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The major solid state principles involved in mineralization in biological systems are discussed. Three major biological control factors of mineralization are described; structural, spatial, and chemical control. Factors determining nucleation on organic surfaces, mineral growth, mineral structure, and morphology are reported. Oriented growth of minerals on organic matrices can occur by three processes; (i) lattice matching (epitaxis) between the organic matrix and depositing crystal faces, (ii) surface structural relationships between the organic matrix and depositing crystal faces, (iii) ordered aggregation of preformed mineral particles. Several a priori reasons indicate that epitaxis is unlikely to be a major process of oriented growth in biomineralization. The presence and importance of biogenic amorphous minerals is also described. [ABSTRACT FROM AUTHOR]

The redox chemistry of chromium(VI) is discussed with respect to the cellular metabolism of the carcinogen chromate in vivo. Possible sites for cellular reduction of chromium(VI) to chromium(III) are considered. The reactions of amino acids, ascorbic acid, carboxylic acids, thiol-containing mole-cules and other small molecules with chromate under physiological conditions are presented. In general only ascorbate and those molecules containing sulfhydryl groups are capable of easily reducing chromate at pH 7.4. Thus, in the cytoplasm, glutathione, cysteine and ascorbate are likely candidates to react with chromate. While most proteins are unreactive toward chromate, certain redox proteins are active in reducing chromate. The heme proteins hemoglobin and cytochrome P-450 possess chromate-reductase activity, whereas cytochrome c and myoglobin are inactive. The NADPH-dependent flavoenzymes glutathione reductase and NADPH-cytochrome P-450 reductase also possess chromate-reductase activity. However, the NAD(P)H enzymes, isocitrate dehydrogenase, glutamate dehyrogenase and malate dehydrogenase do not reduce chromate. Both microsomes and mitochondria possess chromate-reductase activity. The microsomal activity is accounted for by the NADPH-cytochrome P-450 reductase/cytochrome P-450 system. The enzyme(s) responsible for the mitochondrial reduction of chromate have not been identified. Chromium(VI) and its metabolite chromium(III) inhibit the normal activities of enzymes which bind chromium(III) or reduce chromate. The metabolism of chromate involves the generation of reactive intermediates which ultimately bind to cellular constituents and damage their function in the cell. [ABSTRACT FROM AUTHOR]

Reductive elimination of the 2′-hydroxyl group from ribonucleotides to yield 2′-deoxyribonucleotides, the monomeric precursors of DNA, requires an uncommon type of enzyme catalysis in which the transition metals, manganese, iron, or cobalt, and free radical intermediates cooperate. In the group of deoxyadenosylcobalamin (coenzyme B 12)-dependent ribonucleotide reductases the coenzyme supplies a transient radical pair of deoxyadenosyl· and cob(II)alamin whereas in the nonheme-iron group of enzymes a protein subunit carries a stable tyrosyl radical coordinated to a binuclear iron(III) complex; the manganese-dependent enzymes are less precisely known. The radicals are thought to function in hydrogen transfer from cysteine SH to the ribonucleotide substrate, and the metal complexes are apparently needed to generate and stabilize the radicals. In aerobic organisms oxygen also plays a critical role in these processes and hence in DNA synthesis and cell proliferation. In addition to the transition metals, Mg2+ or Ca2+ are required by several ribonucleotide reductases for structural integrity. However the most potent inhibitors of deoxy-ribonucleotide biosynthesis (of potential interest in chemotherapy) are not metal chelators but radical scavengers. Cell cycle arrest and cell death produced by simple chemicals like N-hydroxyurea and hydroxamates can be traced back to their reaction with ribonucleotide reductase. Evidence is accumulating that independent enzymes of deoxyribonucleotide and DNA synthesis are functionally coupled in a novel type of supramolecular structure. [ABSTRACT FROM AUTHOR]

A brief discussion of the vast and increasingly important area of organoselenium chemistry is presented as it relates to the biochemistry of this element. Organo selenoamino acids are the basic building blocks of many selenium-containing macromolecules, which are the primary focus of this article. First to be discussed are synthetic selenium analogues of sulfur-containing natural products such as selenocoenzyme A and selenobiotin. Secondly, are those enzymes which will indiscriminately incorporate selenium and the most important of these are considered. Finally, the most important and interesting class of selenium-containing macromolecules are the naturally occurring selenoproteins, of which there are eight currently known and only three which have been extensively studied. Physical studies which relate to or have the potential to aid in investigations of selenium biochemistry constitute the final topic. [ABSTRACT FROM AUTHOR]