Properties of Phenylalanine Hydroxylase of Cultured Hepatoma

The kinetic and immunologic properties of phenylalanine hydroxylase of adult rat liver were compared to the properties of the similar enzyme present in cultured H4-II-E-C3 hepatoma cells. The enzymes from the two sources could not be distinguished by the K, values for either phenylalanine or 6,7-dimethyltetrahydropterin. Analysis by double immunodiffusion showed that phenylalanine hydroxylase from the two sources had identical immunologic determinants, but immunotitrations revealed a small but significant difference between the enzyme of the normal adult rat liver and the enzyme of cultured hepatoma cells. The results of double immunodiffusion and immunotitration experiments indicated also that the increased levels of phenylalanine hydroxylase seen in the hepatoma cells grown in the presence of hydrocortisone resulted from the accumulation of enzyme protein, but it could not be decided whether this accumulation resulted from an increased rate of synthesis or decreased rate of degradation.