THE CYTOCHROME B561 PROTEINS

In 1971 Scandinavian researchers discovered a b-type cytochrome in bovine chromaffin granule membranes that (1) participated in trans-membrane electron transport but was not a member of the mitochondrial electron transport chain, (2) was essential for the synthesis of neurotransmitters, and (3) needed the presence of ascorbate (vitamin C) for its biological activity. The newly identified protein was named cytochrome b561 (Cyt-b561) protein.

It has recently become obvious that proteins similar to the bovine chromaffin granule Cyt-b561 protein are common in most, if not all, eukaryotic organisms, including plants, animals and humans too. Moreover, these organisms, in general, contain more than one protein with physical and chemical properties very similar to those of bovine Cyt-b561 protein. Such proteins accept electron (e^─) from ascorbate (ASC) on one side of a biomembrane, transfer the electron – across their „protein body” – from one side to the other side of the biomembrane, and donate the electron to some electron acceptor molecule (A_ox) on the other side of the biomembrane. On the basis of their physical and chemical properties, such proteins form the Cyt-b561 protein family. The electron acceptor is ascorbate free radical (A_ox≡ASC*) for the chromaffin granule Cyt-b561; however, the electron acceptors of the other Cyt-b561 proteins are not known. At present 5 members of the Cyt-b561 protein family are known and characterized in some detail. One major aim of the present research is to identify the electron acceptors for and the physical-chemical properties of the new members of Cyt-b561 protein family, since this information is essential and needed for understanding the biological function of the newly discovered Cyt-b561 proteins.

It is already evident that members of the Cyt-b561 protein family take part in vitamin C metabolism. Preliminary laboratory experiments suggest that (1) some Cyt-b561 proteins also play a role in Fe-metabolism, and (2) one member of the Cyt-b561 protein family is expressed with other tumour suppressor proteins in cancerous cells and thus participates in their elimination.

Future possibilities

Since the newly discovered members of the Cyt-b561 protein family are integral trans-membrane, electron-transporting proteins definitely participating in vitamin C metabolism and probably participating in Fe-metabolism, detailed knowledge of their structure, function, and regulation would open new perspectives in plant nutrition, health, medical, and pharmaceutical research.