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Succinate oxidation in scntella of germinating seeds of wheat and maize was investigated. Besides oxidation via succihate dehydrogenase (SDH; EC 1.3.99.1), an alternative path of succinate oxidation insensitive to SDH inhibitors -malonate and thenoyltrifluoroacetone (TTFA) -was revealed. Using isopicnic sucrose gradient it was shown that this path is localized in glyoxysomal membranes. Glyoxysomal succinate oxidase (GSO) converts succinate directly into malate with the production of hydrogen peroxide identified using auxilary enzymes malate dehydrogenase and peroxidase. GSO is most active during the intensive operation of the glyoxylate cycle (3-5 days of germination). Quinacrine, the inhibitor of flavine-containing oxidases, strongly suppressed the activity of GSO. Km for succinate is 18 mM for GSO from maize scutellum. It is concluded that in scutella of cereal seeds the glyoxysomal succinate oxidation non-linked with ATP synthesis operates.

Homogeneous malate dehydrogenase (MDH) with a specific activity of 20-24 units per mg protein was purified from the sulfur bacterium Beggiatoa leptomitiformis strain D-402 grown organotrophically and lithotrophically and from the organotrophic bacterium Beggiatoa alba. MDHs from the B. leptomitiformis strain D-402 grown under organotrophic conditions and from B. alba are homodimers with the subunit molecular weight of 40 kD. Tetrameric MDH is formed in B. leptomitiformis strain D-402 grown under lithotrophic conditions. The dimeric and tetrameric forms of MDH from B. leptomitiformis D-402 display some differences in kinetic properties.

Thermostable dimeric malate dehydrogenase (MDH) was isolated from the microorganism of hydrothermal vents Vulcanithermus medioatlanticus. The enzyme was electrophoretically homogeneous and possessed the specific activity of 6.9 U/mg. The large molecular weight of the subunits (55 kD) is likely to provide the rigidity of the enzyme structure (the activation energy of the enzymatic reaction is 32.6 kJ/mol). The thermophilic MDH differs little from the mesophilic enzyme in terms of kinetic and regulatory characteristics.

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