Tyrosine hydroxylase is the rate-limiting enzyme in catecholamine biosynthesis, and its N-terminus plays a critical role in the intracellular stability of the enzyme. In the present study, we investigated the mechanism by which the N-terminus of human tyrosine hydroxylase type I (hTH I) affects the stability. The results obtained by using N-terminus-deleted hTH I mutants identified the sequence up to Ala 23 as mediating the stability. The down-regulation of 14-3-3 eta proteins in PC12D cells exogenously expressing hTHI, enhanced the stability of the wild-type enzyme and that of the mutant lacking the N-terminus up to Ala 23. However, the stability of the mutant was reduced compared to the wild-type enzyme. The stability of the mutant with the N-terminus deleted up to Glu(43) was not affected by the down-regulation of 14-3-3 eta. These results suggest that the 14-3-3 eta protein regulates hTH1 stability by acting on the N-terminus. (C) 2007 Elsevier Inc. All rights reserved.