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

Synthesis of A7,B7-dicarbainsulin, an analogue with a noncleavable bond between A- and B-chain. II. Synthesis of the A-chain segments

As part of the total synthesis of [A7,B7-L,L-2,7-diaminosuberoyl]-des-(B26-B30)-insulin B25-amide, an insulin analogue containing a non-cleavable bond between A- and B-chain, the chemical synthesis of the A-chain segments is described. The N-terminal sequence A(1-6), Boc-Gly-Ile-Val-Glu(OBut)-Gln-Cys(SBut)-NH-NH2, was synthesized in solution. The middle segment A(8-16), Ddz-Thr(But)-Ser(But)-Ile-Cys(SBut)-Ser(But)-Leu-Tyr- (But)-Gln-Leu-NH-NH2, was obtained by solid phase synthesis according to the Fmoc strategy. The C-terminal segment A(17-21), Bpoc-Glu(OBut)-Asn-Tyr-Cys(Acm)-Asn-OBut, was prepared in solution.

The distribution, abundance and subcellular localization of kinesin

An antiserum which binds kinesin specifically on Western blots was used to determine the distribution and abundance of chicken kinesin by correlated immunoblotting and immunolocalization. Quantitative immunoblotting showed that the abundance of kinesin varied widely in different cell and tissue types, from 0.039% of total protein in epidermal fibroblasts to 0.309% in sympathetic neurons; of the types examined, only red blood cells lacked detectable kinesin. The molar ratio of tubulin/kinesin varied over a narrower range. To analyze the intracellular distribution of kinesin, cultured fibroblasts were fractionated by sequential extraction with saponin-, Triton X-100-, and SDS-containing buffer. Quantitative blotting of the resulting cell fractions indicated that 68% of fibroblast kinesin is in soluble form, 32% is membrane- or organelle-associated, and none is detectable in cytoskeletal fractions. To visualize this distribution, cells treated by the same extraction protocol were immunofluorescently stained with antikinesin and antitubulin. Without extraction, kinesin staining was located throughout cultured neurons and fibroblasts. However, when fibroblasts were extracted with saponin or Brij 58 before fixation, subsequent staining revealed that the remaining kinesin fraction was colocalized with interphase microtubules, but not with mitotic spindles. Prefixation extraction with Triton abolished antikinesin staining. These data suggest that kinesin may play a role in tubovesicular movement but provide no evidence for a role in mitosis.

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.