Serotonin biosynthesis and its regulation in seeds of Juglans regia L

To prevent the concentration of ammonia from rising to toxic levels as moisture is lost during the ripening period, the seeds of Juglans regia reduce their ammonia content by synthesizing organic compounds such as tryptophan (Trp) and serotonin (5-HT). The biosynthetic pathway of 5-HT was investigated using Tritium-Trp and C(14 isotope)-TNH2 as tracers, and reversed-phase HPLC for separation of the indoles involved in 5-HT and IAA metabolism. The first step is the decarboxylation of Trp to TNH2. This step is rate limiting and is catalyzed by the adaptive tryptophan decarboxylase enzyme (E.C. 4.1.1.27). This enzyme is the key control element of 5-HT biosynthesis. The decarboxylase activity is not induced by a high level of ammonia and Trp, but by the cessation of metabolite transfer following abscission of the seeds. Subsequently TNH2 is transformed to 5-HT by a constitutive enzyme tryptamine-5-hydroxylase. The very low rate of 5-HO-Trp biosynthesis demonstrates a high substrate specifi ty of this hydroxylase in vivo. We also discuss IAA biosynthesis in walnuts from Trp via 3-indolepyruvic acid, involving tryptophan transaminase as a constitutive enzyme in these seeds, but not via TNH2. (IFU)