Effect of aging on tryptophan hydroxylase in rat brain: implications on serotonin level

Tryptophan hydroxylase (TrpH) catalyzes a rate-limiting step in the biogenesis of serotonin. The main objective of this study is to investigate the effect of aging on the activity of TrpH in serotonergic-enriched brain regions such as midbrain, pons, and medulla. TrpH activity was monitored by incubating various concentrations of tryptophan in a fixed amount of brain homogenate from midbrain, pons, and medulla of 2-month (young), 12-month (mature), and 24-month (old) rats (pH 7.4, 37 degrees C). The product 5-hydroxytryptophan was quantitated using a reversed phase HPLC equipped with an electrochemical detection system. Michaelis-Menton constants, K(m) and V(max), were calculated using the Lineweaver-Burk plot. The affinity (K(m)) of the enzyme significantly declined in midbrain and pons of old rats (141.1 +/- 2. 6, 126.0 +/- 10.8 microM) relative to mature rats (22.4 +/- 7.7, 38. 2 +/- 4.7 microM). However, no change was observed in medulla of old rats. The V(max) of TrpH in pons of all three age group rats was fairly constant. However, the V(max) of midbrain was significantly elevated, whereas that of medulla was reduced in old rats relative to mature rats. Clearance formation, a ratio of V(max) to K(m), of 5-hydroxytryptophan declined significantly in midbrain, pons, and medulla of old rats relative to mature rats. A combined effect of inefficient phosphorylation and oxidative damage of TrpH enzyme may be responsible for lower TrpH activity in aging brain. Such alterations in TrpH activity may reduce the level of serotonin in brain, which may be linked to late-life depression and other brain disorders, such as Alzheimer and Parkinson diseases.