Conformation-dependent inactivation of tryptophan decarboxylase from <i>Catharanthus roseus</i>

Abstract

Catharanthus roseus tryptophan decarboxylase (TDC) was irreversibly inactivated in crude extracts and in purified form by incubation of the enzyme at 37 °C. The half-life of inactivation of TDC was approximately 0.75 h, whereas addition of Mn 2+ or Mg 2+ cations increased this half-life to 9.5 and to over 30 h, respectively. The evidence presented suggests that TDC could exist in vivo in equilibrium between a dimeric active form, which is stabilized by Mg 2+ cations, and a monomeric form, which is susceptible to irreversible inactivation. When the equilibrium is shifted to monomer production and inactivation, TDC was quickly degraded by a proteolytic process, which could involve ATP. The results suggest that TDC activity may be regulated by posttranslational controls that shift the equilibrium between conformational states, some of which may be susceptible to irreversible inactivation followed by proteolysis.Key words: tryptophan decarboxylase, Mg 2+ -dependent conformational changes, posttranslational modifications.