alpha.-Methyl polyamines: metabolically stable spermidine and spermine mimics capable of supporting growth in cells depleted of polyamines

In order to assess the tolerance of the target enzyme spermine synthase for alpha-substituents on the aminopropyl moiety of the substrate spermidine, 1-methylspermidine (MeSpd, 2) was synthesized. It was determined that MeSpd is a poor substrate for spermine synthase and is not a substrate for spermidine N1-acetyltransferase, suggesting that alpha-methylated polyamines might be metabolically stable and therefore useful tools for studying polyamine effects in intact cells. On the basis of initial cellular results with 2, 1-methylspermine (MeSpm, 3) and 1,12-dimethylspermine (Me2Spm, 4) were also synthesized. When added to cells (L1210, SV-3T3, or HT29) depleted of both putrescine and spermidine by prior treatment with alpha-(difluoromethyl)ornithine (DFMO), these alpha-methylated polyamines were able to restore cell growth to that observed in the absence of DFMO. In accord with the enzyme data noted above, metabolic studies indicated a slow conversion of 2 to 3, but no metabolism of 4 in these cells. It was concluded from these results that the alpha-methylated polyamines are able to substitute for the natural polyamines spermidine and spermine in critical biochemical processes which involve polyamines for continued cell growth. In accord with the hypothesis, preliminary data indicate that MeSpd and Me2Spm are as effective as spermidine and spermine, respectively, in promoting the conversion of B-DNA to Z-DNA.