Oxytocin Analogues Research Articles

The oxytocin analogue carbetocin prevents priming-induced reinstatement of morphine-seeking: Involvement of dopaminergic, noradrenergic and MOPr systems

Relapse to illicit drug-seeking following abstinence is a major challenge for the treatment of addiction as no effective pharmacotherapy is available. We have recently shown that activating the central oxytocinergic system prevents emotional impairment and stress-induced reinstatement associated with opioid withdrawal. Here, we investigated whether the oxytocin analogue carbetocin (CBT) is able to reverse morphine-primed reinstatement of conditioned-place preference (CPP) in mice. The mechanism underlining the behavioural effect of CBT was investigated by assessing the involvement of the striatal noradrenergic and dopaminergic systems in CBT reversal of priming- and stress-induced reinstatement of opioid CPP. In addition, given recent evidence suggesting the presence of oxytocin receptor (OTR)-μ-opioid receptor (MOPr) interactions in the brain, we further explored these interactions by carrying out OTR autoradiographic binding in brain of mice lacking MOPr. CBT administration prevented priming-induced reinstatement of morphine CPP. While an acute effect of CBT in enhancing dopamine turnover was observed following stress- and priming-induced reinstatement, CBT significantly decreased striatal noradrenaline turnover only following priming-induced reinstatement. Moreover, a significant brain region- specific increase in OTR binding was observed in MOPr knockout mice, indicating the presence of a possible OTR–MOPr interaction, which may be involved in the modulation of relapse. These results support the oxytocinergic system as a promising target for the prevention of relapse to opioid use and highlight the differential involvement of monoaminergic systems on the effects of OTR stimulation in preventing stress- and priming-induced reinstatement of opioid CPP behaviour.

Two Birds with One Stone: Possible Dual-Role of Oxytocin in the Treatment of Diabetes and Osteoporosis.

Oxytocin (OT), a hormone most commonly associated with labor and lactation, may have a wide variety of physiological and pathological functions, which makes OT and its receptor potential targets for drug therapy. In this review, we highlight the newly discovered metabolic role of OT in diabetes and its complication, such as diabetic osteopathy. OT may have positive metabolic effects; this is based on the change in glucose metabolism, lipid profile, and insulin sensitivity. It may modify glucose uptake and insulin sensitivity both through direct and indirect effects. It may also cause regenerative changes in diabetic pancreatic islet cells. Moreover, it has an anabolic effect on the bone biology. So, the activation of the OT receptor pathway by infusion of OT, OT analogs, or OT agonists may represent a promising approach for the treatment of diabetes and some of its complications, including diabetic osteopathy.

Lipo-oxytocin-1, a Novel Oxytocin Analog Conjugated with Two Palmitoyl Groups, Has Long-Lasting Effects on Anxiety-Related Behavior and Social Avoidance in CD157 Knockout Mice.

Oxytocin (OT) is a nonapeptide hormone that is secreted into the brain and blood circulation. OT has not only classical neurohormonal roles in uterine contraction and milk ejection during the reproductive phase in females, but has also been shown to have new pivotal neuromodulatory roles in social recognition and interaction in both genders. A single administration of OT through nasal spray increases mutual recognition and trust in healthy subjects and psychiatric patients, suggesting that OT is a potential therapeutic drug for autism spectrum disorders, schizophrenia, and some other psychiatric disorders. Although the mechanism is not well understood, it is likely that OT can be transported into the brain where it activates OT receptors to exert its function in the brain. However, the amount transported into the brain may be low. To ensure equivalent effects, an OT analog with long-lasting and effective blood-brain barrier penetration properties would be beneficial for use as a therapeutic drug. Here, we designed and synthesized a new oxytocin analog, lipo-oxytocin-1 (LOT-1), in which two palmitoyl groups are conjugated at the amino group of the cysteine9 residue and the phenolic hydroxyl group of the tyrosine8 residue of the OT molecule. To determine whether LOT-1 actually has an effect on the central nervous system, we examined its effects in a CD157 knockout model mouse of the non-motor psychiatric symptoms of Parkinson’s disease. Similar to OT, this analog rescued anxiety-like behavior and social avoidance in the open field test with the social target in a central arena 30 min after intraperitoneal injection in CD157 knockout mice. When examined 24 h after injection, the mice treated with LOT-1 displayed more recovery than those given OT. The results suggest that LOT-1 has a functional advantage in recovery of social behavioral impairment, such as those caused by neurodegenerative diseases, autism spectrum disorders, and schizophrenia.

