Modulation of polyamine-biosynthetic activity by S-adenosylmethionine depletion

The methionine-analogue inhibitor of S-adenosylmethionine (AdoMet) synthetase, L-2-amino-4-methoxy-cis-but-3-enoic acid (L-cisAMB), was used to study the early effects of AdoMet depletion on polyamine biosynthesis. In the presence of decreased methionine (30 microM) in the medium, treatment of cultured L1210 cells with 1 mM-L-cisAMB resulted in a near-total (95%) depletion of cellular AdoMet pools by 4 h. This was accompanied by a 3-fold increase in ornithine decarboxylase (ODC) activity, a 2.5-fold increase in AdoMet decarboxylase (AdoMetDC) activity and a 20% decrease in spermidine and spermine pools. The increase in enzyme activities seemed to be partially due to prolongation of enzyme activity half-life, since that of ODC was extended from 30 to 50 min and that of AdoMetDC from 65 to 310 min. By temporal sequence characterization (0-4 h), the onset of elevations of enzyme activity (0.5-1 h) seemed to be causally related to an earlier (0-0.5 h) decline in AdoMet pools, as opposed to the 20% decrease in spermidine and spermine pools, which occurred much later (2-4 h); the latter are known to regulate decarboxylase activities negatively. Drug-induced elevations in ODC and, to a lesser extent, AdoMetDC activities were reversed by later treatment with exogenous AdoMet. However, because the latter also increased spermine pools (which could not be prevented with various enzyme inhibitors), the reversal of elevations in enzyme activities could not be directly linked to AdoMet. Although not definitive, the data raise the interesting possibility that, in addition to being negatively regulated by polyamines, ODC and AdoMetDC activities may also be subject to negative control by cellular AdoMet (or an AdoMet metabolite). The net effect of either or both of these influences would be to conserve polyamine-biosynthetic activity in the face of declining AdoMet supplies.