Abstract
The temperature dependence of the initial velocity, ν0, using substrates benzylamine, tryptamine, tyramine, and serotonin for the multiple enzyme forms of monoamine oxidase in situ in purified intact rat brain mitochondria, was determined in the range between 4° and 45°. In rat brain tissue, the oxidative deamination of serotonin is catalyzed by monoamine oxidase-A-type enzyme exclusively. In contrast, the oxidation of benzylamine is catalyzed by monoamine oxidase-B-type enzyme. However, both enzyme types metabolize tryptamine and tyramine. Arrhenius plots reflecting the functional states of the multiple monoamine oxidase enzyme forms showed clear differences. A straight line was observed when using benzylamine as substrate. Changes in slope at 21-22° and 35-36° were observed when serotonin, tryptamine or tyramine was employed as substrate. The functional state of monoamine oxidase-B enzyme appeared to be homogeneous whereas that of monoamine oxidase-A enzyme appeared to be heterogeneous and strongly temperature dependent. Spin-labeled stearic acids I (12, 3) or I (1, 14) were incorporated as structural probes into the outer mitochondrial membrane. Temperature dependence of order parameter S, indicating fluidity in the vicinity close to the surface region of the membrane, revealed apparent breaks at 21-22° and 32.6-33.6° when I (12, 3) was used. Temperature dependence of the rotational correlation time τ, relating to fluidity in the center region of the membrane, indicated phase transition at 29-30° when I (1, 14) was used. The functional state of monoamine oxidase-B enzyme was apparently independent of the fluidity properties of the membrane lipids. Coincidence of transition temperature at 21-22° was observed only when spin-labeled stearic acid I (12, 3) was used in the ESR study and 14C-labeled substrates serotonin, tyramine or tryptamine was used in the enzymic study. The nature of monoamine oxidase-A enzyme was sensitive to the physical state of the bulk membrane lipids through lipid-protein interactions. The lipid-protein interactions occurring in the region close to the surface of the membrane were effective in modulating the functional state of monoamine oxidase-A enzyme. The break temperature at 35-36° observed using tryptamine, tyramine, or serotonin as substrate for the monoamine oxidase-A enzyme—did not correspond to any of the characteristic phase transition temperatures of the bulk membrane-lipid domain. A temperature-induced conformational change in monoamine oxidase-A at 35-36° may be attributed to an additional lipid-protein interaction originating from the tightly bound lipid shell or an intrinsic alteration in the enzyme protein.
ACKNOWLEDGMENT The author wishes to thank Ms. Donna Miller for typing the manuscript.
- Copyright © 1980 by The American Society for Pharmacology and Experimental Therapeutics
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