A quantitative structure–activity relationship (QSAR) study of peptide drugs based on a new descriptor of amino acids

Quantitative structure-activity relationships (QSAR) approach is used for finding the relationship between molecular structures and the activity of peptide drugs. In this work, stepwise multiple regression, was employed to select optimal subset of descriptors that have significant contribution to the drug activity of 21 oxytocin analogues, 48 bitter tasting threshold, and 58 angiotensin-converting enzyme inhibitors. A new set of descriptor, SVWGM, was used for the prediction of the drug activity of peptide drugs and then were used to build the model by partial least squares method, for it?s estimation stability and generalization ability was strictly analyzed by both internal and external validations, with cross-validation correlation coefficient, correlation coefficient and correlation coefficient of external validation.

The oxytocin receptor impairs ethanol reward in mice

It is well established that oxytocin, and its receptor (OxtR), play a crucial role in addiction and that the stimulation of oxytocin neurotransmission reduces addictive behaviors to ethanol in laboratory animals. However, the impact of OxtR modulation on acquisition, extinction and reinstatement of drug-elicited ethanol-conditioned place preference (EtOH-CPP) has not yet been investigated. In this study, we evaluated the effects of OxtR pharmacological modulation, using the oxytocin analog Carbetocin, and genetic overexpression in the nucleus accumbens (NAcc), using lentiviral-mediated gene transfer technology, of the OxtR on acquisition, extinction and reinstatement of drug-elicited EtOH-CPP in mice. In the first experiment, results showed that Carbetocin administration and NAcc OxtR-overexpression (LV–OxtR) reduced EtOH-CPP establishment. In the second experiment, systemic Carbetocin treatment and OxtR overexpression resulted in decreased time spent in the ethanol-paired compartment following completion of a 7-day extinction protocol. Finally, the third experiment showed that Carbetocin and LV–OxtR suppressed primed reinstatement of EtOH-CPP. It is concluded that pharmacological and genetic modulation of the OxtR can modulate the acquisition, extinction, and reinstatement of conditioned reinforcing effects of ethanol. Taken together, the current findings add to the growing literature on oxytocin neurotransmission modulation in the pharmacotherapy of ethanol addiction and alcoholism.

Changes in Blood Pressure and Cardiac Output During Cesarean Delivery

Background:Little is known about maternal hemodynamics after Cesarean delivery. Uterine contractions may increase cardiac output. Oxytocin is the first-line treatment for uterine atony, although the effects of the long-acting oxytocin analogue carbetocin are comparable with that of oxytocin. The aut

To Determine Biologically Important Mutations in Oxytocin

Oxytocin (OT) has long been deemed the “love hormone” after its important role in social bonding has been documented. In the current study, the relative importance of five OT residues (X2–X5 and X8) was profiled systematically and quantitatively through an integrative approach. In the procedure, eight quantitative sequence-activity models were developed with classical amino acid descriptors and sophisticated random forest (RF) regression based on a panel of 49 known OT analogues. These models were further optimized to achieve both high internal fitting ability and strong external predictive power. The best model, which was built from the combination of VHSE descriptor and RF regression, was then employed to characterize the single position mutation profile of the five investigated OT residues. It is suggested that (i) the polar and charged amino acids presented at positions X2, X3 and X8 are quite unfavorable to OTR–OT binding, and (ii) the positions X4 and X5 have only a moderate effect on the binding as compared to other three positions. Subsequently, three novel OT analogues, i.e. the single-point mutants , and , were designed rationally in terms of the putative mutation profile, and their interaction mechanism with OTR were investigated at molecular level using atomistic molecular dynamics (MD) simulations and molecular mechanics/generalized Born surface area (MM/GBSA) analysis. Further, the agonistic potencies U of OA12 and OA90 were measured to be 0 and 190 IU/mg, respectively, by testing their agonistic capability against rat uterus, which are roughly consistent with the theoretical results arising from computational investigations.

