Abstract:Time courses of the activation‐inactivation sequence in rat midbrain tryptophan hydroxylase after preincubation with calcium, ATP + MgCl2, or sulfhydryl reagents and after freezing and thawing suggest that the activated enzyme is more vulnerable to loss of activity. The sequence induced by calcium was prevented by the protease inhibitor leupeptin, and an accelerated decline in activity after activation by ATP + MgCl2was reduced greatly by increasing levels of tetrahydrobiopterin (BH4) cofactor. The effects of calcium and ATP + MgCl2were additive, which suggests independent mechanisms. The findings suggest that time courses of enzyme activation and inactivation processes may offer a useful way to study the influence of a range of effectors on tryptophan hydroxylase functio