Origin of the ribityl side-chain of riboflavin from the ribose moiety of guanosine triphosphate in Pichia guilliermondii yeast.

In wild-type cells and some riboflavin-deficient mutants of P. guilliermondii GTP is transformed to the ribitylated intermediates 2,5-diamino-6-hydroxy-4-ribitylaminopyrimidine and 5-amino-2,6-dihydroxy-4-ribitylaminopyrimidine of the riboflavin biosynthetic path. We were able to show that these compounds were formed in vitro as well as in permeabilized cells by reactions including a reductive conversion of the product of GTP cyclohydrolase II action upon GTP. In order to analyse the pyrimidine derivates, 6,7-dimethyl-8-ribitylpterin and 6,7-dimethyl-8-ribityllumazine were synthesized by reaction of pyrimidines with diacetyl. The formation of ribitylated pyrimidines was shown to be strictly dependent on the presence of NADPH2. The data obtained indicate that the reductive step is catalyzed by a 2,5-diamino-6-hydroxy-4-ribosylaminopyrimidine-reductase. 6,7-Dimethyl-8-ribitylpterin and 6,7-dimethyl-8-ribityllumazine isolated from the incubation mixtures have been identified by chromatography and by their ultraviolet and fluorescence spectra.