CHARACTERIZATION OF THE Ca2+‐INDUCED PROTEOLYTIC ACTIVATION OF TRYPTOPHAN HYDROXYLASE FROM THE RAT BRAIN STEM

The incubation of the 35,000 g supernatant of a rat brain stem homogenate in the presence of 7.5 mM‐CaC12 for 10 min at 25°C resulted in a more than 2‐fold increase in its tryptophan hydroxylase activity. This activation was irreversible and involved a reduction in the molecular weight of the enzyme, from 220,000 to 160,000. The partially proteolysed tryptophan hydroxylase, in contrast to the native enzyme, could not be activated by trypsin, sodium dodecyl sulphate, phosphatidylserine or phosphorylating conditions; dithiothreitol and Fe2+ were the only compounds whose stimulating effect on the enzymatic activity was not prevented by the Ca2+ ‐induced proteolysis of tryptophan hydroxylase. These findings suggest that the mol. wt. 60,000 fragment removed by the Ca2+ dependent neutral proteinase plays a critical role in the regulatory properties of tryptophan hydroxylase.