Phospholipid-induced activation of tryptophan hydroxylase from the rat brainstem

Abstract

Phospholipids (phosphatidylserine, phosphatidylethanolamine and lysophosphatidylcholine, but not phosphatidylinositol) and gangliosides stimulated the activity of tryptophan hydroxylase extracted from the brainstem of adult rats. This effect was pH dependent; it was much more pronounced at alkaline pH (8.3) than at pH 7.6 (pH for tryptophan hydroxylase assay under standard conditions); this resulted in a shift of the optimal pH for tryptophan hydroxylase activity from 7.6 to 7.9 when phosphatidylserine was included in the assay mixture. Another change concerned the apparent affinity of the enzyme for its cofactor, 6-MPH 4, which doubled in the presence of phosphatidylserine. These alterations did not completely resemble those induced by sodium dodecylsulfate (SDS), suggesting that the detergent properties of phosphatidylserine were not solely responsible for tryptophan hydroxylase activation. This conclusion was strengthened by the observation that phosphatidylserine and SDS exerted opposite effects on tryptophan hydroxylase from young (16 day-old) rats: at pH 7.6, SDS stimulated, whereas the phospholipid largely inhibited the enzymic activity. Pretreatment of the 35,000 g supernatant of an homogenate of brainstem from adult rats with phospholipase A or C resulted in the activation of tryptophan hydroxylase. This suggests that changes in the composition of endogenous phospholipids (notably the formation of lysophosphatides and the removal of possible inhibitory phospholipids) in the vicinity of tryptophan hydroxylase may alter its activity. These results are discussed in relation to the possible role of phospholipids in the control of tryptophan hydroxylase activity under physiological conditions.