Quantitation of immunoadsorbed flavoprotein oxidases by luminol-mediated chemiluminescence

Studies in vitro on the flavinylation of 6-hydroxy-D-nicotine oxidase

The gene of 6-hydroxy-D-nicotine oxidase (6-HDNO), a flavoenzyme from Arthrobacter oxidans with covalently bound FAD, was expressed with the aid of an expression vector in a cell-free coupled transcription-translation system derived from Escherichia coli MZ9. Ultraviolet irradiation of the E. coli extract did not affect synthesis of the 6-HDNO polypeptide nor total protein synthesis but enzymatic 6-HDNO activity could not be detected. Addition of FAD to the irradiated cell extract restored the capability of the transcription-translation assays to synthesize enzymatically active 6-HDNO. However, enzymatic activity could not be restored on addition of FAD plus cell-free extract to the ultraviolet-inactivated assays after completion of apo-6-HDNO synthesis (60 min) nor to immunoprecipitates thereof. Under similar conditions, addition of [14C]FAD did not increase the protein-bound radioactivity. These results indicate that under conditions of limited FAD supply in the in vitro system a flavinless apo-6-HDNO-polypeptide was synthesized. It was, however, not possible to bind the cofactor to the completed polypeptide chain. These findings argue for a cotranslational cofactor binding.

In vivo and in vitro expression of the 6-hydroxy-D-nicotine oxidase gene of Arthrobacter oxidans, cloned into Escherichia coli, as an enzymatically active, covalently flavinylated polypeptide

The 6-hydroxy-D-nicotine oxidase gene of Arthrobacter oxidans was cloned into E.coli with the aid of the expression vector pKK223-3. This enzyme, as well as the E.coli enzymes succinate dehydrogenase and fumarate reductase, bears the cofactor FAD covalently attached to the polypeptide through a His-N3-8 alpha-linkage. The amino acid sequence surrounding the histidine residue involved in FAD binding in 6-hydroxy-D-nicotine oxidase and the two E.coli enzymes, however, show no homology. Nevertheless, 6-hydroxy-D-nicotine oxidase is expressed in E.coli in vivo and in an E.coli-derived coupled transcription-translation system as a covalently flavinylated, enzymatically active polypeptide.