Detection of the substance immunologically cross-reactive with insulin in insulin RIA is an artifact caused by insulin tracer degradation: involvement of the insulin-degrading enzyme

Peptidase inhibitors in the MEROPS database

The MEROPS website (http://merops.sanger.ac.uk) includes information on peptidase inhibitors as well as on peptidases and their substrates. Displays have been put in place to link peptidases and inhibitors together. The classification of protein peptidase inhibitors is continually being revised, and currently inhibitors are grouped into 67 families based on comparisons of protein sequences. These families can be further grouped into 38 clans based on comparisons of tertiary structure. Small molecule inhibitors are important reagents for peptidase characterization and, with the increasing importance of peptidases as drug targets, they are also important to the pharmaceutical industry. Small molecule inhibitors are now included in MEROPS and over 160 summaries have been written.

The pharmacokinetics of pulmonary insulin in the in vitro isolated perfused rat lung: Implications of metabolism and regional deposition

The pharmacokinetics of several lung disposition pathways for pulmonary insulin were studied and modeled in the isolated perfused rat lung (IPRL). Insulin solution was administered by forced instillation into the airways of the IPRL as 0.1 or 0.02 ml doses of coarse spray, with or without bacitracin (BAC), N-ethylmaleimide (NEM) and atrial natriuretic peptide (ANP). Each insulin absorption profile was fitted to a kinetic model that incorporated the distribution fraction of the dose reaching the lobar region (DF) and the rate constants for absorption into perfusate (k(a)) and non-absorptive loss (k(nal)); k(nal) was shown to be due to the sum of mucociliary clearance and metabolism. Insulin absorption occurred largely by passive diffusion with values for k(a) = 0.39-0.50 h(-1). With DF = 0.91 following 0.1 ml doses, 11.9 +/- 3.4% of bioavailabilities were observed in 1h. In contrast, derived values for k(nal) = 2.34-3.45 h(-1) were significantly larger than the rate constant for mucociliary clearance determined previously in this IPRL (0.96-1.74 h(-1)) due to lung metabolism. Indeed, BAC, but neither NEM nor ANP, was found to decrease the value of k(nal), which suggested that BAC-inhibitable lung ectopeptidases, and not insulin degrading enzyme (IDE), were responsible for this pulmonary metabolism. Shallower lung distribution with DF = 0.73 following 0.02 ml doses resulted in reduced values for k(a) = 0.27 h(-1) and k(nal) = 2.79 h(-1), indicating that these kinetic processes may be lung-region dependent, even within this model and emphasizing the likely importance of reliable lung deposition in vivo.

Non-covalent interaction of ubiquitin with insulin-degrading enzyme

Insulin-degrading enzyme (IDE) is a metalloprotease implicated in insulin degradation and suggested to have a variety of additional functions, including the clearance of amyloid beta peptides of Alzheimer's disease. Little is known about endogenous proteins that may interact with and modulate IDE's activity in the cell. We purified and characterized two proteins from mouse leukemic splenocytes that interact with IDE and inhibit its insulin-degrading activity. A protein of 14 kDa was similar to a competitive IDE inhibitor reported previously. The major inhibitor was identified by amino acid sequencing as ubiquitin, a protein that is post-translationally covalently attached to other intracellular proteins and regulates diverse cellular processes. Ubiquitin inhibited insulin-degrading activity of IDE and diminished crosslinking of 125I-insulin to IDE in a specific, concentration-dependent, reversible, and ATP-independent manner. Ubiquitin did not affect the crosslinking of 125I-insulin to insulin receptors or of 125I-atrial natriuretic peptide (ANP) to its receptor guanylate cyclase-A. These findings suggest a novel role for ubiquitin or perhaps proteins with ubiquitin-like domains in regulating the function of IDE.

