Studies on Monoamine Oxidase. Xviii. Enzymic Properties of Placental Monoamine Oxidase

Abstract

Abstract-Enzymicproperties of partially purified monoamine oxidase (MAO) from human placenta were studied with tyramine, serotonin and benzylamine as substrates. The highest activity was obtained with serotonin and almost no activity was observed with benzylamine. These results are similar to those obtained with rat placental MAO, but different from those with rabbit placental MAO. The Km values for serotonin and tyramine were found to be 0.21 mM and 0.23 mM, respectively and the pH optimum was 8.1 with either substrate. The thermal inactivation curves of this enzyme with the two substrates were identical. The pt curves for inhibition of MAO activity by harmine, pargyline and iproniazid were similar and almost the same pI 50 values for the respective inhibitors were obtained with the two substrates. MAO in human placenta differs from that in other organs, such as liver, brain and plasma from the standpoint of the substrate specificity and the inhibitor sensitivity. The possibility that human placenta contains a single form of MAO is discussed on the basis of the present results.In 1948 Luschinsky and Tickner (1) reported that human placenta contained a relatively large amount of monoamine oxidase (MAO). The following year Thompson and Tickner (2) confirmed this observation in rat, rabbit and ginea pig placentas. About twenty years later Youdim and Sandler (3) obtained a partially purified preparation of the enzyme from human placenta. Their final product showed a specific activity of 3,500 and 400-fold puri fication over the original homogenate. There have been many clinical studies (4-8) on human placental MAO obtained at term and postmature deliveries, but characteristics and functional significance of this enzyme are unknown.In this work the enzymic properties of MAO from mitochondria of human placenta were compared with those of preparations from bovine liver and brain. To determine whether or not MAO in human placenta exists as a single enzyme with a broad substrate specificity or as a multiple form of the enzyme, the substrate specificities, pH optimum, sensitivity to inhibitors and effect of temperature on the enzyme were studied with serotonin and tyramine as substrates. MATERIALS AND METHODS Preparation of MAOHuman placental MAO: MAO was partially purified from human placental mito chondria by a slight modification of the method of Youdim and Sourkes (9). Mitochondria were prepared from human placenta by differential centrifugation, suspended in 0.1 M phosphate buffer (pH 7.5) and then were sonicated for 30 min at 20 kHz. Triton X-100 was

cDNA cloning of human liver monoamine oxidase A and B: molecular basis of differences in enzymatic properties.

“…3, lane B). This result is consistent with catalytic activity assays since predominantly MAO A activity has been found in human placenta (10).…”

Human monoamine oxidase A and B genes exhibit identical exon-intron organization.

“…The tissue distribution of MAOA and MAOB mRNAs determined by RNA blot-hybridization (Northern) analysis (9,15) is consistent with enzymatic studies (16). MAOA and MAOB are coexpressed in almost all human tissues except placenta, which predominantly expresses MAOA (11,15,16), and platelets and lymphocytes, which predominantly express MAOB.…”

Gossypol effects on monoamine oxidase(MAO) activity in several organs of term rats.

“…Monoamine oxidase (MAO), derived from the outer membrane of mitochondria in cytoplasma, is considered to be an indispensable enzyme which inactivates catecholamines, indoleamines (Chen et al 1976) and serotonin (Gujrati et al 1985). There are two types of MAO, A and B. Liver has mainly A type, brain has mainly B type and placenta has only A type which is known to have two subunits (Egashira 1976; Chen and Weyler 1988). MPTP, a inhibitor, has been shown to induce Parkinsonism (Langston et al 1984;Fuller and HemrickLuecke 1985).…”