Earlier studies have shown that the abundance of hepatic phenyl-alanine hydroxylase (PAH) diminishes to 60% of control values in rats fed with a diet composed of 40% (w/w) glycerol [Guerin, Walsh, Donlon and Kaufman (1998) Int. J. Biochem. Cell Biol. 30, 1047–1054]. In this experimental model, there are corresponding decreases in the hepatic concentrations of both the hydroxylase cofactor, tetrahydrobiopterin, and the nucleotide guanosine triphosphate. We now show that the cytoplasmic activities of hepatic pterin-4a-carbinolamine dehydratase (PCD) are also lower in these animals, by approx. 50% compared with control values. Immunoblotting confirmed a diminution of protein abundance in vivo. PCD also functions as a dimerization cofactor (DCoH) for the hepatocyte nuclear factor 1α (HNF1α) and the relative abundance of PCD/DCoH in the nucleus is also decreased. There is a small reduction in the mRNA levels for PAH and for PCD/DCoH in the glycerol-fed animals. In the kidney, there is also a diminution in the abundance of both PAH and PCD proteins. Hepatic GTP cyclohydrolase I activity was not altered and the abundance of hepatic HNF1α remained unchanged. HNF1α is required for the expression of PAH in the liver and our results support a role for PCD/DCoH, through its interaction with HNF1α, in regulating the expression of PAH.

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