Enzymatic inactivation of peptide hormones possessing a C-terminal amide group

Clearance of oxytocin and its potentially enzyme resistant analogues in the OXT-receptor compartment of the potassium depolarized rat myometrium

The time–response behaviour of a group of oxytocin analogues structurally modified on potential sites of oxytocin splitting by tissue inactivation enzymes (“enzyme probes”) was investigated ex vivo on the potassium depolarized rat myometrium (at 30°C) and compared with the data obtained in the in vivo experiments. The modified oil‐immersion method by Kalsner and Nickerson was used to record time profiles after cessation of a steady state myometrium contraction triggered by analogues in a high potassium tissue medium. An exchange of the aqueous medium for mineral oil enables to suppress return diffusion of the peptide and to record its irreversible clearance near the corresponding receptor compartment. Response records were analysed by a nonlinear numeric procedure based on combination of steady state and kinetic terms that allows concomitant estimations of affinities from time–response measurements, in the given case for analogues on depolarized myometrium. Potential inactivation‐sensitive sites in the oxytocin chain are the Ν‐terminal peptide bond Cys¹‐Tyr² (aminopeptidase splitting), the intramolecular disulphide bridge (reduction and formation of the practically inactive linear peptide) and the C‐terminal Leu⁸‐GlyNH₂⁹ or the Pro⁷‐Leu⁸ (postprolin cleaving enzyme) bond, respectively. Clearance rate constants of single peptides in the OXT‐receptor compartment were in an interval of 0.025 to 0.28 min⁻¹. The fragment contribution analysis reveals a significant linear additivity of individual structural changes and thus a predictivity of irreversible inactivation rate in the receptor compartment. The most potent inactivation of oxytocin is associated with aminopeptidase splitting; other enzymes may play some though nondecisive role. Less significant differences within the peptide group were found for rate constants for peptide transport between receptor compartment and its external aqueous medium. Besides rate constants, the evaluation of time–response data yields affinity values of the tested peptides and indicates a 25‐times desensitation of depolarized compared with a native state.

C-Terminal lactamization of peptides

Solid-phase synthesis of peptides (SPPS) with release through formation of C-terminal γ-, δ-, or ε-lactams is presented. The natural products ciliatamide A and C were synthesized in up to 90% yield. Peptides carrying C-terminal lactams were shown to possess increased bio-stability and comparable biological activity as compared to the parent non-lactamized peptide amides.

A comparison of the effects of amide and acid groups at the C-terminus on the collision-induced dissociation of deprotonated peptides

The dissociative behavior of peptide amides and free acids was explored using low-energy collision-induced dissociation and high level computational theory. Both positive and negative ion modes were utilized, but the most profound differences were observed for the deprotonated species. Deprotonated peptide amides produce a characteristic c(m-2)(-) product ion (where m is the number of residues in the peptide) that is either absent or in low abundance in the analogous peptide acid spectrum. Peptide acids show an enhanced formation of c(m-3)(-); however, this is not generally as pronounced as c(m-2)(-) production from amides. The most notable occurrence of an amide-specific product ion is for laminin amide (YIGSR-NH(2)) and this case was investigated using several modified peptides. Mechanisms involving 6- and 9-membered ring formation were proposed, and their energetic properties were investigated using G3(MP2) molecular orbital theory calculations. For example, with C-terminal deprotonation of pentaglycine amide, formation of c(m-2)(-) and a 6-membered ring diketopiperazine neutral requires >31.6 kcal/mol, which is 26.1 kcal/mol less than the analogous process involving the peptide acid. The end group specific fragmentation of peptide amides in the negative ion mode may be useful for identifying such groups in proteomic applications.

