THE DAUNORUBICIN-BINDING PROTEIN OF M(R)-54,000 IS AN ALDEHYDE DEHYDROGENASE AND IS DOWN-REGULATED IN MOUSE-LIVER TUMORS AND IN TUMOR-CELL LINES

A rabbit antiserum developed against purified rat liver daunorubicin-b inding protein of M(r) 54,000 (DNR-BP54) cross-reacted with a mouse pr otein of the same molecular weight. This protein was expressed in the liver and several other organs of mice. A series of tumors and cell li nes tested for the presence of the protein were negative. By immunocyt ochemistry, we found that DNR-BP54 was abundantly expressed in the cyt oplasm of normal hepatocytes but was expressed at much lower levels in urethane-induced mouse liver tumors. By immunoscreening of a mouse li ver cDNA library, we cloned the cDNA coding for DNR-BP54 and we found that this protein is aldehyde dehydrogenase-2 (EC 1.2.1.3). This resul t was confirmed by the dehydrogenase activity found in pure preparatio ns of DNR-BP54 from normal rat and mouse livers, assayed with acetalde hyde as substrate and NAD as cofactor. The enzyme activity was inhibit ed by daunorubicin. The inhibition was found to be competitive with re spect to NAD.