Abstract
The presence of an enzyme which cleaves quinolinothiamin in a manner similar to the thiaminase II has been demonstrated in extracts of Euglena gracilis z, an unicellular eukaryote. The enzyme is considered to be localized in the mitochondrial membrane of this species. The pH optimum of the enzyme was shown to be around 9.5 and the optimum temperature was near 38℃. The Km value as calculated by the method of Lineweaver and Burk was found to be 1.7μM for quinolinothiamin. The splitting-activity towards quinolinothiamin was inhibited by 2-methyl-4-amino-5-hydroxymethyl pyrimidine (OMP), the pyrimidine moiety of thiamin. The enzyme activity in cells grown in a thiamin-limiting medium was higher than that in cells grown in a thiamin-rich medium. This might account for the fact that the requirement of this organism for OMP can be replaced by either pyrithiamine or quinolinothiamin. The enzyme appears to be involved in the production of OMP through the hydrolytic cleavage of these compounds.
