Neurocatin-induced inhibition of monoamine oxidase A in rat brain synaptosomes

90 years of monoamine oxidase: some progress and some confusion

It would not be practical to attempt to deal with all the advances that have informed our understanding of the behavior and functions of this enzyme over the past 90 years. This account concentrates key advances that explain why the monoamine oxidases remain of pharmacological and biochemical interest and on some areas of continuing uncertainty. Some issues that remain to be understood or are in need of further clarification are highlighted.

Monoamine oxidase inactivation: from pathophysiology to therapeutics

Monoamine oxidases (MAOs) A and B are mitochondrial bound isoenzymes which catalyze the oxidative deamination of dietary amines and monoamine neurotransmitters, such as serotonin, norepinephrine, dopamine, beta-phenylethylamine and other trace amines. The rapid degradation of these molecules ensures the proper functioning of synaptic neurotransmission and is critically important for the regulation of emotional behaviors and other brain functions. The byproducts of MAO-mediated reactions include several chemical species with neurotoxic potential, such as hydrogen peroxide, ammonia and aldehydes. As a consequence, it is widely speculated that prolonged excessive activity of these enzymes may be conducive to mitochondrial damages and neurodegenerative disturbances. In keeping with these premises, the development of MAO inhibitors has led to important breakthroughs in the therapy of several neuropsychiatric disorders, ranging from mood disorders to Parkinson's disease. Furthermore, the characterization of MAO knockout (KO) mice has revealed that the inactivation of this enzyme produces a number of functional and behavioral alterations, some of which may be harnessed for therapeutic aims. In this article, we discuss the intriguing hypothesis that the attenuation of the oxidative stress induced by the inactivation of either MAO isoform may contribute to both antidepressant and antiparkinsonian actions of MAO inhibitors. This possibility further highlights MAO inactivation as a rich source of novel avenues in the treatment of mental disorders.

Imidazoline/guanidinium binding domains on monoamine oxidases. Relationship to subtypes of imidazoline-binding proteins and tissue-specific interaction of imidazoline ligands with monoamine oxidase B

Pharmacologically active compounds with an imidazoline and/or guanidinium moiety are recognized with high affinity by a family of membrane-bound proteins collectively known as imidazoline binding sites or imidazoline/guanidinium receptive sites. Two such receptive sites may correspond to imidazoline binding domains identified on the A and B isoforms of monoamine oxidase (MAO), but the detection of monoamine oxidase isoforms in multiple tissues contrasts with the restricted expression of imidazoline-binding proteins. To address these issues, we determined the relationship between monoamine oxidase isoforms and subtypes of imidazoline-binding proteins in human tissues known to express one or both isoforms of MAO. 2-(3-Azido-4-[125I]iodophenoxy)methylimidazoline ([125I]A-ZIPI), a photoaffinity adduct that selectively labels imidazoline-binding proteins, photolabeled an M(r) = approximately 59,000 peptide in liver and an M(r) = approximately 63,000 peptide in placenta, consistent with the M(r) of the MAO isoforms identified by immunoblots in these tissues. The photolabeled species in liver was immunoprecipitated with MAO-B selective antibodies, whereas the photolabeled species in placenta was immunoprecipitated by MAO-A selective antibodies consistent with the isoform of MAO predominantly expressed in these tissues. The imidazoline/guanidinium ligands interact with the enzyme at a site distinct from the substrate recognition domain, and the immunoprecipitated peptides in liver and placenta display distinct ligand recognition properties consistent with those reported for subtypes of imidazoline binding sites. However, the imidazoline binding domain was not detected in platelet membrane preparations containing amounts of MAO-B equivalent to those in the photolabeled liver membranes indicating that recognition of this domain is tissue-restricted. Restricted access to the imidazoline binding domain on platelet MAO-B was not altered by membrane washing with 500 mM KCl or by solubilization and partial purification of the enzyme suggesting that there are distinct subpopulations of MAO. Identification of a binding domain on MAO that recognizes this class of pharmacologically active compounds suggests a novel mechanism for regulation of substrate oxidation/selectivity or that the enzyme may subserve an as yet undefined function.

Effect of neurocatin on the activity of monoamine oxidase B in rat brain synaptosomes

Neurocatin, a small (about 2,000 Dalton) neuroregulator isolated from mammalian brain, is a powerful effector of monoamine oxidase B in rat brain synaptosomes. Incubation of intact synaptosomes with neurocatin caused an inhibition of the enzyme dependent on the concentration of neurocatin. This inhibition became statistically significant at a neurocatin concentration of 10 ng/200 microliters and was significant at all higher neurocatin concentrations. At 40 ng/200 microliters, neurocatin inhibited monoamine oxidase B activity by about 60%. This inhibitory effect was almost completely abolished by breaking the synaptosomal membrane by hypotonic buffer prior to incubation with neurocatin. In addition, incubation of the synaptosomes in calcium free medium almost completely abolished the inhibitory effect of neurocatin on monoamine oxidase B. The inhibition appeared to involve covalent modification of the enzyme mediated by a neurocatin receptor(s). Measurements of the kinetic parameters of the enzyme showed that 20 ng of neurocatin caused a statistically significant decrease in Vmax (by 20%) with no significant change in KM, compared to controls. Inhibition of monoamine oxidase by neurocatin is potentially of great clinical importance because this enzyme plays a major role in catabolism of the biogenic amines and alterations in its activity is believed to contribute to several neurological disorders.

The effect of neurocatin on protein phosphorylation in striatal synaptosomes from rat brain

Neurocatin, a neuroregulatory factor isolated from mammalian brain, is a powerful affector of protein phosphorylation in rat striatal synaptosomes. Two major synaptosomal phosphoproteins of approximately 80 and approximately 60 kDa, possibly synapsin I and tyrosine hydroxylase, were especially sensitive to neurocatin. Immunoprecipitation experiments confirmed that the 60-kDa protein is the enzyme tyrosine hydroxylase. At low concentrations of neurocatin (to approximately 7.5 ng/100 microliters of suspension), incorporation of 32P orthophosphate into these proteins increased with increasing neurocatin concentration. At 7.5 ng of neurocatin, incorporation of the label into the two proteins increased by 22 and 26%, respectively. Concentrations of neurocatin > 7.5 ng/100 microliters caused progressive decrease in incorporation of 32P into many synaptosomal proteins; by a concentration of neurocatin of approximately 45 ng/100 microliters, the level of 32P incorporation into many proteins was < or = 70% of control. The effects of neurocatin on synaptosomal protein phosphorylation were also dependent on the time of incubation. At a constant concentration of approximately 7.5 ng/100 microliters of neurocatin, increased incorporation of 32P into many proteins was measurable within 0.5 min and was maximal by 1 min. Incubation times > 2.0 min, showed progressive decrease in 32P incorporation. Removing extrasynaptosomal Ca2+ with EGTA attenuated the increased 32P incorporation induced by low neurocatin concentrations, suggesting that calcium plays a role in neurocatin-induced phosphorylation of rat striatal synaptosomal proteins. The reduced incorporation of label induced by high neurocatin concentrations, however, was not calcium dependent. The effects of neurocatin on the level of 32P incorporation into proteins were observed only in intact synaptosomes, consistent with this compound acting through receptors on the plasma membrane.