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Abstract-The inhibitory effects of some detergents commonly used in biochemical research on carp liver mitochondrial monoamine oxidase were examined. Sodium dodecylsulfate, octyl-(3-D-glucopyranoside, sodium cholate and Triton X-100 at relatively low concentrations caused strong dose-dependent inhibition of the activity towards tyramine, but digitonin caused only weak inhibition. Sodium dodecylsulfate, octyl-Q D-glucopyranoside and sodium cholate caused almost complete inhibition of activity in the concentration ranges tested. The extent of inhibition by Triton X-100 was greater after preincubation at 37'C for 30 min than that without preincubation, but with or without preincubation, the inhibition was not substrate-selective and was not complete at a relatively high concentration (2%) of Triton X-1 00. Without preincubation, the mode of inhibition by Triton X-1 00 was competitive and reversible with respect to the oxidations of 5-HT, tyramine and PEA, but after preincubation (37°C for 30 min), it became non competitive and irreversible, depending on the concentration of detergent used. These findings suggest that it had different actions on the enzyme depending on preincubation. Triton X-100 also slightly changed enzyme sensitivity towards clorgyline and deprenyl, regardless of the preincubation time or the substrate used. Some possible mechanisms of the inhibitory effect of Triton X-100 are discussed. Many tissues of various species examined so far have been shown to contain both forms of MAO in different proportions or only one of these two forms of MAO (4, 5). However, our recent studies on carp (Cyprinus carpio) liver mitochondria suggest the existence of a single form of MAO with the properties of both forms of mammalian MAO, but differing from both in showing the same sensitivity to clorgyline and deprenyl (6).This MAO may be a classical mitochondrial MAO, presumably with FAD as its prosthetic group, since we also observed time dependent change in its mode of inhibition, which was first reversible and then irreversible, like that of mammalian MAO by clorgyline and deprenyl (6).The inhibitory effect of Triton X-100 on MAO activity was noticed during solubili zation of carp liver mitochondrial MAO with this non-ionic detergent (7). In the present study, we examined the inhibitory effects of various commonly used detergents on carp liver mitochondrial MAO. Special attention was paid to the inhibitory effects of Triton X-100.

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