Characterization of two thioredoxins in pig heart including a new mitochondrial protein.

Heart tissue contains two different thioredoxins. One is a specific mitochondrial protein and is best prepared from pre-isolated, intact heart mitochondria (mt-thioredoxin) whereas mitochondria-depleted tissue homogenates contain the major cellular thioredoxin of cytoplasmic origin (c-thioredoxin). Both heat-stable proteins are clearly differentiated chromatographically. They exhibit slightly different molecular weights (12,300 vs. 12,000) and isoelectric points (4.7 vs. 4.8) but differ remarkably in their cysteine content: mt-Thioredoxin has two cysteine residues like the bacterial proteins, and c-thioredoxin possesses six cysteines. Heart extracts were also shown to contain a NADPH-specific thioredoxin reductase of the known mammalian type. A specific function or target enzyme of mt-thioredoxin has not as yet been established.