Abstract
Treatment of male mice with excess androgens increased the activity of renal ornithine decarboxylase 60-fold in the BALB/c strain and 4-fold in the CD-1 strain. Part of the increase in the activity of ornithine decarboxylase was due to a decreased rate of degradation of the enzyme since activity declined more slowly (t1/2 80 min) in androgen-treated BALB/c mice than in controls (t1/2 20 min) when protein synthesis was inhibited by cycloheximide. When ornithine decarboxylase protein was labeled in vivo by injection of [5-14C]difluoromethylornithine, the rate of disappearance of the labeled protein was exactly the same as the rate of loss of ornithine decarboxylase activity when protein synthesis was inhibited by cycloheximide, confirming that ornithine decarboxylase protein does turn over rapidly in vivo. The half-life of another rapidly turning over enzyme important in polyamine metabolism, S-adenosylmethionine decarboxylase, was also increased in the mouse kidney by androgen treatment. These results indicate that steroid hormones can affect the level of certain proteins by changing the rate of degradation and that the labeling of ornithine decarboxylase by reaction with radioactive alpha-difluoromethylornithine in vivo provides a useful method for studying the degradation of this protein.
Highlights
Treatment of male mice with excess androgens in- these antisera was not f i y established [6, 18, 19]
The half-life of another rapidly turningresponsiblefor the degradation of ornithine decarboxylase over enzyme important in polyamine metabolism, S- and in developing in vitro systems carrying out this reaction adenosylmethionine decarboxylase, waaslso increased [6]
In order to testwhether the decreased degradation rate was a general phenomenon, the effect of androgen treatment on the turnover of S-adenosylmethionine decarboxylase was measured. This enzyme turns over rapidly in mammalian tissues [41, 42, 44] and the loss of enzyme activity when protein synthesis was inhibited occurred with a half-life of about 33 min in control mice and 72 min after androgen treatment (Fig. 2)
Summary
STUDIES USING LABELING OF THE ENZYME BY REACTION WITH [’4CJ~-DIFLUOROMETHYLORNITHINE*. One of the most interesting properties of mammalian ornithine decarboxylase is its apparently very rapid rate of turnover as indicated by the precipitous fall in activity onexposure to inhibitors of protein synthesis [7,8,9,10,11,12,13,14,15]. These results have been widely accepted as indicating thatthe enzyme has a high rate of the enzyme protein becomes labeled as activity is lost [32]. Synthesis and degradation and this would be consistent with the rapid fluctuation of enzyme activities in response to stim-
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