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

The minimal enzymatically functional form of purified rat hepatic phenylalanine hydroxylase (PAH) is a dimer of identical subunits. Radiation target analysis 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 implicates a large multimeric structure, minimally a tetramer of PAH subunits. Preincubation of the samples with phenylalanine prior to irradiation abolished this inhibition component without affecting the minimal functional unit target sizes of the enzyme in both preparations. The characteristics of rat hepatic PAH determined by studies of the purified enzyme in vitro may not completely represent the properties of PAH in vivo.