Oxitocina frente a carbetocina para prevenir hemorragias posparto tras cesárea

Antecedentes: La hemorragia posparto (HPP) es la causa aislada más importante de mortalidad materna y su causa más frecuente es la atonía uterina. El mejor tratamiento es su prevención. La oxitocina reduce el riesgo de HPP. Se ha comercializado en España la carbetocina, análogo de la oxitocina humana que tiene inicio de acción rápido y una vida media 4 a 10 veces más larga que la oxitocina, produciendo una contracción tónica que reduce la pérdida hemática posparto. Objetivos: Comparar efectos de carbetocina y oxitocina para prevenir hemorragias posparto tras cesárea. Diseño: Estudio observacional retrospective de comparación. Institución: Servicio de Obstetricia y Ginecología, Hospital General Universitario de Elche, Alicante, España. Participantes: Gestantes con factor de riesgo de atonía uterina tras cesárea. Metodología: Se ha incluido 247 mujeres con algún factor de riesgo de atonía uterina tras cesárea, estudiadas desde abril de 2010 hasta marzo de 2012. Se ha comparado los efectos de administrar 100 microgramos de carbetocina y 10 UI de oxitocina tras el alumbramiento. Se ha evaluado las cifras de hemoglobina previas a la cesárea y tras 24 a 48 horas de la misma, y el porcentaje de transfusiones realizadas. Principales medidas de resultados: Anemia, transfusiones sanguíneas, hierro intravenoso. Resultados: En el grupo carbetocina, la hemoglobina tras la cesárea descendió menos (P=0,56) y existió menos casos de anemia tras la misma (P=0,852). Las transfusiones y el hierro intravenoso no aportaron datos concluyentes. Conclusiones: No existieron diferencias estadísticamente significativas entre carbetocina y oxitocina para prevenir HPP tras cesáreas.

The role of arginine vasopressin and oxytocin as immunomodulatory hormones on experimental autoimmune encephalomyelitis (876.1)

Arginine vasopressin (AVP) and oxytocin (OXT) are pleiotropic hormones released from the neurohypophysis. We demonstrated that neurointermediate pituitary lobectomy (NIL) in rats significantly protects against experimental autoimmune encephalomyelitis (EAE), a demyelinating T cell‐mediated disease of the central nervous system (CNS), whereas administration of desmopressin (DP, an AVP synthetic analog), restores the susceptibility of the NIL animals to EAE. Since immune cells have AVP and OXT receptors, we investigated whether OXT beside AVP, has also an immunomodulatory role on EAE. A comparative study of the effects of NIL, NIL+ DP and NIL+ carbetocin (CBT, a synthetic OXT analog) on clinical symptoms of EAE and on the number of CNS infiltrating lymphocytes were assessed. Animals were immunized for EAE one week after starting the administration of DP and CBT. EAE clinical signs were evaluated daily and CNS lymphocytes were separated with Percoll gradient. In NIL rats DP restored susceptibility to EAE and increased lymphocytic infiltration in the CNS while CBT resulted in very mild EAE symptoms and no changes in CNS lymphocytes. In conclusion, AVP has more powerful immunomodulatory effects than OXT.

Conformational studies of neurohypophyseal hormones analogues with glycoconjugates by NMR spectroscopy

Two glycosylated peptides have been studied using NMR spectroscopy supported by molecular modeling. Peptide I is an oxytocin (OT) analogue in which glutamine 4 was replaced by serine with attached α-d-mannose through the oxygen β atom, whereas peptide II is a lysine-vasopressin (LVP) analogue with lysine 8 side chain modified by the attachment of glucuronic acid through an amide bond. Both peptides exhibit very weak uterotonic effect and are less susceptible to proteolytic degradation than the mother hormones. Additionally, peptide II reveals very weak pressor and antidiuretic activities. Our results have shown that the conformational preferences of glycosylated analogues are highly similar to those of their respective mother hormones. OT glycosylated analogue (I) exhibits a 3,4 β-turn characteristic of OT-like peptides, and vasopressin-glycosylated analogue (II) exhibits β-turns typical of vasopressin-like peptides. Therefore, the lack of binding of the glycosylated analogues to the receptors can be attributed to a steric interference between the carbohydrate moieties and the receptors. We also consider this to be the reason of the very low activity of the analyzed glycopeptides. We expect that results from these studies will be helpful in designing new OT-like and vasopressin-like drugs.

