Choline oxidase from Alcaligenes sp. catalyzed the oxidation of choline and betaine aldehyde to betaine with concomitant consumption of oxygen and production of hydrogen peroxide. The values of K_m for choline and betaine aldehyde were 0.87 and 6.2mM, respectively. The molecular weight of the enzyme was estimated to be 66, 000 by SDS-gel electrophoresis and 72, 000 by gel-filtration using a high performance liquid chromatograph.
The prosthetic group of the enzyme was identified as 8α-[N (3)-histidyl]-FAD from the electrophoretic mobility at pH 6.25 of the hydrolysate of the methylated histidylflavin. The visible absorption spectrum of the enzyme showed peaks at 358 and 453 nm and a shoulder at about 480 nm. The covalently bound FAD was reduced on addition of either choline or betaine aldehyde under anaerobic conditions and was reoxidized by aeration. The enzyme was found to contain 1 mol of FAD per mol enzyme.
Amino acid analysis of a purified flavin peptide gave the following molar ratios of amino acids to flavin: Pro (1), Asp+Asn (3), Ser (1), His (1), and Arg (1). Aspartic acid was the N-terminal amino acid. The partial sequence of amino acids in the flavin peptide was as follows:
Asp-Asn-Pro-Asn (His FAD, Ser, Arg)