The noncompetitive S-adenosylhomocysteine (AdoHcy) hydrolase antagonist adenosine dialdehyde (AD) has been shown to suppress the growth of cultured C-1300 murine neuroblastoma (MNB) cells. The enzymatic sites at which AD and other nucleoside analogues exert their cytotoxic effects have been postulated to include protein carboxylmethyltransferase (PCM), AdoHcy hydrolase, and ribonucleotide reductase.

AD (10-5m) increased PCM activity 350% in suspensions prepared from disrupted cells after 72 h of drug exposure; in contrast, 3-deaza-adenosine (10-4m) increased PCM activity 57%, whereas AdoHcy and sinefungin had no effect. When intact MNB cells were incubated with AD for varying time periods up to 72 h and then pulse labeled with the S-adenosylmethionine precursor l-[3H]-methionine, AD (10-8 to 5 × 10-6m) produced a concentration-dependent inhibition of protein carboxylmethylation which persisted for up to 6 h. Following extended periods of AD treatment (48 to 72 h), AD (10-6 to 10-5m) produced a 250% increment in protein carboxylmethylation, similar in magnitude to that observed in disrupted cell preparations. This increase in carboxylmethylation was observed at timepoints when AdoHcy hydrolase activity remained suppressed. The inhibitory effect of AD on AdoHcy hydrolase activity was maximal within 4 h and still apparent after 72 h of incubation. In contrast, AD treatment had no effect on the ribonucleotide reductase activity of MNB cells.

These data suggest that the cytotoxic effect of AD on MNB cells results directly from its inhibition of AdoHcy hydrolase activity and indirectly through its suppression of methyltransferase enzyme systems. The potential linkage between the observed long-term elevations in PCM activity and AD-induced cytotoxicity remains to be defined.

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This investigation was supported by Grants NS-17194, National Institute of Neurological and Communicable Diseases and Stroke, GM33776, National Institute of General Medical Science, and the University of Minnesota Institutional Award from the American Cancer Society.

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