Monoamine Oxidase (MAO) in Human Peripheral Tissues
Section snippets
INTRODUCTION
Monoamine oxidase (MAO; EC 1.4.3.4) is a flavoenzyme found tightly associated with the outer membrane of mitochondria, bound via a C-terminal transmembrane polypeptide segment (Mitoma and Ito, 1992) and inserted in the membrane by means of ubiquitin, with energy provided by ATP (Zhuang et al., 1988, Zhuang et al., 1992). There are two isoenzymes, MAO-A and MAO-B, encoded by separate genes located on the X chromosome; each gene consists of 15 exons with identical intron–exon organisation,
MAO Activities in Tissue and Cell Homogenates
The methods available to study MAO expression in human tissues have evolved over the years, and this has influenced what has been known about the distribution of MAO both in the brain and, more recently, the localisation of MAO in non-CNS tissues. Early work on the distribution of human MAO enzymes relied on the measurement of enzyme activities in crude tissue or cell homogenates, using the preferred substrates for MAO-A (viz. NE and 5-HT) and MAO-B (PEA) coupled with low concentrations of
CORRELATION BETWEEN MAO mRNA AND MAO PROTEIN EXPRESSION
There are very few studies of MAO mRNA in human peripheral tissues. In one of the few that exist, Grimsby et al. (1990) used Northern blot analyses to monitor the levels of MAO-A and MAO-B transcripts in various foetal tissues. High levels of MAO-A transcript were detected in placenta, lung and muscle, followed by kidney, liver, spleen and adrenal (all in order); foetal heart and thymus appeared to express no MAO-A transcripts. The relative concentrations of a 3 kb MAO-B mRNA transcript were:
DISCUSSION
The concentrations of MAO-A and MAO-B protein molecules in a given cell or tissue are not necessarily a measure of the relative importance of the two enzymes in substrate deamination in vivo. This is because the molecular turnover (nmol substrate deaminated/mol enzyme) of MAO-A with all substrates, other than PEA and benzylamine, is much higher than that of MAO-B (Riley and Denney, 1991). This explains why the activity data in Table 1 would suggest that MAO-A activity is dominant in peripheral
Acknowledgements
The author wishes to thank Dr. R. Earl for helpful discussions, Dr. S. Sivasubramaniam for assistance with the figures and The Wellcome Trust and Nottingham Trent University for financial support.
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