The polyamine-dependent modulation of phenylalanine hydroxylase phosphorylation state and enzymic activity in isolated liver cells

The role of polyamines in the control of phenylalanine hydroxylase phosphorylation state and enzymic activity was investigated. Pre-treatment of liver cells with spermine (1 mM) abolishes the glucagon (1 nM)-stimulated increase in hydroxylase phosphorylation. Concurrently there is a decrease in phenylalanine hydroxylation flux, reflecting decreased enzyme activity; 50% inhibition occurs at approx. 10 microM-spermine. These results are discussed in the context of reports concerning the properties of protein phosphatase 2A.