Ectopeptidases Responsible for the Inactivation of Enkephalins. In: Kreutzberg GW, Reddington M, Zimmermann H, editors. Cellular Biology of Ectoenzymes. Berlin: Springer Berlin Heidelberg; 1986. pp. 272-84. [DOI: 10.1007/978-3-642-70664-6_21]

High-performance liquid chromatographic-colorimetric assay for glycine carboxypeptidase activity

A rapid and sensitive assay method for the determination of glycine carboxypeptidase activity has been reported. This method is based on the monitoring of the absorption at 460 nm of 4-dimethylaminoazobenzene-4'-sulfonyl-Gly-L-Phe, enzymatically formed from the substrate 4-dimethylaminoazobenzene-4'-sulfonyl-Gly-L-Phe-Gly, after separation by high-performance liquid chromatography (HPLC) using a TSK gel ODS-80TM reversed-phase column by isocratic elution. This method is sensitive enough to measure 4-dimethylaminoazobenzene-4'-sulfonyl-Gly-L-Phe at concentrations as low as 1 pmol and yield highly reproducible results and requires less than 7.5 min per sample for separation and quantitation. The pH optimum for glycine carboxypeptidase activity was 4.8 to 5.4. The Km and Vmax values were respectively 21.1 micromol and 3.73 pmol/microg/h with the use of enzyme extract obtained from bovine pituitary. Glycine carboxypeptidase activity was strongly inhibited by Ag+, Cu2+ and p-chloromercuriphenylsulfonic acid. Among the organs examined in a mouse, the highest specific activity of the enzyme was found in testis. The sensitivity and selectivity of this method will aid in efforts to examine the physiological role of this peptidase.

The 140-kDa protein of blood-brain barrier-associated pericytes is identical to aminopeptidase N

Although ample evidence has been accumulated on the structure and functional role of endothelial cells in blood-brain barrier mechanisms, little is known about the contribution that cerebral pericytes provide to this phenomenon. We have reported recently on a monoclonal antibody specific for cerebral pericytes at blood-brain barrier sites. To confirm the pericytic localization of this antigen, and in order to elucidate its biochemical identity, we have performed immunocytochemical, biochemical, and molecular biological studies. By immunocytochemistry on the light microscopic as well as electron microscopic level, we provide definite evidence that the 140-kDa antigen recognized by this monoclonal antibody is confined to cerebral pericytes, whereas endothelial cells are devoid of this antigen. N-Terminal sequencing of the corresponding immunocrossreacting renal protein revealed that the protein detected by the monoclonal antibody is identical to aminopeptidase N. By means of the reverse transcriptase-polymerase chain reaction, the identity of the 140-kDa antigen as aminopeptidase N could also be verified for cerebral microvascular cells. Cerebral pericytic aminopeptidase N may be involved in neurotransmitter (enkephalin) metabolism at the blood-brain interface. By taking into account that brain pericytes have been found to express further plasma membrane-bound enzymes, these results strongly suggest the contribution of cerebral pericytes in the metabolic concert of the homeostatic balance regulated by the blood-brain barrier.

Localization of neprilysin (EC 3.4.24.11) mRNA in rat brain by in situ hybridization

The distribution of the messenger RNA (mRNA) coding for neprilysin (EC 3.4.24.11) has been studied by in situ hybridization in the adult rat brain. A markedly heterogenous distribution among various brain regions was found. A strong signal was observed in the glomerular layer of the olfactory bulb, the olfactory tubercle, the caudate putamen, the habenular, anterior pretectal, interpeduncular, red, dorso tegmental, pontine, and vestibular nuclei, the mammillary bodies, the Purkinje cells, and the choroid plexus of the fourth ventricle. A large number of areas such as the cortex, the dentate gyrus, the hippocampus, the medial terminal nucleus of the accessory tract, the accumbens and the arcuate nuclei, the superior and inferior colliculi, and a few regions in the thalamus at the mesencephalic level exhibited a moderate or low signal of hybridization. The majority of these regions are also known to contain the neprilysin protein. On the other hand, the globus pallidus, the substantia nigra, and the central gray matter, which show a high or a moderate amount of neprilysin, did not contain any neprilysin mRNA. Comparison of the regional distribution of neprilysin mRNA with that of its translation product provides insight into neprilysin neuronal pathways in the central nervous system.

Characterisation of enkephalinase (EC 3.4.24.11) activity on various leukemic cells expressing the common acute lymphocytic leukemia antigen (CALLA)

The deduced amino acid sequences of CALLA, a cell surface marker of human acute lymphocytic leukemia, and human enkephalinase (neutral endopeptidase, EC 3.4.24.11) were recently reported to be almost identical. We show that membranes of CALLA+ cells of the REH lymphoblastic cell line as well as blast cells derived from the blood or bone marrow of patients with acute lymphocytic leukemia display high enkephalinase activity. This activity was abrogated by several enkephalinase inhibitors at concentrations closely similar to those required to inhibit pure human enkephalinase. However, these compounds did not significantly modify the rate of REH cell proliferation in vitro. Hence, the functional role, if any, of the high peptidase activity in lymphoblastic cells remains to be established.

Histochemistry of proteases in ependyma, choroid plexus and leptomeninges

Aminopeptidase M (APM), aminopeptidase A (APA), dipeptidyl peptidase IV (DPP IV) and gamma-glutamyl transferase (GGT) were demonstrated histochemically in cryostat sections of the rat brain to show the reaction pattern of ependyma, choroid plexus and leptomeninges. GGT was only demonstrable in the cell membranes of ependymal cells and in the leptomeninges; however, APA, APM and DAP IV showed a variable degree of activity in the capillary endothelium of the choroid plexus as well as in the leptomeninges. On the basis of these results, it is postulated that peptides in the cerebrospinal fluid can be cleaved extraventricularly by the enzymes demonstrated in the leptomeninges.