Mitochondrial NADH-cytochrome b(5) reductase plays a crucial role in the reduction of D-erythroascorbyl free radical in Saccharomyces cerevisiae.

The relevance of NADH-cytochrome b(5) reductase to the NADH-dependent reduction of D-erythroascorbyl free radical was investigated in Saccharomyces cerevisiae. MCR1, which is known to encode NADH-cytochrome b(5) reductase in S. cerevisiae, was disrupted by the insertion of URA3 gene into the gene of MCR1. In the mcr1 disruptant cells, the activity of NADH-D-erythroascorbyl free radical reductase almost disappeared and the intracellular level of D-erythroascorbic acid was about 11% of that of the congenic wild-type strain. In the transformant cells carrying MCR1 in multicopy plasmid, the intracellular level of D-erythroascorbic acid and the activity of NADH-D-erythroascorbyl free radical reductase increased up to 1.7-fold and 2.1-fold, respectively. Therefore, it indicated that the MCR1 product, mitochondrial NADH-cytochrome b(5) reductase, plays a key role in the NADH-dependent reduction of D-erythroascorbyl free radical in S. cerevisiae. On the other hand, the mcr1 disruptant cells were hypersensitive to hydrogen peroxide and menadione, and overexpression of MCR1 made the cells more resistant against oxidative stress. These results suggested that the mitochondrial NADH-cytochrome b(5) reductase functions as NADH-D-erythroascorbyl free radical reductase and plays an important role in the response to oxidative damage in S. cerevisiae.

Copper- and zinc-containing superoxide dismutase and its gene from Candida albicans.

Cytosolic copper- and zinc-containing superoxide dismutase was purified 136-fold with an overall yield of 2.5% to apparent electrophoretic homogeneity from the dimorphic pathogenic fungus, Candida albicans. The molecular mass of the native enzyme was 39.4 kDa and the enzyme was composed of two identical subunits with a molecular mass of 19.6 kDa. The enzyme was stable in the range of pH 4.0-9.0 and up to 55 degrees C. The ultraviolet-visible absorption spectrum of the enzyme showed the absorption band of copper- and zinc-containing superoxide dismutase at 660 nm. The atomic absorption analysis revealed that the enzyme contained 0.87 g-atom of copper and 0.79 g-atom of zinc per mole of subunit. The N-terminal amino acid sequence alignments up to the 40th residue showed that copper- and zinc-containing superoxide dismutase from C. albicans has high similarity to other eukaryotic copper- and zinc-containing superoxide dismutases. The sod1 encoding copper- and zinc-containing superoxide dismutase has been cloned using a polymerase chain reaction fragment as a probe. Sequence analysis of the sod1 predicted a copper- and zinc-containing superoxide dismutase that contains 154 amino acids with a molecular mass of 16143 Da and displayed 79%, 69%, and 57% sequence identity to the homologues of Neurospora crassa, Saccharomyces cerevisiae, and bovine, respectively. The cloned sod1 contained an intron of 245 nucleotides, which was verified by reverse transcription-polymerase chain reaction.

Manganese-containing superoxide dismutase and its gene from Candida albicans.

Mitochondrial manganese-containing superoxide dismutase was purified around 112-fold with an overall yield of 1.1% to apparent electrophoretic homogeneity from the dimorphic pathogenic fungus, Candida albicans. The molecular mass of the native enzyme was 106 kDa and the enzyme was composed of four identical subunits with a molecular mass of 26 kDa. The enzyme was not sensitive to either cyanide or hydrogen peroxide. The N-terminal amino acid sequence alignments (up to the 18th residue) showed that the enzyme has high similarity to the other eukaryotic manganese-containing superoxide dismutases. The gene sod2 encoding manganese-containing superoxide dismutase has been cloned using a product obtained from polymerase chain reaction. Sequence analysis of the sod2 predicted a manganese-containing superoxide dismutase that contains 234 amino acid residues with a molecular mass of 26173 Da, and displayed 57% sequence identity to the homologue of Saccharomyces cerevisiae. The deduced N-terminal 34 amino acid residues may serve as a signal peptide for mitochondrial translocation. Several regulatory elements such as stress responsive element and haem activator protein 2/3/4/5 complex binding sites were identified in the promoter region of sod2. Northern analysis with a probe derived from the cloned sod2 revealed a 0.94-kb band, which corresponds approximately to the expected size of mRNA deduced from sod2.