Protein disulfide-isomerase synthesis

The hbraries of enzyme substrates were obtained by spht-pool synthesis to yield one-bead-one-compound hbraries. The substrate assay was performed with a range of proteolytic enzymes such as subtilisin Carlsberg [26], cruzipain [27], protein disulfide isomerase [28-29], matrix metalloprotease MM P-9 [30], papain [31],... 

Protein phosphatases 544, 646 Protein S 634 Protein sequenators 118 Protein sequences from genes 119 Protein synthesis 3, 538, 539 Protein tyrosine kinases 544 Protein-disulfide isomerase 83 Protein-DNA interactions 266 Proteinase. See Protease Proteoglycan(s) 181,182. See also Glycosami-noglycans... 

In ABL, an early step in apoB lipoprotein assembly shared by intestinal and liver cells is defective. The net result is near absence of all plasma apoB lipoproteins. ApoB synthesis from a mRNA transcript occurs, but its successful assembly into the mature lipoprotein particle does not. The inability to assemble apoB into lipoproteins was shown to be due to a defect in the mttp gene in affected individuals (Wetterau et al., 1992). Its translational product is an 894-amino acid, 97-kd, polypeptide that exists in the ER complexed with a 55-kd protein disulfide isomerase which is believed to maintain solubility, physiologic activity, and ER retention of the 97-kd peptide. The heterodimeric complex of the 97-kd and 55-kd subunits is referred to as microsomal triglyceride transfer protein (MTP) (Wetterau et al., 1992). 

By analogy with animal systems, the function of signal sequences in plant storage proteins is to facilitate the translocation of the storage protein into the lumen of the endoplasmic reticulum (ER) as the first step in intracellular transport. Protein folding and disulfide bond formation are considered to occur within the lumen of the endoplasmic reticulum, and may be assisted by molecular chaperones and by the enzyme protein disulfide isomerase respectively [83]. The precise mechanism of intracellular transport of storage proteins from their site of synthesis to their site of deposition are still largely unknown but a two-way hypothesis has been proposed by Shewry [45]. 

Whether oxidants affect the lipase action or promote the synthesis and release of TG-rich lipoproteins has not been determined. It is also possible that TG containing peroxidized fatty acid components are resistant to lipolysis. Earlier studies have demonstrated that antioxidants enhanced the (hepatic) lipase actions [66] however, these studies have not been corroborated or confirmed by more robust studies. In contrast, phospholipases seem to act more robustly on peroxidized or oxidatively tailored phospholipids [67-70] although the enzyme itself could be inactivated by oxidants [70], Recently it was reported that the protein disulfide isomerase, a redox-sensitive enzyme, could contribute to the endoplasmic reticulum-associated degradation of apoB through its chaperone activity [71],... 

It is possible to reduce enzymatically both AFR and DHA, regenerating ascorbate and thus decreasing nutritional requirements in animals unable to carry out the synthesis de novo. Enzymatic systems that reduce AFR usually utilize NADH as an electron donor (one-electron reduction pathway or NADH-ascorbate free radical reductase). Reduction of DHA to ascorbate involves two electrons and has been attributed to thioltransferase (glutaredoxin) and protein disulfide isomerase (Wells et aL, 1990). However, taking into consideration the reported values for DHA (millimolar range) it is unlikely that the latter enzymes are involved in the maintenance of ascorbate in its reduced state in vivo and that function is carried out by NADH-AFR reductase (Minetti et aL, 1992). 

Clearly, the action of prolyl isomerases is not restricted to the slow folding of polypeptide chains with intact disulfides, but they also accelerate the oxidative folding of reduced proteins, which resemble more closely the nascent polypeptide chains as they occur in the endoplasmic reticulum. The simultaneous presence of PPI markedly enhances the efficiency of PDI as a catalyst of disulfide bond formation. Both enzymes act according to their specificity and catalyze the isomerization of prolyl peptide bonds and the formation of disulfide bonds, respectively, in the folding protein chains. It remains to be demonstrated that a similar concerted action of the two enzymes can take place in the course of de novo synthesis and folding of proteins in the cell. 

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