Addition of putrescine of spermidine prevents the increase in ornithine decarboxylase activity in cultures of 3T3 cells brought about by pituitary growth factors and results in a rapid, specific, and reversible reduction of enzyme activity in cultures previously stimulated by the growth factors. These effects are not due to polyamine toxicity and do not require other organic medium components. The amines apparently share a single carrier-mediated transport system in 3T3 cells. Methylglyoxal bis(guanylhydrazone), an inhibitor of spermidine synthesis from putrescine was found to also inhibit uptake of each amine. Studies with this drug indicate that each amine is effective without further metabolism. Since ornithine decarboxylase activity decays more rapidly in the presence of each polyamine after addition of camptothecin, the major locus of amine action appears to be in the cytoplasm. However, direct inhibition of the enzyme in vivo by assimilated amines appears to account for at most a small part of the reduction in activity, a conclusion supported by the inability to recover activity in vitro. Also, neither amine seems to act by accelerating enzyme inactivation. When amines are removed from the medium, the subsequent recovery of enzyme activity is totally prevented by trichodermin, an inhibitor of protein synthesis, but is only slightly reduced by camptothecin. It is suggested that both putrescine and spermidine reduce ornithine decarboxylase activity by selectively inhibiting translation.

There are two forms of ornithine decarboxylase with respect to pyridoxal 5'-phosphate (pyridoxal-P) affinity in exponentially-growing Swiss 3T3 mouse fibroblasts: form I (K, % 10 pM) accounts for 30% of the total activity, and form I1 (K, z 0.4 pM) the remainder.Each form of the enzyme is in rapid equilibrium with ornithine and pyridoxal-P; neither form recognizes the Schiff base between ornithine and pyridoxal-P as a substrate.Total pyridoxal-P concentrations indicate that both forms may normally be at least partially active in vivo.Upon stimulation of 3T3 cells by pituitary growth factors, form I becomes undetectable within 4 h. As total activity increases over 10-fold during this time and continues to increase thereafter, a possible conversion of form I to form I1 could account for this increase only if the K, change reflects other changes in preexisting enzyme.The rates of cofactor dissociation are apparently the same for each form and neither rate changes with the growth state. Since rapid equilibrium kinetics apply, the forms apparently differ in their rate of cofactor association.The half-lives of the two forms in vivo are the same in unstimulated cells when measured concurrently. Also, the half-life of total activity decreases markedly upon stimulation as form I1 becomes dominant. These and other observations are not consistent with pyridoxal-P serving a major protective function for the enzyme in vim.

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