Abstract
In most flavoproteins FAD and FMN can be readily liberated as free flavin nucleotides on denaturation of the protein. During the past decade it has been reported that a variety of plant and animal tissues also contain flavoproteins whose prosthetic groups are not extracted by acidification or boiling but require proteolytic digestion for extraction. In addition to succinate dehydrogenase, other mitochondrial enzymes such as monoamine oxidase and sarcosine dehydrogenase and Chromatium cytochrome c-552 have been shown to contain this new form of flavin, which subsequently became known as "covalently bound flavin". Recently it has been demonstrated that the covalently-bound FAD in these enzymes is attached to the peptide backbone via the 8α-position of the flavin ring system, and that the immediate substituent to 8α-position is histidine in succinate dehydrogenase in C-N bond from an imidazole N(3) to the 8α-CH_2 and cysteine in monoamine oxidase in thioether linkage. Chemical and physical studies on covalently bound flavin are briefly reviewed.
