BINDING OF THYROXINE ANALOGS TO HUMAN LIVER ALDEHYDE DEHYDROGENASES

A fragment of a cytosolic thyroid-hormone-binding protein from Xenopus liver was reported to be 92-100% identical to residues 236-258 in sev eral cytosolic aldehyde dehydrogenases [Yamauchi, K. & Tata, J. R. (19 94) fur: J. Biochem. 225, 1105-1112], which have been proposed to form part of the hinge region necessary to bind the adenosine moiety of NA D. Here we investigated the effects of two thyroxine analogs, 3,3',5-t riiodo-L-thyronine and 3,3',5-triiodothyroacetic acid, on purified hum an liver mitochondrial and cytosolic aldehyde dehydrogenases. The comp ounds were found to be competitive inhibitors against NAD and uncompet itive inhibitors with respect to aldehyde. At pH 7.4, the apparent K-i values were in the micromolar range when the concentration of NAD was varied. The inhibition against recombinantly expressed mutant forms o f aldehyde dehydrogenase, which possessed diminished NAD binding, was determined. Essentially no differences were found between the native e nzyme and the mutants, showing that the analog binding was not affecte d by altering the NAD-binding site. Furthermore, the analogs could dis place NAD but not NADH from the enzyme. These findings indicated that the binding of NAD differed from that of NADH, and that aldehyde dehyd rogenases, like other dehydrogenases, can be inhibited by thyroxine an alogs.