The effect of age on the induction of tyrosine aminotransferase and tryptophan oxygenase genes by physiological stress.

Expression of the genes coding for tyrosine aminotransferase and tryptophan oxygenase in rats is induced in response to cold stress. We have studied the effect of ageing on this induction. The induction of tyrosine aminotransferase activity in young adult rats (10 months) was about twice that observed with old rats (25 months). This difference between the two age groups was also observed when the steady-state level of tyrosine aminotransferase mRNA was measured by hybridization with a specific DNA probe. However, when the transcription rate of the gene was measured by in vitro elongation of nascent RNA in isolated nuclei, no difference was detected. In contrast to the results with tyrosine aminotransferase, induced tryptophan oxygenase enzyme and mRNA levels did not show an age-dependent difference. These results suggest that there is, with ageing, an impairment in post-transcriptional regulation of the synthesis of tyrosine aminotransferase. The regulation of tryptophan oxygenase, on the other hand, is similar in the two age groups.