Oxytocin analogues have potential in relieving chronic abdominal pain

Oxytocin analogues have potential in relieving chronic abdominal pain

Carbetocin versus oxytocin for prevention of postpartum haemorrhage: a randomised controlled trial

BackgroundPostpartum haemorrhage (PPH) is the leading cause of maternal death worldwide. Prophylactic uterotonics are effective in reducing PPH, and the drug of choice is oxytocin. Carbetocin, a newer analogue of oxytocin, has a greater biological effect and longer half-life. It is also more heat-stable than oxytocin, which is of crucial importance to resource-poor settings. We compared the efficacy of carbetocin with oxytocin. MethodsIn this randomised controlled trial in a tertiary maternity hospital in Mexico, pregnant women with at least one risk factor for PPH were randomly assigned by a central computer-generated list of random numbers in a 1:1 ratio with no masking either to carbetocin 100 μg as a single intravenous bolus or to oxytocin 20 IU as a 6-h infusion, both administered immediately after childbirth. The primary outcome was PPH with blood loss exceeding 500 mL. Secondary outcomes were the volume of blood loss, severe PPH (blood loss >1000 mL), change in haemodynamic and clinical variables within 24 h of childbirth, and the need for additional uterotonic treatment. Comparisons were done by intention-to-treat analysis. This trial is registered with Instituto Mexicano del Seguro Social (IMSS), number R-2011-3606-1. Findings1210 women were included with 602 assigned to carbetocin and 608 assigned to oxytocin. Blood loss exceeding 500mL was lower in women assigned to carbetocin than in women assigned to oxytocin (18·4% [111/602] vs 25·8 [157/208], relative risk [RR] 0·67, 95% CI 0·54–0·83; number needed to treat [NNT] 14, 95% CI 8–37). Mean blood loss was less with carbetocin than with oxytocin (366 mL [SE 7·8] vs 400 [7·6], p<0·001). The frequency of blood transfusion was similar in the two groups (1·7% [10/602] vs 2·6% [16/608]; RR 0·67, 95% CI 0·31–1·38). The frequency of severe PPH did not differ between the two groups (1·3% [8/602] vs 1·6% [7/608]; 1·15, 0·42–3·16). Fewer participants receiving carbetocin than receiving oxytocin required additional uterotonics (1·5% [9/602] vs 5·8% [35/608], adjusted RR 0·3, 95% CI 0·14–0·61), and fluid resuscitation (20·6% [124/602] vs 24·2% [147/608]; 0·77, 0·62–0·95). No significant difference in the haemodynamic variables was found. InterpretationTo our knowledge, this is the largest trial comparing carbetocin with oxytocin. An updated meta-analysis, combining the results from six randomised trials, including this study, found that carbetocin was associated with a reduction of PPH compared with oxytocin. FundingAmmalife Charity, Ferring Pharmaceuticals.

Selenoether oxytocin analogues have analgesic properties in a mouse model of chronic abdominal pain

Poor oral availability and susceptibility to reduction and protease degradation is a major hurdle in peptide drug development. However, drugable receptors in the gut present an attractive niche for peptide therapeutics. Here we demonstrate, in a mouse model of chronic abdominal pain, that oxytocin receptors are significantly upregulated in nociceptors innervating the colon. Correspondingly, we develop chemical strategies to engineer non-reducible and therefore more stable oxytocin analogues. Chemoselective selenide macrocyclization yields stabilized analogues equipotent to native oxytocin. Ultra-high-field nuclear magnetic resonance structural analysis of native oxytocin and the seleno-oxytocin derivatives reveals that oxytocin has a pre-organized structure in solution, in marked contrast to earlier X-ray crystallography studies. Finally, we show that these seleno-oxytocin analogues potently inhibit colonic nociceptors both in vitro and in vivo in mice with chronic visceral hypersensitivity. Our findings have potentially important implications for clinical use of oxytocin analogues and disulphide-rich peptides in general.

Oxytocin as a novel therapeutic option for type I diabetes and diabetic osteopathy.

The aim of the present study was to highlight the newly discovered metabolic role of oxytocin (OT) in the type I diabetic rats. Previous studies have demonstrated that OT has a beneficial role on bone physiology and therefore, the OT effect on the diabetic osteopathy will be assessed as well. Induction of the type I diabetes was carried out by an intraperitoneal injection of 60 mg/kg body weight of streptozotocin. The metabolic role of OT on diabetic rats after OT treatment with intramuscular injection of 40 µIU/kg body weight for 6 weeks was assessed. Histological and ultrastructural studies of rat pancreas samples, before and after the OT injection, were performed and compared with the obtained physiological results. Oxytocin treatment had positive metabolic effects in diabetic rats. This is based on the change in glucose metabolism, lipid profile, and insulin sensitivity in experimental animals. In addition, OT treatment showed histological regenerative changes of pancreatic islet cells of diabetic rats. Moreover, OT administration showed that it has an anabolic effect on the bone biology. The results suggest that activation of the oxytocin receptor (OTR) pathway by infusion of OT, OT analogs, or OT agonists may represent a promising approach for the treatment of diabetes and some of its complications, including diabetic osteopathy.

