Immunochemical Characterization of Ferredoxin from Halobacterium of the Dead Sea

An immunological approach, based on both antigen binding tests and radioimmunoassay, has been used for the comparative study of ferredoxin from Halobacterium of the Dead Sea and ferredoxins from a variety of sources. The halophilic ferredoxin was not cross‐reactive with the plant, and the mammalian ferredoxins tested. On the other hand, it exhibited up to 80% of antigenic cross‐reactivity with a ferredoxin from another extreme halophilic organism, Halobacterium halobium. It is therefore proposed that these two ferredoxins represent a class of halophilic 2‐Fe ferredoxins. distinct from the plant and algal classes. The halophilic apo‐ferredoxin in its reduced and alkylated form is antigenically indistinguishable from the holo‐protein, whereas the non‐reduced form of the apo‐ferredoxin reacts better than the holo‐protein with antibodies towards the latter. This phenomenon is attributed to the fact that upon removal of the prosthetic group, the apo‐ferredoxin becomes aggregated and thus its antigenic valence is increased. Competition experiments indicated that the apo‐ferredoxin contains all the antigenic determinants present on the holo‐protein and that the prosthetic group apparently does not play any direct role in the antigenic specificity of the ferredoxin molecule.