Effect of dexamethasone on biopterin levels and tyrosine hydroxylase activity in PC-12 cells.

Culture of PC-12 cells in 1 microM dexamethasone for 24, 48 or 72 hr did not alter significantly PC-12 cell total biopterin levels, although tyrosine hydroxylase activity in extracts of cell homogenates was increased 2- to 3-fold. Increasing the concentration of dexamethasone to 10 microM did not change biopterin levels or result in further increases in tyrosine hydroxylase activity measured after 72 hr. Culture of cells in dexamethasone markedly decreased the ratio of biopterin concentration to tyrosine hydroxylase activity. The molar ratio of tyrosine hydroxylase subunits to biopterin in control cells can be estimated to be approximately one. Therefore, following induction of the enzyme by dexamethasone, there appears to be an excess of enzyme molecules relative to cofactor molecules in these cells.