Tissue specificity of mitochondrial enzyme of higher plants.

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We have investigated the tissue specificity of cytochrome c oxidase which is an important element of the respiratory chain of mitochondria. The enzyme consists of the mitochondrial-DNA encoded and nuclear-DNA encoded subunits. The tissues of higher plants can be divided into the above- and below-ground parts and they grow in the different condition of temperature. The leaf and stem tissues of tropical and subtropical crops are exposed to an extremely high temperature, while roots are kept alive at a comparatively moderate temperature. Cytochrome c oxidases of the leaf and stem tissues were resistant to heat inactivation than the enzyme of the root tissue. This is due to the difference in the enzyme molecules rather than to the difference in membrane lipids species. Because, cytochrome c oxidase purified from the hypocotyl (above-ground tissue) of mung bean seedlings was more resistant to inactivation by heat than the enzyme from the root.Then, we have analyzed the variation in the molecular form of cytochrome c oxidase. The structure of mitochondrially made subunits (I,II and Iv) are very conservative, which suggests no existence of the tissue-specific isoform of the subunits. While a nuclear-DNA encoded subunit (subunit VC) of cytochrome c oxidase in mung bean hypocotyls differed immunologically from that in the roots. The result suggests that at least in mung bean, a nuclear-encoded subunit of cytochrome c oxidase is synthesized tissue-specifically. In the case of sweet potato, there was difference in the number of the subunit Vc in one enzyme complex between the enzymes from root and shoot tissues. We thought that the variation of the molecular form of cytochrome c oxidase may cause the difference in thermostagility of the enzyme.