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Evidence for existence of A and B form monoamine oxidase in mitochondria from dog platelets
T Obata1, T Egashira, Y Yamanaka
1Department of Pharmacology, Medical College of Oita, Japan.
Abstract:
It is known that platelet MAO appears to behave more like the B-form enzyme than the A-form enzyme based on inhibitor sensitivity and substrate specificity. However, dog platelets showed a different substrate specificity such as high activity with 5-HT and beta-PEA as substrates. Moreover, dog platelet MAO was sensitive to the drugs clorgyline and harmaline with 5-HT as the substrate, while it was sensitive to the drug deprenyl with beta-PEA as the substrate. These results also indicate the existence of two forms of MAO in dog platelets unlike in other platelets such as those from humans. A-form MAO from dog platelets was more stable against heat treatment at 55 degrees C than A-form MAO from dog liver and brain. On the other hand, there was no difference in the heat resistance of the three enzymes with beta-PEA as the substrate. After digestion with trypsin at 37 degrees C for 30 min, it was found that MAO from dog platelets, brain and liver were mostly inhibited with 5-HT as the substrate. In contrast, MAO in brain and liver were inhibited about 10%, but platelet MAO was inhibited about 50% with beta-PEA as the substrate. From these results, it is considered that dog platelet MAO exists as the two forms of the enzyme and has different enzymic properties in comparison with those of MAO from dog liver and brain mitochondria.
Insights
Dog platelets contain two forms of monoamine oxidase (MAO), differing in substrate specificity and inhibitor sensitivity compared to other mammals. These findings suggest unique enzymatic properties in canine platelet MAO.
Area of Science:
- Biochemistry
- Enzymology
- Pharmacology
Background:
- Platelet monoamine oxidase (MAO) typically exhibits characteristics of the B-form enzyme.
- Previous studies suggest MAO-B is predominant in platelets based on inhibitor sensitivity and substrate specificity.
Purpose of the Study:
- To investigate the enzymatic properties and forms of MAO present in dog platelets.
- To compare dog platelet MAO with MAO from dog liver and brain mitochondria.
Main Methods:
- Enzyme kinetics using various substrates (5-HT, beta-PEA).
- Inhibitor sensitivity assays with clorgyline, harmaline, and deprenyl.
- Thermal stability assays at 55°C.
- Enzyme digestion studies with trypsin at 37°C.
Main Results:
- Dog platelets displayed high activity with 5-HT and beta-PEA, unlike typical MAO-B.
- Dog platelet MAO showed differential sensitivity to inhibitors (clorgyline, harmaline, deprenyl) depending on the substrate.
- A-form MAO from dog platelets was more heat-stable than from dog liver and brain.
- Platelet MAO exhibited distinct trypsin inhibition patterns compared to liver and brain MAO, especially with beta-PEA.
Conclusions:
- Dog platelets possess two distinct forms of MAO, challenging the typical MAO-B dominance observed in other species.
- Dog platelet MAO exhibits unique substrate specificities and inhibitor sensitivities.
- Enzymatic properties of dog platelet MAO differ significantly from those of MAO in dog liver and brain mitochondria.