Iron-dependent regulation of rat liver phenylalanine hydroxylase activity in vivo, in vitro, and in perfused liver.

The evidence presented shows that in vivo, in adult well fed rats, 20-4576 of liver phenylalanine hydroxylase is in an inactive state. This enzyme can be activated in a liver extract by addition of iron (II) and dithiothreitol and in situ by perfusion of rat liver with a defined medium. Activation during perfusion is not dependent on protein synthesis or affected by amino acid levels; activation is blocked by perfusion with serum and, most importantly, simply by addition of transferrin to the defined perfusion medium. Enzymes activated by perfusion and in vitro by iron are indistinguishable. Iron activation increases the specific activity of phenylalanine hydroxylase in a stable manner, and the enzyme can be purified completely with no loss of activation. Pure iron-activated enzyme has an increased turnover number (about 720 mol of tyrosine formed/ min/mol of enzyme subunit in our assay) compared to unactivated enzyme, and it also has been shown (Gottschall, D., Dietrich, R. F., Benkovic, S. J., and Shiman, R. (1982) J. Biol. Chem. 257,845-849) to have a proportionately increased iron content. In vivo there is a change with development of the rat of the state of iron activation of phenylalanine hydroxylase, and enzyme in newborn animals cannot be stimulated with iron. We have also found, using light-dark and timed feeding trained rats, a diurnal variation in the iron activatability of the enzyme. The overall implication is that phenylalanine hydroxylase activity can be controlled by iron and that in vivo this represents a functionally reversible means of regulating this enzyme’s activity.