Plasma levels of vasoactive regulatory peptides in patients receiving regular hemodialysis treatment

The fasting plasma levels of 10 vasoactive regulatory peptides were measured by radioimmunoassay in 23 stable patients with chronic renal failure receiving regular hemodialysis treatment (RDT) and compared with those of healthy controls. The plasma concentrations of arginine vasopressin, atrial natriuretic peptide, beta-endorphin, methionine-enkephalin, motilin, neuropeptide Y, substance P, and vasoactive intestinal peptide were increased. The plasma level of calcitonin gene-related peptide was not statistically different from that of the controls. The plasma concentration of gamma 2-melanocyte-stimulating hormone was lowered in the RDT-patients. The arterial blood pressure correlated with the plasma levels of motilin and neuropeptide Y. We conclude that patients with chronic renal failure receiving RDT have increased concentrations of 8 out of 10 measured vasoactive regulatory peptides. The elevated levels of vasoactive peptides may contribute to the adaptation of the cardiovascular system to impaired renal function.

Further investigations of intestinal hormonal polypeptides

Attempts are described to identify additional polypeptides of hormonal nature in a concentrate of intestinal polypeptides shown previously to contain secretin, cholecystokinin-pancreozymin (CCK), motilin, gastric inhibitory polypeptide (GIP), vasoactive intestinal polypeptide (VIP), enteroglucagon and chymodenin. The probable amino acid sequence of a variant form of CCK is disclosed. The possibility of using characteristic fragments of polypeptides for the quantitation of the polypeptides themselves in crude preparations is briefly discussed.

The purification and specificity of a neutral endopeptidase from rabbit kidney brush border

1. A neutral peptidase, previously shown to be located in the brush border of the proximal tubule, and assayed by its ability to hydrolyse [(125)I]iodoinsulin B chain was purified from rabbit kidney. 2. The starting material for the purification was a microsomal pellet prepared from a homogenate of cortical tissue. The membrane-bound enzymes were solubilized by treatment with toluene and trypsin. About half the neutral peptidase activity was released by this treatment in a form that no longer sedimented with the microsomal pellet and which penetrated polyacrylamide gels when subjected to disc electrophoresis. Other treatments with detergents or proteolytic enzymes either inactivated the peptidase or failed to convert it into a genuinely soluble form. 3. Chromatography with successive columns of Sephadex G-200, DEAE-cellulose and hydroxyl-apatite yielded an enzyme that was free of other brush-border peptidase activities and which was homogeneous on disc electrophoresis and ultracentrifugation. 4. The purified enzyme attacked [(125)I]iodoglucagon at a rate comparable with that for [(125)I]iodoinsulin B chain. It did not appear to attack proteins (insulin, albumin and casein) that had been similarly iodinated. 5. Unlabelled insulin B chain and unlabelled glucagon were substantially hydrolysed by the endopeptidase, whereas insulin and albumin released only trivial amounts of ninhydrin-reacting material. The resistance of insulin to attack by endopeptidase, even after prolonged incubation, was confirmed by biological and immunoassay. 6. The specificity of the peptidase was determined by analysis of the products after incubating unlabelled insulin B chain, and some oligopeptide substrates, including pentagastrin, with the enzyme. All of the bonds readily cleaved were those involving the alpha-amino group of hydrophobic residues, i.e. x-Leu-, x-Val-, x-Tyr-, x-Phe- and x-Met-, provided that the residues were not C-terminal. 7. The enzyme showed only endopeptidase activity. Substrates suitable for aminopeptidases, carboxypeptidases or esterases were not attacked.

Dissociation of the behavioural and endocrine effects of lysine vasopressin by tryptic digestion

1. Lysine vasopressin induces resistance to extinction of active avoidance behaviour (De Wied, 1971).2. Digestion of lysine vasopressin with trypsin almost completely destroys the pressor-, antidiuretic-, oxytocic- and corticotrophin-releasing factor activities of lysine vasopressin, but does not materially influence its effect on the maintenance of an avoidance response.

Leucylglycinamide released from oxytocin by human uterine enzyme

Uteri of pregnant and nonpregnant women contain enzymic activities which inactivate oxytocin. A potent enzyme, which has been partially purified from uterine homogenates, cleaves the prolyl-leucyl peptide bond of oxytocin. This findinig associates for the first time the release of the dipeptide leucylglycinamide with the degradation of neurohypophyseal hormones.