Cytosolic cofactors and dihydrolipoamide stimulate hepatic microsomal 5'-deiodination.

The effect of rat hepatic cytosolic constituents on microsomal 5'-deiodinase (5'-DI) activity in the presence of the dihydrolipoamide-generating system or dihydrolipoamide itself has been studied. Reconstitution experiments have demonstrated for the first time the essential role of cytosol as well as lipoamide and NADH in the activation of 5'-iodothyronine DI to a level equivalent to that attained in the presence of 5 mM dithiothreitol. Omission of NADH, lipoamide, or cytosol reduced the DI activity by 86%. By gel filtration chromatography of the cytosol, a fraction of intermediate mol wt (approximately 13,000) was shown to be highly stimulatory of microsomal DI in washed microsomes, provided either a dihydrolipoamide-generating system (lipoamide dehydrogenase, as well as NADH and lipoamide) or dihydrolipoamide alone was added. These observations suggest a possible role for a heretofore unreported reducing system influencing hepatic 5'-deiodination, wherein cytosolic cofactors appear to function as efficient intermediaries which enhance the action of a dihydrolipoamide in the activation of isolated hepatic microsomal 5'-DI.