SP-22 is a thioredoxin-dependent peroxide reductase in mitochondria

Area of Science:

• Biochemistry
• Mitochondrial Biology
• Antioxidant Research

Background:

• SP-22 is a known mitochondrial antioxidant protein in bovine adrenal cortex.
• It shares homology with thioredoxin peroxidase and other antioxidant enzymes.
• SP-22 protects enzymes from oxidative damage induced by radical-generating systems.

Purpose of the Study:

• To purify and characterize a second mitochondrial protein that cooperates with SP-22.
• To elucidate the mechanism by which SP-22 exerts its antioxidant function.
• To identify SP-22 as a specific type of antioxidant enzyme.

Main Methods:

• Purification of a novel mitochondrial protein (mt-Trx).
• Amino acid and nucleotide sequencing of mt-Trx and its cDNA.
• Assay of reductase activity using NADPH and 5,5'dithiobis(2-nitrobenzoic acid) (Nbs2).
• Protection assays for oxyhemoglobin and enzyme activity in the presence of peroxides.

Main Results:

• A second mitochondrial protein, designated mt-Trx, was purified and identified as a thioredoxin family member.
• mt-Trx contains a mitochondrial targeting signal.
• SP-22, in conjunction with mt-Trx and a partially purified Nbs2 reductase, protected oxyhemoglobin from damage.
• The SP-22/mt-Trx system catalyzed peroxide-dependent NADPH oxidation, indicating a role in peroxide reduction.

Conclusions:

• SP-22 functions as a thioredoxin-dependent peroxide reductase in bovine adrenal cortex mitochondria.
• The identified pathway involves SP-22, mt-Trx, and a thioredoxin reductase system.
• This study clarifies the antioxidant mechanism of SP-22 within the mitochondrial environment.