Biosynthesis of riboflavin in vitro. Isotopic incorporation studies in Pichia guilliermondii extracts.

In the riboflavin-deficient mutant rib3 and in wild-type cells of P. guilliermondii, GTP is transformed into 2,5-diamino-6-hydroxy-4-ribitylaminopyrimidine and riboflavin, respectively. We were able to demonstrate the partial in vitro synthesis of these compounds, including a reductive conversion step of the product of GTP cyclohydrolase II action upon labelled [14C]GTP. In order to analyse the pyrimidine derivatives formed, 6,7-dimethyl-8-ribitylpterin and riboflavin were synthesized by addition of diacetyl. The data obtained indicate that the radiocarbon from the ribose moiety of GTP was transformed into the ribityl sidechain of 2,5-diamino-6-hydroxy-4-ribitylaminopyrimidine as well as riboflavin without any dilution. Therefore, the ribityl sidechain of the riboflavin formed originates from the ribose moiety of GTP.