Microbial metabolism of quinoline and related compounds. XVI. Quinaldine oxidoreductase from Arthrobacter spec. Rü 61a: a molybdenum-containing enzyme catalysing the hydroxylation at C-4 of the heterocycle.

Quinaldine oxidoreductase from Arthrobacter spec. Rü 61a converts quinaldine to 1H-4-oxoquinaldine. The enzyme was purified 70-fold to apparent homogeneity in a 5-step procedure with a recovery of 4%. The molecular mass of the native enzyme was calculated to be 340,000 Da by gel filtration. SDS-polyacrylamide gel electrophoresis of the enzyme revealed 3 protein bands corresponding to 82,000 Da, 35,000 Da and 22,000 Da. The enzyme contained 1.6 atoms of molybdenum, 8 atoms of iron, 8 atoms of acid labile sulfur, 2 molecules of FAD and as part of the molybdenum cofactor, molybdopterin cytosine dinucleotide. Due to the composition of the cofactors the quinaldine oxidoreductase belongs to the class of molybdo-iron/sulfur-flavoproteins. Cyanide, arsenite and 4-hydroxymercuribenzoate were effective inhibitors whereas the enzyme was not affected by methanol.