A Thioester Analogue of an Amino Acetylenic Aldehyde Is A Suicide Inhibitor of Aldehyde Dehydrogenase and an Inducer of Apoptosis in Mouse Lymphoid Cells Overexpressing the bcl2 Gene

Abstract

We have previously shown that methional, CH₃SCH₂CH₂CHO, an endogeneous cellular aldehyderived by the oxidative decarboxylation of 4-methylthio-2-oxobutanoic acid (B), an intermediate in the methionine salvage pathway is a potent inducer of apoptosis when added to cultures of mouse lymphoid cells BAF₃ bo (Quash et al., 1995). MTOB a00 μM was also capable of alleviating the methionine dependence shown by transfed cells (Ogier et al., 1993) because of its ready transamination to methionine, with gamine as amine donor (Backlund et al., 1982). Further, the inhibition of this MTOB transaminase with novel transition state inhibitors such as methionine-ethyl ester- pyridoxal induced apoptosis in BAF₃ bo cells but not in BAF₃ bcl₂ which had been trans- fected with the human bcl₂ gene (Roch et al., 1996). When the reasons for this refractory behaviour of BAF₃ bcl₂ were investigated using[14C] MTOB it was found that the production of [14methional from [14C] MTOB was reduced by 50% in BAF₃ bcl₂ and that even the direct addition of 600μM methional could not induce apoptosis in BAF₃ bcl₂ (Roch and al, 19 Methional is itself further metabolised intracellularly [Figure 1] by 3 pathways: reduction by aldehyde reductase (ALR) to methionol, oxidation by aldehyde dehydro- genaseLDH1) to methyl thiopropionic acid and β-hydroxylation to malondialdehyde. We therefore tried to see whether the inhibition of the reductase or the dehydrogenase would uce apoptosis in methional treated BAF₃ bcl₂ cells. Using quercitin as an inhibitor of ALR increase in apotosis was observed in BAF₃ bcl₂, but this result is to be interpreted with caution as quercitin also induces DNA strand breaks directly.With disulfiram as an ibitor of ALDH a small increase was seen but experimentation could be carried out only with low concentrations (< 100μM) due to the intrinsic toxicity of this compound. It was therefore clear that more specific inhibitors of ALDH were required to try to determine if ALDH played a role in the resistance to the apoptosis-inducing activity of methio.