The activity and purification of membrane-associated thioredoxin reductase from human metastatic melanotic melanoma

Electron spin resonance spectroscopy has been used to measure the reduction of nitroxide radicals on a spin-labeled quaternary ammonium substrate by plasma membrane-associated thioredoxin reductase (EC 1.6.4.5) at the surface of cutaneous and subcutaneous melanoma metastases from one patient (B.M.). Enzyme activity in these metastases was shown to be hyperactive compared to normal skin and was subject to inhibition by calcium. From the remainder of the tissue (50.6 g), plasma membrane-associated thioredoxin reductase has been isolated and its molecular properties were compared with the same enzyme purified from the cytosol of rat liver and Escherichia coli. The enzyme from melanoma possessed an identical molecular weight to that from rat liver as determined by SDS-polyacrylamide gel electrophoresis (Mr 58,000). Upon fluorescence spectroscopic examination, the enzyme from melanoma was shown to contain flavin adenine dinucleotide as previously shown in the enzymes from E. coli and rat liver. The increased activities in plasma membrane-associated thioredoxin reductase in metastases of malignant melanotic melanoma are discussed in terms of the cellular functions of this important enzyme.