Control of ketogenesis from amino acids: III. In vitro and in vivo studies on ketone body formation, lipogenesis and oxidation of tyrosine by rats

Ketone body formation from tyrosine was studied in rat liver in vitro with special references to the activities of tyrosine aminotransferase (EC 2.6.1.5) and p-hydroxyphenylpyruvate hydroxylase (EC 1.14.2.2). Liver was obtained from rats which had been given a high protein diet or cortisol to induce various levels of tyrosine aminotransferase. The enzyme activities of the preparations were plotted against the amounts of ketone body formed from tyrosine. It was found that over a low range of tyrosine aminotransferase activities, activity was proportional to the amount of ketone body formed. However, above this range, ketone body formation ceased to increase and p-hydroxyphenylpyruvate started to accumulate. This inhibition of ketone body formation and accumulation of the p-hydroxyphenylpyruvate could be prevented by addition of ascorbate. These results suggest that the primary factor regulating metabolism of tyrosine in vitro is tyrosine aminotransferase and when the activity of this is high so that it is no longer rate limiting, p-hydroxyphenylpyruvate hydroxylase becomes the rate limiting step because its activity is inhibited by the accumulation of p-hydroxyphenylpyruvate.For in vivo studies rats were given a high protein diet or cortisol to induce various levels of tyrosine aminotransferase and then injected with a tracer dose of U- or 1-¹⁴ C]tyrosine. Then their respiratory ¹⁴CO₂ and the incorporation of ¹⁴C into total lipids of liver were measured. The amounts of radioactivity in CO₂ and lipids were found to be proportional to the tyrosine aminotransferase activity and were not affected by the free tyrosine concentration in the liver. After injection of U-¹⁴C] acetate the radioactivities in CO₂ and lipids were not proportional to the tyrosine aminotransferase activity. These results indicate that the enzyme activity also regulates tyrosine metabolism in vivo. In vivo studies gave no evidence of the participation of p-hydroxyphenylpyruvate hydroxylase in regulation of tyrosine metabolism.