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Biomedical subjects

M Tien

Publications and source records attributed to M Tien.

77 records · Page 5Linked to original sources
PubMed1981

Superoxide dependent lipid peroxidation.

Rat liver microsomal NADPH-dependent lipid peroxidation and xanthine oxidase-promoted lipid peroxidation were reviewed and compared to see if a unified mechanism is involved in each system. These systems were also compared to hydroxyl radical-dependent lipid peroxidation in order to determine the physiological significance of the different mechanisms of lipid peroxidation. Fenton's reagent very readily promotes lipid peroxidation, which is inhibited by catalase and hydroxyl radical traps but not by superoxide dismutase. However, the addition of ADP to Fenton's reagent results in a type of lipid peroxidation that is not inhibited by hydroxyl radical traps and the amount of hydroxyl radical spin trap adducts formed is much less. Xanthine oxidase-promoted lipid peroxidation is not inhibited by catalase and is greatly stimulated by ADP. Microsomal NADPH-dependent lipid peroxidation is also dramatically stimulated by ADP in Tris buffer but not in phosphate buffer. Hydroxyl radical traps are without effect in both microsomes and xanthine oxidase-promoted lipid peroxidation. These results suggest several in vitro mechanisms for the initiation of lipid peroxidation but do not support the hydroxyl radical for a role in physiological lipid peroxidation.

Animals↗
PubMedDate not supplied

Redox cycling and lipid peroxidation: the central role of iron chelates.

Toxicities associated with redox cycling, including lipid peroxidation, are often attributed to the hydroxyl radical through a superoxide-driven, iron-catalyzed Haber-Weiss reaction. However, other cellular reducing agents more prevalent than superoxide, i.e., glutathione, ascorbate, cysteine and certain enzymes, can also reduce chelated iron and thereby initiate lipid peroxidation which is not inhibited by superoxide dismutase. The autoxidation of ferrous chelates yields partially reduced oxygen intermediates similar to those produced during the iron-catalyzed Haber-Weiss reaction. The mechanism of their formation, and the nature of the strong oxidant responsible for the initiation of lipid peroxidation are poorly understood, but are apparently influenced by the chelation of the iron. These conclusions stem from experiments which demonstrated variable amounts of inhibition of lipid peroxidation, dependent on the autoxidation of various ferrous chelates, by added superoxide dismutase or catalase. In addition, certain ferrous chelates are unable to initiate peroxidation of lipids in a liposomal configuration, yet capable of initiating peroxidation of lipids which were dispersed with a detergent. The results of these studies point out the need to isolate and identify the predominant physiological iron chelates for assessment of their ability to participate in redox related toxicities within the cell.

Adenosine Diphosphate↗
PubMedDate not supplied

Cloning and sequencing of a cDNA for a ligninase from Phanerochaete chrysosporium.

Lignin is a complex polymer of phenylpropanoid subunits. It is an essential component of woody tissue, to which it imparts structural rigidity. Lignin is remarkably resistant to degradation by most microbes; nevertheless, a few species of white-rot fungi are able to catalyse its oxidation to CO2. Its biodegradation is of great ecological significance because, next to cellulose, lignin is the most abundant renewable polymer on Earth. The first step in lignin degradation is depolymerization, catalysed by the lignin peroxidase isozymes (ligninases). These isozymes are secreted, along with hydrogen peroxide (H2O2) by the fungus Phanerochaete chrysosporium Burds, under conditions of nutrient (nitrogen) limitation. Ligninases are not only important in lignin biodegradation, but are also potentially valuable in chemical waste disposal because of their ability to degrade environmental pollutants. We have undertaken the cloning of the ligninase genes to understand further their regulation and enzymology. We report here the isolation and characterization of a ligninase complementary DNA clone with a full-length insert. The cDNA sequence shows that the sequence of the mature ligninase is preceded by a 28-residue leader, and the mature protein is predicted to have a relative molecular mass of 37,000 (Mr 37K). Consistent with the classification of ligninase as a peroxidase certain residues thought to be essential for peroxidase activity can be identified and near these residues the ligninase shows homology with other known peroxidases. Our cDNA clone has also allowed us to show that expression of ligninase is regulated at the messenger RNA level.

Agaricales↗