Ascorbate (AsA) is contained at high levels in photosynthetic organisms and serves as a scavenger of active oxygen species (AOS). Furthermore, it acts as an enzyme activator by maintaining prosthetic metal ions in the reduced form. Photosynthetic organisms are prone to oxidative stress because of the highly energetic reactions of photosynthesis and an abundant 0_2 supply. The imposition of biotic and abiotic stress conditions can give rise to excess concentrations of AOS, resulting in oxidative damage at the cellular level. Therefore, photosynthetic organisms have developed antioxidant defense systems. Ascorbate peroxidase (APX) isoenzymes are key enzymes for AOS-scavenging systems and are widely distributed in chloroplasts, microbodies, mitochondria, and cytosol of photosynthetic organisms. One of the most characteristic properties of APX isoenzymes is the instability in the absence of AsA. To determine the structure-function relation and to obtain the stereochemical information about the mechanisms of the rapid inactivation of APX isoenzymes, we carried out an X-ray crystallographic analysis for the recombinant stromal soluble APX (sAPX) of tobacco plants.

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Selenium is an essential trace element for mammals, birds, and some bacteria. Its remarkable biological effects in eukaryotes may be related to unique functions of various selenoproteins. A^<75>Se-labeled protein was purified from a human lung adenocarcinoma cell line, NCI-H441 . A homodimer of 57-kDa subunits contained selenium in the form of selenocysteine. The selenoprotein contained FAD as a prosthetic group and catalyzes NADPH-dependent reduction of insulin in the presence of thioredoxin. The subunit composition and catalytic properties were identical to those of mammalian thioredoxin reductase (TrxR). This was the first report on the biochemical relevance between selenium and Trx-mediated metabolic regulation. Interestingly, human lung adenocarcinoma cells produced the selenoenzyme TrxR over hundred times that of normal mammalian cells. A Iabile selenium donor compound monoselenophosphate, required for selenoprotein biosynthesis, is synthesized from selenide and ATP by selenophosphate synthetase (SPS).

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The insulinomimetic activities of Zn(II) complexes with various kinds of organic compounds in rat adipocytes treated with epinephrine in terms of inhibition of free fatty acid release have been studied. For the in vitro mechanism, such Zn(II) complexes have been revealed to act exclusively on the glucose transporter (GLUT 4) of the cells. Also, the improvement of glucose tolerance for KK-A^y mice with type 2 diabetes mellitus was obtained by i.p. administrations of Zn(II) complexes, following the finding of in vitro insulinomimetic activity. Furthermore, we found that the complex formation between Zn^<2+> ion and organic compounds reduced the toxicity of Zn^<2+> ion.

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