Carboxypeptidase M in Brain and Peripheral Nerves

Abstract

Carboxypeptidase M (CPM), a plasma membrane‐bound enzyme, cleaves C‐terminal basic amino acids with a neutral pH optimum. We studied its distribution in human, baboon, and dog brain and in dog peripheral nerves. Areas were dissected, homogenized, centrifuged, and assayed for activity with dansyl‐Ala‐Arg. The corpus callosum and the pyramidal and optic tract were especially rich in CPM, whereas basal ganglia and cortex had low activity. The identity of the basic carboxypeptidase activity with CPM was shown by similarities in subcellular localization, membrane attachment, substrate hydrolysis, inhibition by a specific basic carboxypeptidase inhibitor, and cross‐reaction with anti‐human CPM antiserum. This antiserum immunoprecipitated an average of 85% of the activity in human and baboon brain and ∼66% in dog brain. CPM copurified with myelin extracted from the brain. Consistent with results obtained in placenta and cultured kidney cells, CPM in the brain appears to be membrane‐bound via a phosphatidylinositol glycan anchor. In the peripheral nerves, the specific activity in dog sciatic nerve and in vagus was high (98 and 149 nmol/h/mg of protein, respectively). In immunohistochemical studies, glia in the brain, which appear to be oligodendrocytes or astrocytes, and the outer aspects of myelin sheaths and Schwann cells in sciatic and vagus nerves were stained. We conclude that in some areas of the CNS and the PNS, CPM is closely associated with myelin and myelin‐forming cells. Northern blot analysis revealed the presence of mRNA coding for CPM in the brain, showing that the enzyme is indeed synthesized there.

Angiotensin I–Converting Enzyme Inhibitors Are Allosteric Enhancers of Kinin B1 and B2 Receptor Function

“…B1R signaling was recently reported to prevent homing of encephalitogenic T-lymphocytes into the CNS, which was enhanced in B1R -/- mice 75. CPM, closely associated with myelin centrally and peripherally 76, should contribute by generating B1R ligands. The report mentioned that ACE inhibitor also suppresses inflammation in the CNS 75.…”

Crystal Structure of Human Carboxypeptidase M, A Membrane-bound Enzyme that Regulates Peptide Hormone Activity

“…13,14 The highest levels of CPM have been found in human lung and placenta, but significant amounts are present in kidney, blood vessels, intestine, brain and in peripheral nerves. 2,13,15,16 CPM has been found in soluble form in various body fluids, including amniotic fluid, seminal plasma and urine. 3,17,18 Human CPM exhibits maximal proteolytic activity at neutral pH, contrasting with CPE or CPD, which have acidic pH optima.…”

Functional roles of cell surface peptidases in reproductive organs

“…Bradykinin, anaphylatoxins, enkephalin hexapeptides, epidermal growth factor and dynorphin A have been reported to be naturally occurring substrates for CP-M. 33 The removal of the carboxy-terminal arginine or lysine results in the modification or inactivation of peptide hormone activity, and can also change the physical properties of the proteins. 35 CP-M has been reported to be distributed in organs and biological fluids such as the placenta, lungs, kidneys, intestines, brain, peripheral nerves, amniotic fluid and so on, 36 but there is no physiological role of CP-M in these tissues and fluids that has been clearly defined to date.…”