Degradation of proinsulin and isolated C-peptide by rat kidney neutral metallo-endopeptidase

Marked Increase in Urinary C-peptide Levels after Treatment with Sacubitril/Valsartan in Patients with Type 2 Diabetes Mellitus and Hypertension

Sacubitril/valsartan, a novel therapy in chronic heart failure (CHF), inhibits the breakdown of various peptides. However, whether or not sacubitril/valsartan administration affects urinary C-peptide levels is unclear. We herein report a 70-year-old man with type 2 diabetes mellitus (T2DM) and hypertension coexisting with CHF and nephrotic syndrome. The patient's urinary C-peptide levels dramatically increased after sacubitril/valsartan administration and decreased after discontinuation of the drug. Furthermore, sacubitril/valsartan administration to five other patients with hypertension and T2DM markedly increased urinary C-peptide levels. Thus, the insulin secretory capacity of patients with T2DM receiving sacubitril/valsartan may be overestimated when their urinary C-peptide level is measured.

Degradation of porcine relaxin by glutathione-insulin transhydrogenase and a neutral peptidase

The susceptibility of porcine relaxin and 125I-polytyrosyl-porcine relaxin to degradation by 3 purified enzymes involved in the degradation of insulin and proinsulin was examined. Rat liver glutathione-insulin transhydrogenase (GIT), which cleaves disulfide bonds in insulin, catalyzed a time- and concentration-dependent increase in trichloroacetic acid (TCA)-soluble radioactivity of relaxin. The Sephadex G-50 profile of the reaction products revealed conversion to the A- and B-chains. Relaxin competitively inhibited the degradation of insulin by GIT; however, kinetic analysis revealed insulin to be preferred over relaxin as a substrate. Rat liver cytosol neutral thiol peptidase (NTP) catalyzed a time- and concentration-dependent increase in the TCA solubility of relaxin and a shift in the Sephadex G-50 radioactivity profile to low molecular weight products. Kinetic analysis revealed that insulin and B-chain are preferred over relaxin as substrates for NTP. A third enzyme, rat kidney neutral metalloendopeptidase, which degrades proinsulin and insulin C-peptide but not insulin, also did not degrade porcine relaxin.

Purification and characterization of a rat liver cytosol neutral thiol peptidase that degrades glucagon, insulin, and isolated insulin A and B chains

A thiol peptidase that catalyzes at near neutral pH the hydrolysis of insulin, the isolated A and B chains of insulin, and glucagon was purified from rat liver cytosol by fractionation on Sephadex G-200, Affi-Gel Blue, and Spherogel TSK-G 3000 SW. The purified enzyme showed a single component by chromatography on a Spherogel TSK column and by gel filtration on a Sephadex G-200 column. The native enzyme has a molecular weight of approximately 180,000 and consists of two subunits having pI's of 5.9 and 6.3. Studies on its substrate specificity showed that the purified enzyme degrades glucagon, insulin, insulin B chain, and insulin A chain, but it does not degrade proinsulin, ACTH, or denatured hemoglobin. Kinetic analyses were performed on three substrates. The Km values were: 34 nM for insulin, 276 nM for insulin B chain, and 3.5 microM for glucagon. The kcat and Vm/Km values were glucagon greater than B chain greater than insulin. Thus, the enzyme has the highest affinity/lowest efficiency for insulin, an intermediate affinity/intermediate efficiency for B chain of insulin and the lowest affinity/highest efficiency for glucagon. The effect of several potential activators and inhibitors on the enzyme's activity was investigated. The enzyme activity was markedly inhibited by N-ethylmaleimide, p-chloromercuribenzoic acid, iodoacetamide, and Np-tosyl-L-phenylalanine chloromethyl ketone (TPCK), and was partially inhibited by dithiothreitol, by the chelating agents EDTA and EGTA, and by phenylmethylsulfonyl fluoride (PMSF). Bacitracin inhibited the activity of the enzyme, but the protease inhibitors aprotinin, leupeptin, pepstatin, and phosphoramidon had little or no effect. Reduced glutathione, iodoacetate, and N alpha,p-tosyl-L-lysine chloromethyl ketone (TLCK) also had little or no effect on the enzyme activity.

Partial purification and characterization of a rat kidney neutral endopeptidase that hydrolyzes succinyl trialanine-4-nitroanilide

The activity for the hydrolysis of succinyl trialanine-4-nitroanilide was higher in kidney homogenates of female rats and mice than in those of male rats and mice. An enzyme hydrolyzing the above substrate was extracted from female rat kidney homogenate and partially purified by means of gel filtration on Sepharose 4B, anion-exchange chromatography on DEAE-Sepharose CL-6B and affinity chromatography on carbobenzoxy-L-Ala-L-Ala-D-Ala-polylysine-agarose. The purified enzyme cleaved the bond between succinyl dialanine and alanine-4-nitroanilide of the substrate and showed a Km value of 3.3 mM at the optimal pH of 7.5. The activity was increased by Ca2+ and Mg2+, but inhibited by EDTA. With oxidized insulin B chain as a substrate, the enzyme cleaved the carbonyl bonds of Ala-14, Tyr-16 and Gly-23 efficiently, and those of His-5 and His-10 less efficiently.

Cleavage of p-nitroanilides of N-acylated tri- and tetrapeptides by alanine endopeptidase from the brush border membranes of rat enterocytes

The activity of the alanine endopeptidase from the intestinal brush border was studied using chromogenic substrates of the general formula Sc-Ala2-X-pNA. Sc-Y-Z-Ala-pNA and W-Ala3-pNA respectively. Substrates with C-terminal Leu or Nle are hydrolyzed more readily than Ala-analogues. At least one Ala-residue in one of the positions adjacent to the C-terminus is necessary for the enzyme activity. An Na-substituent has no effect on the activity.

In vitro existence of a trypsin dependent C-peptidase in human plasma. Discussion of its possible role in vivo

The C-peptide immunoreactivity (CPR) is markedly increased after a short incubation of human plasma with trypsin. Three experiments (study of the action of trypsin-treated plasma on labelled CPR, precipitation of plasma proteins with polyethylene glycol, CPR measurement with three different radioimmunoassays kits) were made in order to account for this phenomenon. The concordant results obtained and the inhibitory action of aprotinin observed in these experiments led us to conclude to the existence in plasma of a trypsin dependent C-peptidase with a specificity for the COOH terminus of the complete CPR (Arg - Arg - C-peptide - Lys - Arg). The role of this protease is probably minor in the C-peptide degradation process but could have an effect on the insulin catabolism through the existence of the alpha 2 - macroglobulin - trypsin complexes and insulin protease. This suggests a possible influence of the exocrine pancreas on the endocrine pancreas.