Identification of the Mouse Aldehyde Dehydrogenases Important in Aldophosphamide Detoxification1

Aldophosphamide, the penultimate cytotoxic metabolite of cyclophosphamide, can be detoxified by an oxidation reaction catalyzed by certain aldehyde dehydrogenases. The selective toxicity of cyclophosphamide is due, at least in part, to a greater expression of the relevant aldehyde dehydrogenase activity in normal cells relative to that expressed in certain tumor cells. Not known at the onset of this investigation was which of the several known mouse aldehyde dehydrogenases catalyze this reaction. Twelve enzymes that catalyze the NAD(P)-linked oxidation of aldophosphamide, acetaldehyde, hen/aldehyde, and/or octanal were chromatographically resolved from mouse liver. Four of these appear to be novel; four others were determined to be betaine aldehyde dehydrogenase, succinic semialdehyde dehydrogenase, glutamic 7-semialdehyde dehy drogenase, and xanthine oxidase (dehydrogenase). An additional aldehyde dehydrogenase, namely AllÃ-)--1,was semipurified from stomach. The stomach enzyme and nine of the hepatic enzymes catalyze the oxidation of aldophosphamide. Ä,„ values for these reactions range from 16 MMto 2.5 HIM.The relevant aldehyde dehydrogenase of major importance varies with the tissue. In the liver, the major cytosolic aldehyde dehydrogenase, namely AHO-2, accounts for >60% of total hepatic aldehyde dehydrogenase-catalyzed aldophosphamide (160 MM)detoxification. Succinic sem ialdehyde dehydrogenase (AHI)-12) and three of the novel hepatic alde hyde dehydrogenases, namely AHD-8, AHD-10, and AHD-13, also contribute significantly to total hepatic aldehyde dehydrogenase-catalyzed aldophosphamide detoxification. In the stomach, AH D-4 and A111)8 account for approximately 86% of total aldehyde dehydrogenasecatalyzed aldophosphamide (160 ¿IM) detoxification. AHD-2 was not found in this tissue. Of all the aldehyde dehydrogenases examined, AHD2 and AHD-8 were estimated to be the most efficient catalysts of aldophosphamide oxidation. Thus, these enzymes would seem most likely to be operative when tumor cells acquire aldehyde dehydrogenase-mediated cyclophosphamide resistance.