AbstractTo investigate whether the metabolism of the polyamines putrescine, spermidine and spermine is related to cellular growth rate, we have measured the activities of L‐ornithine decarboxylase and S‐adenosyl‐L‐methionine decarboxylase as well as the levels of the polyamines in rat brain tumor cells at various stages of a 7‐day in vitro growth period and correlated them with the continuous changes in specific growth rate (dNt/dt/Nt). L‐Ornithine decarboxylase and S‐adenosyl‐L‐methionine decarboxylase both exhibited their maximal activities at the time of most rapid growth. A high positive correlation between the activities of these enzymes and the specific growth rate of the tumor cells during the entire growth period was demonstrated statistically. The pattern of fluctuation of the spermidine content during the culture cycle was similar to those of the enzyme activities and likewise showed a high positive correlation with the specific growth rate of the tumor cells during the entire growth period. The putrescine content exhibited a low positive correlation, whereas the spermine content exhibited a somewhat higher, but negative correlation with the specific growth rate.The high correlation between the specific growth rate of the tumor cells and the synthesis of the polyamines indicates that these events are primarily associated with processes involved in cell replication. Putrescine and spermidine are thought to participate in the regulation of cellular growth rate; a high content may augment, and a low content may restrain, cel
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