Abstract
A study of the diurnal variation of the activities of the first three enzymes of the tyrosine catabolic pathway in rat liver was carried out. Tyrosine α-ketoglutarate transaminase (TKT), as had been shown previously, exhibited a large diurnal variation in activity. The second enzyme, p-hydroxyphenylpyruvate hydroxylase also had a diurnal variation which qualitatively paralleled that of TKT, although of a much lower amplitude. The third enzyme, homogentisate oxygenase did not exhibit change in its activity. Further studies were carried out on the induction of these three enzymes by tyrosine. It was found that in the adrenalectomized animal tyrosine injection produced an increase in p-hydroxyphenylpyruvate activity to the same extent as was observed during the diurnal rhythm studies.
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