Biosynthesis of thiamine in plants. I. Enzymic formation of thiamine from pyrimidine and thiazole moieties.

Abstract

To elucidate the biosynthetic pathway of thiamine in plants, the nature of the activation and condensation reactions of pyrimidine (2-methyl-4-amino-5-hydroxymethylpyrimidine) and thiazole (4-methyl-5, β-hydroxyethylthiazole) moieties were studied with cell-free extracts prepared from acetone powders of Japanese Radish and Rape. Investigations of the effects of preliminary incubation and of the early stage of the reactions using phosphorylated substrates suggested that both pyrimidine and thiazole had to be activated by ATP in the presence of Mg2+ before their condensation to form thiamine-P (thiamine monophosphate). The rate of thiamine-P synthesis in the presence of ATP and Mg2+ increased in the order of pyrimidine-PP>pyrimidine-P>pyrimidine in respect of pyrimidine moieties, and thiazole-P was more active substrate than thiazole. The nature of thiamine synthesized was determined as thiamine-P by the bioautographic method using a thiamine-less mutant of Lactobacillus fermenti. From these results it was concluded that thiamine-P in plants is synthesized through the pathway proposed in the cases of microorganisms.It was found that thiaminephosphate pyrophosphorylase (EC 2.5.1.3) activity was inhibited by ATP, ADP, and EDTA, and that the inhibition was reversed almost completely by the addition of Mg2+ of about the equimolar concentration as each of them.