Distinct behavior of beta-endorphin and corticotropin toward leucine aminopeptidase action

Measurement of beta-endorphin in human plasma by high-performance liquid chromatography with electrochemical detection: validation of a method employing the simultaneous purification and concentration of beta-endorphin

The simultaneous purification and concentration of synthetic human beta-endorphin from plasma is described, which when used together with an appropriate isocratic high-performance liquid chromatographic-electrochemical detection (HPLC-ED) system allows the determination of elevated physiological levels of beta-endorphin. Purification of plasma was gained by flash-freezing in liquid nitrogen, acidifying with 100 microliters of trifluoroacetic acid (10%, v/v) per ml of plasma, thawing at 4 degrees C and centrifuging to remove any precipitate. Solid-phase extraction with silica sorbent was utilised, which allowed further isolation of the analyte, a method of concentration and a procedure whereby beta-endorphin could be transferred to the HPLC mobile phase. Silica sorbent demonstrated greater selectivity than C18 for synthetic human beta-endorphin and, in addition, provided improved recovery of this analyte when utilising elution volumes of 500 microliters or less. Proteolytic degradation and heparin-induced high-affinity binding in plasma were shown not to effect the recovery of beta-endorphin if blood was rapidly chilled and plasma quickly obtained, frozen and acidified. Validation of this purification/concentration method using [125I]beta-endorphin demonstrated a recovery of 85.6% which was not jeopardised when concentrating the sample twenty-fold. This provided an increase in the sensitivity of detection, when used in conjunction with HPLC-ED, from 5 ng/ml to 250 pg/ml.

Human brain leucyl aminopeptidase: isolation, characterization and specificity against some neuropeptides

In order to obtain a greater understanding of the role of aminopeptidases in the degradation of peptides and proteins in the nervous system, we have isolated and characterized leucyl aminopeptidase (EC 3.4.11.1) from human cerebral cortex and studied its action on some physiologically important neuropeptides. The enzyme has a low specificity constant for the hydrolysis of Leu-7-amido-4-methylcoumarin (69s-1M-1) but the peptides Tyr-Gly-Gly and Tyr-Gly-Gly-Phe-Leu (Leu5-enkephalin) were much better substrates (specificity constants 8300 and 18050s -1M-1 respectively). Optimum activity for the degradation of Leu-enkephalin was obtained at pH10.5 in the presence of 5mM-Mn++. A sharp drop in specificity constant occurred with increasing chain length in the series Leu-enkephalin, dynorphin 1-8, 1-10 and 1-13, suggesting that the enzyme functions only as an oligopeptidase. Other neuropeptides were poor substrates (cholecystokinin octapeptide, angiotensin-I) or not hydrolysed at all (somatostatin, Arg8-vasopressin).

Degradation of low-molecular-weight opioid peptides by vascular plasma membrane aminopeptidase M

Since both aminopeptidases and angiotensin I-converting enzyme are reported to degrade circulating enkephalins, we have examined the degradation of low-molecular-weight opioid peptides by a vascular plasma membrane-enriched fraction previously shown to contain both angiotensin I-converting enzyme (EC 3.4.15.1) and aminopeptidase M (EC 3.4.11.2). Except for an enkephalin analog resistant to amino-terminal hydrolysis, [D-Ala2]enkephalin, the purified vascular plasma membrane preferentially degraded low-molecular-weight opioids by hydrolysis of the N-terminal Tyr-1--Gly-2 bond. Enkephalin degradation was optimal at pH 7.0 and was inhibited by the aminopeptidase inhibitors amastatin (I50 = 0.08 microM), bestatin (9.0 microM) and puromycin (80 microM). Maximal rates of hydrolysis, calculated per mg plasma membrane protein, were highest for the shorter peptides (18.3, 15.6 and 16.6 nmol/min per mg for Met5-enkephalin, Leu5-enkephalin and Leu5-enkephalin-Arg6, respectively) and decreased with increasing peptide length (0.7 nmol/min per mg for dynorphin (1-13)). No significant hydrolysis of beta- and gamma-endorphin was detected. Km values decreased significantly with increasing peptide length (Km = 72.9 +/- 2.7, 43.6 +/- 4.7 and 21.4 +/- 0.9 microM for Met5-enkephalin, Leu5-enkephalin-Arg6 and Met5-enkephalin-Arg6-Phe7, respectively). However, no further decreases were seen with even larger sequences, i.e., dynorphin(1-13). Other peptides hydrolyzed by the plasma membrane aminopeptidase (angiotensin III, kallidin and hepta(5-11)-substance P) inhibited enkephalin degradation in a competitive manner. Thus, localization, specificity and kinetic data are consistent with identification of aminopeptidase M as a vascular enzyme with the capacity to differentially metabolize low-molecular-weight opioid peptides within the microenvironment of vascular cell surface receptors. Such differential metabolism may play a role in modulating the vascular effects of peripheral opioids.

Preparation of [125I-Tyr27,Leu5]beta h-endorphin and its use for crosslinking of opioid binding sites in human striatum and NG108-15 neuroblastoma-glioma cells

A radioligand suitable for crosslinking studies to opioid receptors has been obtained by radioiodination and purification of the monoiodotyrosine-27 derivative of the synthetic human beta-endorphin (beta h-endorphin) analogue [5-leucine]beta h-endorphin. The derivative, [27-[125I]monoiodotyrosine,5-leucine]beta h-endorphin, was crosslinked to human striatal (caudate and putamen) and NG108-15 neuroblastoma-glioma cell membranes by using disuccinimidyl suberate. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis under reducing conditions revealed four specifically labeled bands at 68, 40, 30, and 25 kDa for both human caudate and putamen, whereas NG108-15 cell membranes gave specifically labeled bands at 92, 56, 38, and 23 kDa.