Mechanisms protecting plasma peptides from enzyme hydrolysis: a comparative study

Comparative in vitro degradation of the human hemorphin LVV-H7 in mammalian plasma analysed by capillary zone electrophoresis and mass spectrometry

The human hemorphin LVV-H7 (L32VVYPWTQRF41) is a hemoglobin-beta, -gamma, -delta or -epsilon chain derived cationic decapeptide of the micro-opioid receptor binding family. It exhibits potential pharmacological value relevant, for example, for blood pressure regulation, learning performance and Alzheimer's disease. The regulatory potency is strictly dependent on the length of the amino acid sequence which is sensitive towards proteinases from tissues and plasma. To analyse LVV-H7 in vitro degradation in mammalian plasma, a novel multi-component quantitative capillary zone electrophoretic (CZE) procedure was applied, combined with qualitative metabolite profiling by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In all types of plasma, LVV-H7 was N-terminally truncated generating four metabolites (M1-M4) with an intact C-terminus: M1 (V33VYPWTQRF41), M2 (V34YPWTQRF41), M3 (Y35PWTQRF41) and M4 (W37TQRF41). In EDTA plasma these degradation products were detected exclusively, whereas in citrate and heparin plasma four further metabolites appeared resulting from additional C-terminal cleavage of the dipeptide R40F41: M5 (L32VVYPWTQ39), M6 (V33VYPWTQ39), M7 (V34YPWTQ39) and M8 (Y35PWTQ39). In the presence of selective proteinase inhibitors aminopeptidase M and angiotensin-converting enzyme (for N- and C-terminal truncation, respectively) were identified as plasma enzymes responsible for hemorphin degradation. Furthermore, striking inter-mammalian species distinctions were detected revealing strongly differing degradation velocities but similar metabolite patterns.

In vitro degradation of the antimicrobial human peptide HEM-γ 130–146 in plasma analyzed by a validated quantitative LC–MS/MS procedure

In stability studies during preclinical drug development, the human antimicrobial peptide hHEM-gamma 130-146 shows progressive N-terminal degradation in plasma. To determine this effect, we developed and validated a selective and quantitative muHPLC-MS/MS procedure for this compound. Following deproteinization by precipitation, reversed-phase separation is performed with a time-saving two-column design online coupled to an ion trap mass spectrometer for electrospray ionization MS detection. Using a linear calibration curve obtained with synthetic external standards ranging nearly two orders of magnitude, we achieved good precision (repeatability and reproducibility: 5-15%), accuracy (-3 to 15%), and ruggedness with a lower limit of quantification at 0.29 microg/ml plasma (0.15 microM). Because of good linearity (r2>0.999), the recovery (84+/-3%) and ion suppression (86+/-4% remaining intensity) were calculated from specifically prepared calibration curves. The developed procedure was applied to human and animal plasma samples. Incubations in the presence and absence of proteinase inhibitors revealed at least an aminopeptidase M activity for the initial N-terminal truncation of tryptophan (W130) and a putative glutaminyl-peptide cyclotransferase activity for the resulting intermediate starting with the bared glutamine residue (Q131). The calculated periods of half-change demonstrated exceeding interspecies variations, whereas the intraspecies variations were only between 20 and 30%. The current procedure is valuable as a generic method for pharmaceutical purposes, and data give important information for further development toward a potential natural drug candidate.

Peripheral modulation of learning and memory: enkephalins as a model system

Extensive research on the effects of enkephalins on conditioning is reviewed and used as the basis for a model of peripheral modulation of learning and memory. An overall theme emphasized throughout our discussion is that these peptides can influence the strength with which a memory is acquired and stored by acting outside the blood-brain barrier. This assertion is supported by research on the behavioral effects of systemically administered enkephalins and opioid antagonists, the rapid hydrolysis of circulating enkephalins in vivo, and the limited ability of these peptides to penetrate the blood-brain barrier. A consideration of the extensive distribution of enkephalins throughout peripheral autonomic systems leads to the proposal that enkephalins may act to modulate learning and memory by altering peripheral autonomic function; autonomic afferents may then communicate with the memory trace in the CNS through a central modulatory pathway outlined herein. Evidence that some stressful experiences may lead to increases in circulating enkephalins also is discussed. The sites of action of these circulating enkephalins may involve peripheral autonomic sites, or additionally may involve the circumventricular organs. As a further regulatory mechanism, circulating enkephalin levels may be controlled by experience-dependent alterations of the activity of enzyme systems that participate in their breakdown. Finally, it is emphasized that the mechanisms of enkephalin action postulated herein may be applicable to the actions of other peripheral hormones, peptides, and neurotransmitters that participate in the modulation of learning and memory storage processes.

Experience-dependent regulation of Leu-enkephalin hydrolysis in rat plasma

We examined experience-dependent alterations in the production of individual enkephalin metabolites during Leu-enkephalin hydrolysis in rat plasma in vitro. High performance liquid chromatography separation and electrochemical detection were used to quantify accumulation of the primary N-terminal products of enkephalin hydrolysis, Tyr and Tyr-Gly-Gly. Blood was sampled from rats through indwelling femoral artery catheters before, during, and after active avoidance training or appropriate control treatments. Avoidance training increased Tyr production, as did handling and exposing animals to the novel avoidance chamber without foot-shock training. On the other hand, Tyr-Gly-Gly production was elevated by blood sampling alone, by handling/novelty, and by training, but the effect of training was significantly greater than the effects of either of the other treatments. Interestingly, Tyr-Gly-Gly production was highly correlated (r = -0.885) with a measure of escape performance. Since the enzymes that participate in enkephalin hydrolysis act upon a number of peptides, regulation of enzyme activity may be one mechanism whereby an organism can modulate levels of circulating peptides in response to environmental experiences; this may contribute to the expression of appropriate behavioral responses to those environmental experiences.