Rat brain stem tryptophan hydroxylase: mechanism of activation by calcium.

Abstract

Abstract— The activity of soluble tryptophan hydroxylase from rat brain stem was increased in presence of mm concentrations of calcium. Similarly to that observed by treating the enzyme with sodium dodecyl sulphate or trypsin, this activation resulted mainly from an increased affinity of tryptophan hydroxylase for both its substrate, tryptophan, and the cofactor 2‐amino‐4‐hydroxy‐6‐methyl‐5,6,7,8‐tetrahydropteridine (6‐MPH4). In addition, the optimal pH for the enzymic activity was shifted from 7.6 to 7.9 following activation by calcium, sodium dodecyl sulphate or trypsin.Under the assay conditions used for measuring tryptophan hydroxylase activity, calcium also stimulated a neutral proteinase. This latter enzyme could be eliminated from the solution of tryptophan hydroxylase by filtration through Sephadex G 200. The resulting partially purified tryptophan hydroxylase could be activated by calcium only when the neutral proteinase was included in the assay mixture. In support of this conclusion, the effect of calcium on tryptophan hydroxylase was very small in the new born rat when the activity of the neutral proteinase was low. In addition, the activating effect of Ca2+ could be antagonized not only by a chelating agent like EGTA but also (partially) by specific inhibitors of proteinases such as benzethonium and PMSF.These results strongly suggest that the activation of tryptophan hydroxylase by calcium is the consequence of a partial proteolysis of the enzyme by the calcium‐dependent neutral proteinase. Therefore, the physiological significance of this irreversible effect is doubtful.