Selected nonapeptides in terahertz light

Corresponding author: przemyslaw.jarzab@pwr.edu.plEight synthetic histidine analogues of oxytocin and vasopressin are subject of investigations.The spectra of the peptides have been investigated in the terahertz band. The results are obtainedin the terahertz time-domain spectroscopy arrangement.Keywords: terahertz spectroscopy, peptides, oxytocin, vasopressin, derivatives.

The Oxytocin Analogue Carbetocin Prevents Emotional Impairment and Stress-Induced Reinstatement of Opioid-Seeking in Morphine-Abstinent Mice

The main challenge in treating opioid addicts is to maintain abstinence due to the affective consequences associated with withdrawal which may trigger relapse. Emerging evidence suggests a role of the neurohypophysial peptide oxytocin (OT) in the modulation of mood disorders as well as drug addiction. However, its involvement in the emotional consequences of drug abstinence remains unclear. We investigated the effect of 7-day opioid abstinence on the oxytocinergic system and assessed the effect of the OT analogue carbetocin (CBT) on the emotional consequences of opioid abstinence, as well as relapse. Male C57BL/6J mice were treated with a chronic escalating-dose morphine regimen (20-100 mg/kg/day, i.p.). Seven days withdrawal from this administration paradigm induced a decrease of hypothalamic OT levels and a concomitant increase of oxytocin receptor (OTR) binding in the lateral septum and amygdala. Although no physical withdrawal symptoms or alterations in the plasma corticosterone levels were observed after 7 days of abstinence, mice exhibited increased anxiety-like and depressive-like behaviors and impaired sociability. CBT (6.4 mg/kg, i.p.) attenuated the observed negative emotional consequences of opioid withdrawal. Furthermore, in the conditioned place preference paradigm with 10 mg/kg morphine conditioning, CBT (6.4 mg/kg, i.p.) was able to prevent the stress-induced reinstatement to morphine-seeking following extinction. Overall, our results suggest that alterations of the oxytocinergic system contribute to the mechanisms underlying anxiety, depression, and social deficits observed during opioid abstinence. This study also highlights the oxytocinergic system as a target for developing pharmacotherapy for the treatment of emotional impairment associated with abstinence and thereby prevention of relapse.

Physiology of invertebrate oxytocin and vasopressin neuropeptides

New findings• What is the topic of this review?This article describes the discovery and function of invertebrate oxytocin and vasopressin neuropeptides.• What advances does it highlight?The novel discovery of oxytocin-like peptides in arthropods is described. An up-to date overview is gven of the functional role (physiology and behaviour) of oxytocin and vasopressin signalling. The application of natural peptides for drug development is discussed.Neuropeptides and regulatory peptide hormones control many developmental, physiological and behavioural processes in animals, including humans. The nonapeptides oxytocin and arginine vasopressin are produced and released by the pituitary gland and have actions on many organs and tissues. Receptive cells possess particular receptors to which the peptides bind as ligands, leading to activation of G-protein-coupled receptors, hence cellular responses. In humans and other mammalian species, oxytocin and vasopressin mediate a range of peripheral and central physiological functions that are important for osmoregulation, reproduction, complex social behaviours, memory and learning. The origin of the oxytocin/vasopressin signalling system is thought to date back more than 600 million years. All vertebrate oxytocin- and vasopressin-like peptides have presumably evolved from the ancestral nonapeptide vasotocin by gene duplication and today are present in vertebrates, including mammals, birds, reptiles, amphibians and fish. Oxytocin- and vasopressin-like peptides have been identified in several invertebrate species, including molluscs, annelids, nematodes and arthropods. Members of this peptide family share high sequence similarity, and it is possible that they are functionally related across the entire animal kingdom. However, it is evident that not all animals express oxytocin/vasopressin neuropeptides and that there is little information available about the biology and physiology of this signalling system of invertebrates and, in particular, of insects, which represent more than half of all known living organisms. This report describes the discovery of novel oxytocin- and vasopressin-like peptides in arthropods and summarizes the status quo of the functional relevance of this neuropeptide signalling system in invertebrates, which will have beneficial implications for the design of selective and potent ligands to human oxytocin and vasopressin receptors.

