Radiation target analysis indicates that phenylalanine hydroxylase in rat liver extracts is a functional monomer

The minimal enzymatically functional form of purified rat hepatic phenylala nine hydroxylase (PAH) is a dimer of identical subunits, Radiation target a nalysis of PAH revealed that the minimal enzymatically active form in crude extracts corresponds to the monomer, The 'negative regulation' properties of the tetrahydrobiopterin cofactor in both crude and pure samples implicat es a large multimeric structure, minimally a tetramer of PAH subunits, Prei ncubation of the samples with phenylalanine prior to irradiation abolished this inhibition component without affecting the minimal functional unit tar get sizes of the enzyme in both preparations. The characteristics of rat he patic PAH determined by studies of the purified enzyme in vitro may not com pletely represent the properties of PAH in vivo. (C) 1999 Federation of Eur opean Biochemical Societies.