Riboflavin 5'-hydroxymethyl oxidation. Molecular cloning, expression, and glycoprotein nature of the...

Vitamin B2-aldehyde-forming enzyme catalyzes oxidation of the 5'-hydroxymethyl of riboflavin to the formyl group. We have purified the enzyme from the culture media of Schizophyllum commune (ATCC 38719) by modifying the procedure of Tachibana and Oka (Tachibana, S., and Oka, M. (1981) J. Biol. Chem. 256, 6682-6685) for cell-free extract. By SDS-polyacrylamine gel electrophoresis, the enzyme appears to be 78 kDa. The enzyme has a blocked amino terminus, so fragments were obtained by cleaving the purified enzyme with lysyl endopeptidase. Selected peptides were sequenced from their amino termini. We have isolated a full-length cDNA clone using a DNA hybridization probe amplified by polymerase chain reaction with two degenerate oligonucleotide primers, the design of which was based on one of the partial amino acid sequences. From the cDNA clone, it is evident that the enzyme has a Ser/Thr-rich fragment near the COOH-terminal Asp. The enzyme was determined to be a glycoprotein; however, O-deglucosylation only slightly affects activity. Computer searches showed that the B2-aldehyde-forming enzyme has little homology with other proteins, but domain motifs may reflect N-myristoylation of a dehydrogenase with a signature similar to 4Fe-4S ferredoxins. The enzyme cDNA was subcloned into a Pichia expression vector pPIC9K to produce a recombinant protein which exhibited B2-aldehyde-forming enzyme activity.