Properties of a thioredoxin purified from rabbit bone marrow which fails to serve as a hydrogen donor for the homologous ribonucleotide reductase.

A thioredoxin has been highly purified from rabbit bone marrow. This thioredoxin is heat-stable, has a molecular weight of approximately 13,000, and contains 4 half-cystines. It is a substrate for the NADPH-dependent thioredoxin reductase of rabbit bone marrow, catalyzes the reduction of insulin disulfides by dithiothreitol, and is a hydrogen donor for methionine sulfoxide reductase of yeast. Although active as a hydrogen donor for ribonucleotide reductase of Lactobacillus leichmannii, this activity could not be demonstrated when the bone marrow thioredoxin was tested with the ribonucleotide reductases of rabbit bone marrow and Corynebacterium nephridii. A regulatory role for the bone marrow thioredoxin was investigated by determining its ability to activate two of the enzymes of spinach chloroplasts known to require thioredoxin for their activation. The bone marrow thioredoxin effectively activates spinach NADP-malate dehydrogenase but not spinach fructose 1,6-diphosphatase. These observations suggest that instead of serving as a hydrogen donor for ribonucleotide reduction in bone marrow, this thioredoxin may be involved in the regulation of the activity of bone marrow enzyme(s).