Changes in Blood Pressure and Cardiac Output during Cesarean Delivery

Little is known about maternal hemodynamics after Cesarean delivery. Uterine contractions may increase cardiac output. Oxytocin is the first-line treatment for uterine atony, although the effects of the long-acting oxytocin analogue carbetocin are comparable with that of oxytocin. The authors analyzed the effects of i.v. oxytocin 5 U, carbetocin 100 µg, and placebo on hemodynamics, uterine tone, adverse events, and blood loss after Cesarean delivery. This was a randomized, double-blinded, placebo-controlled, parallel-group comparison of carbetocin and oxytocin after elective Cesarean delivery of singletons under spinal anesthesia (n = 76). Continuously measured invasive systolic arterial pressure was the primary outcome measure. The mean systolic arterial pressure decrease was 28 mmHg (95% CI, 22-34) after oxytocin and 26 mmHg (95% CI, 20-31) after carbetocin. The decrease was greatest after 80 (95% CI, 71-89) and 63 s (95% CI, 55-72), respectively (P = 0.006). The differences were nearly undetectable after 2.5 min, although the effect of carbetocin was slightly greater than placebo (P < 0.001). The group differences in systolic arterial pressure decreased over 5 min and were gone at 1 h. Heart rate and cardiac output increased in all three groups. Stroke volume increased after oxytocin and carbetocin but was unchanged after placebo. The hemodynamic side effects of oxytocin 5 U and carbetocin 100 µg were comparable. The lack of an increase in stroke volume in the placebo group challenges the theory that uterine contraction causes autotransfusion of uterine blood, leading to an increase in preload.

Aqueous Microwave-Assisted Solid-Phase Peptide Synthesis Using Fmoc Strategy. II. Racemization Studies and Water Based Synthesis of Cysteine- Containing Peptides

We have developed a microwave (MW)-assisted peptide synthesis using Fmoc-amino acid nanoparticles in water previously. It is an organic solvent-free, environmentally friendly method for peptide synthesis. In this study, we have investigated the racemization of cysteine during an aqueous based coupling reaction with MW irradiation. Under our MW-assisted protocol using WSCI and DMTMM, the coupling reaction can be performed with low levels of racemization of cysteine. We also demonstrated the synthesis of the nonapeptide oxytocin analogue, Cys(Acm)-Tyr-Ile-Gln-Asn- Cys(Acm)-Pro-Leu-Gly-NH2 using our water based MW-assisted protocol with Fmoc-amino acid nanoparticles.

On the mechanism of degradation of oxytocin and its analogues in aqueous solution

Oxytocin (OT) is a cyclic nonapeptide containing one internal disulfide bond between its Cys(1) and Cys(6) residues. Although OT is one of the most commonly used peptidic drugs, the mechanism of its degradation in aqueous solution and the identity of its degradants have not been fully elucidated. To investigate the pathways and products of OT degradation in slightly acidic to neutral solutions, we prepared the peptides: OT, [D-Cys(1)]OT, a series of N-alkylated OT analogues, [[(13)C3,(15) N]Cys(1)]OT, and OT where each sulfur atom was systematically replaced by either methylene, (34)S, or Se. The peptides were incubated at 40°C and the degradation products studied by HPLC, LCMS, and (13)C-NMR. Our findings suggest that the degradation begins with β-elimination of the disulfide linkage to form a putative intermediate linear peptide containing an S-thiocysteine (a persulfide) in position 6 and a dehydroalanine in position 1. This intermediate persulfide appears to donate a sulfur atom to an intact OT molecule to form OT trisulfide and higher monomeric polysulfides, while the dehydroalanine residue is hydrolyzed with loss of the N-terminal amino group to yield a linear N-pyruvoylated octapeptide containing a reduced Cys(6). Based on the MS and (13)C-NMR data of the products from degradation of [[(13)C3,(15)N]Cys(1)]OT, we postulate that the ultimate degradation products of OT are dimers composed of two pyruvoylated octapeptides held together by one disulfide bridge between the two Cys(6) residues and by one more, non-reducible, linkage resulting from an aldol-type condensation between the two N-terminal pyruvoyl groups.