Summary:The process of aging presents itself with various alterations in physiological events. Although the turnover of catecholamines increases with aging, there is a lack of response to catecholamines in target tissues. One of the key enzymes in catecholamine metabolism is monoamine oxidase. It has been suggested that tissue and serum monoamine oxidase activities show pathological alterations in various diseases while physiological fluctuations can also be detected in normals. The aim of this study is to determine the sex and age related changes of platelet and serum monoamine oxidase in healthy volunteers.In this study, 75 healthy volunteers of different ages (21-80 a) and sexes (40 females, 35 males) were included. Serum and platelet monoamine oxidase determinations were performed spectrophotofluorometrically by Tufoesson's (Scand J Clin Lab Invest 1970; 26:151-4) and Kraml's (Biochem Pharmacol 1965; 14:1684-6) modified methods, respectively.While there was no significant difference in serum monoamine oxidase activities related to age and sex, platelet monoamine oxidase manifested a significant increase in females compared to males (p < 0.05) and the mean values in both sexes showed an increase with age (p < 0.001).The results of this study imply that platelet monoamine oxidase shows an age related increase which is more prominent in females.
Abstracl~ Some parameters affecting the activity of monoamine oxidase (MAO) in purified beef brain mitochondria were investigated. and diversities in enzyme properties were found as a function of sub->(rate. The deamination of the biogenic amines: serotonin. dopamine, tyramine, tryptamine, phenylethylilminc and two non-physiological amines, kynuramine and rn-iodobenzylamine, was studied. Anions in high concentrations inhibited enzyme activity with kynuramine being the substrate most affected. Among the biogenic amines, the activity with the indolalkylamines showed greater sensitivity to monovdent anions such as chloride than to polyvalent ions such as phosphate whereas the opposite was true with the phenylalkylamines. However, pyrophosphate ion had little or no effect on M A 0 activity, regardless of substrate. The inhibition of kynuramine and serotonin deamination was non-competitive but mixed competitive inhibition was found with tyramine and phenylethylamine.The activity of M A 0 was markedly affected by pH, and it had been previously reported that the substrates showed different pH optima in their oxidation. The effect of pH on activity has been attributed in part to changes in the ionization of the substrate and the hypothesis that the true substrate is the non-protonated amine. This was reflected in kinetic studies showing high substrate inhibition with increased pH. It was calculated that phenylethylamine would have the highest percentage of un-ionized amine at pH 8.2 and 9.1. At these pHs, there was more pronounced inhibition with high substrate concentrations of phenylethylamine than with the other substrates. In contrast. there was little inhibition with high substrate concentrations of tyramine which was the most ionizable of the substrates tested. When K, values obtained at pH 7.4, 8.2 and 9.1 were corrected for ionization of the substrate, the corrected K , was lowest at pH 7.4 for all substrates.Less than 50'" of M A 0 activity was lost when beef brain mitochondria was heated at 5 0 C for 20 min. However. there was only a slight variation with substrate in the thermal inactivation experiments. It is concluded that the mitochondria1 membrane environment surrounding the enzyme imposes certain restrictions on the enzymatic activity with respect to the different substrates which, in turn, are also affected by such parameters as pH and ions. The results are discussed in terms of the relationship of these factors to the question of enzyme multiplicity.
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