Ornithine decarboxylase modification and polyamine-stimulated enzyme inactivation in HTC cells

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Abstract
Ornithine decarboxylase (ODC) isolated from HTC cells was separated into 2 distinct charged states by salt-gradient elution from DEAE-Sepharose columns. This charge difference between the enzyme forms was maintained in partially purified preparations, but enzyme form II changed to form I in a time-dependent polyamine-stimulated fashion in crude cell homogenates. The enzyme modification that produces this charge diversity between the alternative enzyme states was further investigated for its role in enzyme activity induction, protein stability and rapid turnover. Inhibition of new protein synthesis by cycloheximide resulted in a much more rapid loss of form I enzyme than of form II, suggesting that during normal enzyme turnover the latter enzyme state may be derived from the former. Culture conditions that favor the stabilization of this usually labile enzyme generally induced an increased proportion of the enzyme in the form II charge state. In particular, inhibitors of synthesis of spermidine and spermine induced the stabilization of cellular ODC and promoted a marked accumulation in form II. Conversely, polyamines added to the cells in culture induced a very rapid loss in both forms of the enzyme, an effect that could not be attributed merely to an inhibition of new enzyme synthesis. The polyamines, but not putrescine, may be an essential part of the rapid ODC inactivation process; they may function in part by stimulating the conversion of the more stable enzyme form II into the less stable enzyme state, form I.

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