Correlation of rat hepatic phenylalanine hydroxylase, with tetrahydrobiopterin and GTP concentrations.

Hepatic phenylalanine hydroxylase is reported to be more abundant in experimentally-diabetic rats; whereas livers of animals fed a high protein diet, where gluconeogenesis also prevails, have normal amounts of this enzyme. In this study, in addition to seeking an explanation for this effect of experimental diabetes, we also examined the effects of providing alternative dietary gluconeogenic substrates. In rats fed a diet composed of 40% (w/w) glycerol, the specific activities of hepatic phenylalanine hydroxylase are decreased to about 60% of control values. There is no effect on the apparent state of phosphorylation of the enzyme. However, studies on the incorporation of radiolabelled leucine into liver phenylalanine hydroxylase suggested that there was a decreased rate of synthesis. Similarly, animals fed a diet containing 85% (w/w) fructose also have diminished phenylalanine hydroxylase activities. Under all of the above circumstances and also in streptozotocin-induced diabetic animals, alterations in the concentrations of the hydroxylase cofactor, tetrahydrobiopterin and of GTP closely correlate with the effects on the enzyme activities. They are elevated in livers of diabetic animals and significantly diminished in livers of rats fed diets rich in glycerol or fructose. These observations suggest that in adult rat both liver tetrahydrobiopterin concentrations and the expression of hepatic phenylalanine hydroxylase are regulated by GTP [210].