Profiling and in vivo Quantification of Proteins by High Resolution Mass Spectrometry: The Example of Goserelin, an Analogue of Luteinizing Hormone-Releasing Hormone

Proteins are essential biomolecules which are frequently involved in major pathological syndromes and are widely used as diagnostic markers or therapeutic agents. The emergence of proteomics will doubtless further increase the significance of proteins both in the clinic and in the life sciences in general. Our main objective is to offer innovative solutions to what we like to call the "post-proteomics era". To achieve our goal, we intend to develop novel approaches and technologies for in vivo metabolic studies of proteins using mass spectrometry (MS), focusing on pharmacokinetics and pharmacodynamics. Using goserelin as a model, we have successfully developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the detection and quantification of an intact analogue of luteinizing hormone-releasing hormone (LHRH) in small volumes of rat plasma samples at concentrations ranging from 0.3 to 405.0 microg/l. To this end, a microbore reversed-phase-HPLC system was coupled on-line to a tandem high resolution quadrupole time-of-flight (Q-TOF) instrument fitted with an electrospray ion source and operated in LC-MS/MS mode. External calibration was used and the high resolution was crucial to discard contaminating signals, which would not have been possible with the more conventional triple quadrupole mass spectrometers operated in a static mode. For low sample amounts, calibration curves were constructed corresponding to rat plasma levels of 0.3 to 16.4 microg/l and found to be of third order with a coefficient of determination greater than 0.999. The relative standard deviation was found to be lower than 15%. A lower limit of detection (LLOD) of 0.17 microg/l and a lower limit of quantification (LLOQ) of 0.3 microg/l were determined.

Insulin is degraded extracellularly in wounds by insulin-degrading enzyme (EC 3.4.24.56)

The exact mechanism by which insulin reverses impaired wound healing is unknown. Previous investigators have shown that insulin is degraded in experimental wounds, suggesting that the action of insulin may be locally modified. The following study corroborates these findings and identifies the major proteinase responsible for insulin degradation in wound fluid (WF). Adult male Fisher rats were wounded by subcutaneous implantation of polyvinyl alcohol sponges while under pentobarbital sodium anesthesia. WF and serum were collected on 1, 5, 10, and 14 days postinjury. Decreased insulin concentration in late WF correlated with an increased insulin-degrading activity. Multiple proteinases appear to participate in the overall degradation of insulin in WF. However, the primary enzyme responsible for insulin degradation in WF was characterized by immunoprecipitation and immunoblotting and identified as the neutral thiol-dependent metalloproteinase, insulin-degrading enzyme (EC 3.4.24.56). Exogenous steroid administration caused a decrease in WF insulin-degrading activity. Glucagon and adrenocorticotrophin degradation was also observed, whereas minimal degradation of insulin-like growth factors I and II and epidermal growth factor was detected in WF. The ability to extracellularly degrade insulin may represent a unique mechanism for the regulation of this hormone's role in healing wounds.

Development of an isotope dilution assay for precise determination of insulin, C-peptide, and proinsulin levels in non-diabetic and type II diabetic individuals with comparison to immunoassay

We describe the application of a stable isotope dilution assay (IDA) to determine precise insulin, C-peptide, and proinsulin levels in blood by extraction from serum and quantitation by mass spectrometry using analogues of each target protein labeled with stable isotopes. Insulin and C-peptide levels were also determined by immunoassay, which gave consistently higher results than by IDA, the relative difference being larger at low concentrations. Insulin, C-peptide, and proinsulin levels were all shown by IDA to be higher in type II diabetics than in non-diabetics, with mean values rising from 22 (+/- 2) to 92 (+/- 8), 335 (+/- 11) to 821 (+/- 24), and 6 (+/- 1) to 37 (+/- 3) pM, respectively. Interestingly, the ratio between IDA and immunoassay values for insulin levels increased from 1.3 in non-diabetics to 1.7 in type II diabetics. The ratio between proinsulin and insulin levels by IDA increased from 0.24 in non-diabetics to 0.36 in type II diabetics, whereas the ratio between C-peptide and insulin levels by IDA decreased from 17.6 to 10.7. This disproportionate change in protein levels between different types of individuals has implications for the metabolism of insulin in the diabetics studied (type II) and suggests that C-peptide levels are not always a reliable guide as to pancreatic insulin secretion. In addition, levels of the 33-residue C-peptide (partially trimmed form) were shown to be less than 10% that of the fully trimmed 31-residue C-peptide levels, and we tested IDA in a clinical context by two post-pancreatic graft studies. IDA was shown to give direct, positive identification of the target protein with unrivaled accuracy, avoiding many of the problems associated with present methodology for protein determination.