Rat brain aryl acylamidase: Multiple forms and inhibition effects of LSD, serotonin and related compounds

Melatonin metabolism in the central nervous system

The metabolism of melatonin in the central nervous system is of interest for several reasons. Melatonin enters the brain either via the pineal recess or by uptake from the blood. It has been assumed to be also formed in some brain areas. Neuroprotection by melatonin has been demonstrated in numerous model systems, and various attempts have been undertaken to counteract neurodegeneration by melatonin treatment. Several concurrent pathways lead to different products. Cytochrome P₄₅₀ subforms have been demonstrated in the brain. They either demethylate melatonin to N-acetylserotonin, or produce 6-hydroxymelatonin, which is mostly sulfated already in the CNS. Melatonin is deacetylated, at least in pineal gland and retina, to 5-methoxytryptamine. N¹-acetyl-N²-formyl-5-methoxykynuramine is formed by pyrrole-ring cleavage, by myeloperoxidase, indoleamine 2,3-dioxygenase and various non-enzymatic oxidants. Its product, N¹-acetyl-5-methoxykynuramine, is of interest as a scavenger of reactive oxygen and nitrogen species, mitochondrial modulator, downregulator of cyclooxygenase-2, inhibitor of cyclooxygenase, neuronal and inducible NO synthases. Contrary to other nitrosated aromates, the nitrosated kynuramine metabolite, 3-acetamidomethyl-6-methoxycinnolinone, does not re-donate NO. Various other products are formed from melatonin and its metabolites by interaction with reactive oxygen and nitrogen species. The relative contribution of the various pathways to melatonin catabolism seems to be influenced by microglia activation, oxidative stress and brain levels of melatonin, which may be strongly changed in experiments on neuroprotection. Many of the melatonin metabolites, which may appear in elevated concentrations after melatonin administration, possess biological or pharmacological properties, including N-acetylserotonin, 5-methoxytryptamine and some of its derivatives, and especially the 5-methoxylated kynuramines.

On the active site for hydrolysis of aryl amides and choline esters by human cholinesterases

Cholinesterases, in addition to their well-known esterase action, also show an aryl acylamidase (AAA) activity whereby they catalyze the hydrolysis of amides of certain aromatic amines. The biological function of this catalysis is not known. Furthermore, it is not known whether the esterase catalytic site is involved in the AAA activity of cholinesterases. It has been speculated that the AAA activity, especially that of butyrylcholinesterase (BuChE), may be important in the development of the nervous system and in pathological processes such as formation of neuritic plaques in Alzheimer's disease (AD). The substrate generally used to study the AAA activity of cholinesterases is N-(2-nitrophenyl)acetamide. However, use of this substrate requires high concentrations of enzyme and substrate, and prolonged periods of incubation at elevated temperature. As a consequence, difficulties in performing kinetic analysis of AAA activity associated with cholinesterases have hampered understanding this activity. Because of its potential biological importance, we sought to develop a more efficient and specific substrate for use in studying the AAA activity associated with BuChE, and for exploring the catalytic site for this hydrolysis. Here, we describe the structure-activity relationships for hydrolysis of anilides by cholinesterases. These studies led to a substrate, N-(2-nitrophenyl)trifluoroacetamide, that was hydrolyzed several orders of magnitude faster than N-(2-nitrophenyl)acetamide by cholinesterases. Also, larger N-(2-nitrophenyl)alkylamides were found to be more rapidly hydrolyzed by BuChE than N-(2-nitrophenyl)acetamide and, in addition, were more specific for hydrolysis by BuChE. Thus, N-(2-nitrophenyl)alkylamides with six to eight carbon atoms in the acyl group represent suitable specific substrates to investigate further the function of the AAA activity of BuChE. Based on the substrate structure-activity relationships and kinetic studies, the hydrolysis of anilides and esters of choline appears to utilize the same catalytic site in BuChE.

Brain aryl acylamidase

1. The serotonin (5-HT) sensitive brain aryl acylamidase (AAA) has received considerable attention due to its potential involvement in 5-HT action mechanism in CNS. 2. Multiple forms, AAA-1 and 2, have been separated by ammonium sulfate precipitation of brain extract and subsequent gel filtration. 3. Their chemical properties have been characterized and differentiated by effects of several classes of drugs including d-LSD, 5-HT, 5-HT related compounds and tetrahydro-beta-carbolines on their enzyme activities. 4. In the rat brain, AAA-1 shows highest specific activity in corpus striatum and lowest activity in cerebellum whereas AAA-2 shows highest specific activity in cerebellum and lowest activity in corpus striatum. 5. Subcellularly, AAA-1 exhibits highest specific activity in synaptosomal fraction of rat corpus striatum, lowest activity in mitochondrial fraction and no activity in nuclear fraction while AAA-2 exhibits highest specific activity in microsomal fraction and lowest activity in nuclear fraction. 6. Triton X-100 treatment altered the subcellular distribution pattern of both AAA-1 and AAA-2. 7. AAA-2 is possibly associated with true acetylcholinesterase (AChE) in brain based on its inhibition by neostigmine but its identity with AChE needs further elucidation. 8. To determine the physiological role(s) for brain AAA, naturally occurring aromatic alkylamines other than melatonin need to be tested as possible substrate(s) for the enzyme activity.

Rat brain aryl acylamidase: further characterization of multiple forms

1. Two fractions of aryl acylamidase (EC 3.5.1.13) were further separated from rat brain extracts at pH 7.5 by ammonium sulfate precipitation and Bio-Gel chromatography. 2. 1,2,3,4-Tetrahydro-beta-carboline competitively inhibited (67%) fraction-1 but slightly inhibited (13%) fraction-2. Tetrahydroharman, 6-hydroxy-tetrahydroharman and harminic acid slightly inhibited both fractions. Harmalol inhibited fraction-1 but enhanced fraction-2. 6-Methoxy-harman, 6-methoxy-harmalan and harmaline enhanced both fractions. 3. Pargyline did not affect either fraction. Methiothepin, cyproheptadine and chlorimipramine inhibited fraction-1 but stimulated fraction-2. 4. Neostigmine moderately (30%) inhibited AAA-2 but did not have any significant effect on AAA-1. 5. These results indicate that the beta-carboline compounds might play a role in regulating activity of AAA-1 and 2 in brain. 6. Both fractions might be related to serotonergic neurons but only AAA-2 might be associated with acetylcholinesterase.

Subcellular localization of chicken kidney aryl acylamidase activity

The intracellular localization of aryl acylamidase (aryl-acylamide amidohydrolase, EC 3.5.1.13) in chicken kidney was investigated. By separation on density gradients of the silica sol Ludox AM, the enzyme was localized in the mitochondrial fraction. This mitochondrial fraction was shown to be substantially free of lysosomal contamination. Subfractionation of the purified mitochondria indicates that the enzyme is located on the outer membrane, can be solubilized, and may be a suitable marker enzyme for kidney mitochondria.