Formal Demonstration of the Phosphorylation of Rat Brain Tryptophan Hydroxylase by Ca2+/Calmodulin‐Dependent Protein Kinase

Abstract: Tryptophan hydroxylase is activated in a crude extract by addition of ATP and Mg2+. This activation is reversible and requires in addition both Ca2+ and calmodulin. Thus, phosphorylation by an endogenous calmodulin‐dependent protein kinase has long been suspected. Now that we have prepared a specific polyclonal antibody to rat brain tryptophan hydroxylase, we have been able to prove that this hypothesis is correct. After incubation of purified tryptophan hydroxylase with Ca2+/calmodulin‐dependent protein kinase together with [γ‐32P]ATP, Mg2+, Ca2+, and calmodulin, followed by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis and blotting of the enzymes onto nitrocellulose sheets, we could label the band of tryptophan hydroxylase by the antiserum and the peroxidase technique and show by autoradiography that 32P was incorporated into this band. By measuring the radioactivity, we calculated that about 1 mol of phosphate was incorporated per 8 mol of subunits of the enzyme (2 mol of native enzyme). Because the concentration of ATP which we employed (50μM) gives about half‐maximal activation in crude extract compared to saturating ATP conditions (about 1 mM), this result indicates that the incorporation of at least 1 mol of phosphate/mol of tetramer of native tryptophan hydroxylase is required for maximal activation.