Some characteristics of a peptidyl dipeptidase (kininase II) from rat CSF: differential effects of NaCl on the sequential degradation steps of bradykinin

The effect of halogenation on blood-brain barrier permeability of a novel peptide drug

The utility of a drug depends on its ability to reach appropriate receptors at the target tissue and remain metabolically stable to produce the desired effect. To improve central nervous system entry of the opioid analgesic [D-Pen2, L-Pen5, Phe6] Enkephalin (DPLPE-Phe), our research group synthesized analogs that had chloro, bromo, fluoro, and iodo halogens on the para positions of the phenylalanine-4 residue. This study reports on investigation of the effect of halogenation on stability, lipophilicity, and in vitro blood-brain barrier permeability of a novel enkephalin analog DPLPE-Phe. The stability of each halogenated DPLPE-Phe analog as well as the amidated and nonamidated parent peptide was tested in plasma and brain. All peptides tested had a half-time disappearance >300 min except for DPLPE-Phe-NH2, which was found to have a half-life of 30 min in plasma. Octanol/saline distribution studies indicated addition of halogens to DPLPE-Phe-OH significantly increased lipophilicity except for p-[F-Phe4]DPLPE-Phe-OH. p-[Cl-Phe4]DPLPE-Phe-OH exhibited the most pronounced increase in lipophilicity. Para-bromo and para-chloro halogen additions significantly enhanced in vitro blood-brain barrier permeability, providing evidence for improved delivery to the central nervous system.

Presystemic bradykinin metabolism in sheep and rat nasal homogenates

Bradykinin (BK) and its fragments BK(1-8), BK(1-7), and BK(1-5) were incubated with sheep nasal homogenates to investigate the extent of peptide metabolism within the nasal mucosa. The products for both bradykinin and BK(1-8) degradation were found to be BK(1-7) and BK(1-5). BK(1-7) was metabolized to BK(1-5) alone. The patterns of degradation suggest that the Pro7-Phe8 bond of bradykinin was hydrolyzed first, then BK(1-7) was further hydrolyzed to form BK(1-5). The metabolism of bradykinin in rat nasal homogenates and plasma was also investigated. BK(1-5) was the only metabolite measurable in the rat nasal homogenates, likely due to the activity of an endopeptidase. The reduction in the bradykinin degradation rate resulting from the inhibition of angiotensin converting enzyme (ACE) or carboxypeptidase N indicates that these enzymes participate in mucosal bradykinin metabolism to some degree. In comparison, the products of bradykinin hydrolysis in rat plasma were found to be BK(1-8), BK(1-7), and BK(1-5). These results indicate that the enzyme populations or/and activities vary significantly between different species and between different tissues within the same species. Although significant aminopeptidase activities were detected in the sheep nasal homogenates, bradykinin was not affected by their presence, since the N-terminal sequence of bradykinin is not susceptible to hydrolysis by most aminopeptidases.

Bradykinin metabolism in rat and sheep nasal secretions

The nasal secretions are the first barrier that nasally administered drugs encounter. Therefore, the characterization of peptide metabolism in the nasal secretions is essential to predict nasal peptide bioavailability. Metabolism of bradykinin was measured in rat and sheep nasal secretions to estimate the extent of degradation of nasally administered peptide compounds. A single-pass, in situ nasal perfusion technique was employed to collect secretions for the investigation of peptide metabolism in rat nasal secretions. The protein content, mucin concentration, and degree of bradykinin metabolism in perfusate aliquots collected over a 2-h period showed that the early perfusate fractions contained most of the active secretory materials. Evidence of continuous mucus secretion and plasma extravasation was found in the nasal perfusate throughout the entire collection period. Sheep nasal secretions were collected with a cotton pledget inserted into the nasal cavity. Bradykinin and its fragments were degraded by carboxypeptidases and endopeptidases present in both rat and sheep nasal secretions. Hydrolysis of Phe5-Ser6 was the major metabolism pathway of bradykinin in the rat nasal perfusate, whereas in sheep nasal secretions, hydrolysis of the Pro7-Phe8 and Phe8-Arg9 bonds also occurred. Evidence of angiotensin converting enzyme, carboxypeptide N, and aminopeptidase activity was identified in the rat nasal perfusate with specific substrates and inhibitors. The activity of these and other enzymes in the nasal secretions may significantly limit the bioavailability of nasally administered peptide drugs prior to their exposure to the nasal mucosal tissues.

Elevated angiotensin-converting enzyme (kininase II) in the cerebrospinal fluid of neuroleptic-treated schizophrenic patients

Disturbances of water homeostasis have frequently been reported in schizophrenia. Water homeostasis is regulated by arginine vasopressin (AVP), the renin-angiotensin system and natriuretic hormones. The aim of this study was to determine the activity of the central renin-angiotensin system in schizophrenia by measuring levels of angiotensin-converting enzyme (ACE) in the cerebrospinal fluid (CSF) and blood in 14 in-patients with schizophrenia on neuroleptic medication and in 9 healthy volunteers. The levels of CSF ACE were significantly higher in the schizophrenia group. There were no correlations between CSF ACE and gender, age, age at first episode, duration of illness, term of hospitalization or neuroleptic dosage. No correlations between CSF ACE and serum ACE were found in either group. The authors suggest an activated central renin-angiotensin system in schizophrenia at least during antipsychotic drug treatment, which may cause 'psychogenic' polydipsia in some patients. ACE and the brain renin-angiotensin system may also play a role in the regulation of neuron growth and differentiation in schizophrenia.

Cardiovascular responses elicited by intrathecal kinins in the conscious rat

In the conscious, unrestrained rat, intrathecal (i.t.) injection of 0.81 pmol-81 nmol bradykinin (BK), kallidin (KD) and T-kinin at the T-9 spinal cord level produced transient (less than 10 min) increases in mean arterial pressure (MAP) and longer lasting decreases in heart rate (HR). These effects were dose-dependent and similar with respect to intensity and time course for the three kinins. The des-Arg9-BK fragment, a selective agonist for B1 receptors, was active only at 81 nmol. The pressor response induced by BK was enhanced by propranolol and by transection of the cervical spinal cord but was converted to a vasodepressor effect by prazosin. The bradycardia was converted to tachycardia by prazosin, atropine, pentolinium, capsaicin and in spinal transected rats. However, the cardiovascular responses to BK remained unaffected by diphenhydramine plus cimetidine, morphine, indomethacin, adrenal medullectomy, i.t. idazoxan and after bulbospinal noradrenaline deafferentation with 6-hydroxydopamine. These results suggest that the increase in MAP induced by i.t. BK is mediated by the sympathoadrenal system while the decrease in HR is ascribable to a vagal reflex involving sensory C-fibers and a spinobulbar pathway. This pharmacological evidence therefore supports a role for kinins in cardiovascular regulation in the spinal cord.