pancreas and kidney) except the spleen, which demonstrated two immunoreactive protein bands of Mr 56,000 and 51,000. The same immunoblots, when probed with the IgG2b antibody, demonstrated the same immunoreactive protein banding pattern as above plus an additional immunoreactive protein band of Mr 67,000 in all tissues. Studies with spleen extracts from steptozotocin-induced diabetic rats demonstrated that there was a loss of the 51,000 immunoreactive band in diabetes. This 51,000 protein was restored upon insulin treatment of the diabetic rats and nullified upon concomitant administration of cycloheximide or actinomycin D with insulin. Immunoblots of human liver, adipose and skeletal muscle extracts indicated that each monoclonal antibody cross-reacted with the human form of the enzyme which had a molecular weight of Mr 63,000; a second minor immunoreactive band of 67,000 was detected with the IgG2b antibody. The physiological significance of additional molecular forms of the enzyme (i.e., 67,000 and 51,000) remains to be determined.