DISTRIBUTION OF ACCUMULATED IRON IN BIFIDOBACTERIUM-THERMOPHILUM

Bifidobacterium thermophilum (ATCC 25866) is capable of absorbing large amounts of Fe2+ from a buffer solution at pH 5 containing glucose if the organism had been grown in an iron-poor medium. An investigation of the distribution of such iron in cell was undertaken, where the cells were disrupted by sonication for 4 min. Iron was thus measured on the cell surface, in its particulate fraction and its soluble fraction. Iron distribution depended on total cellular iron content: at low iron loads, the Fe2+ was present largely on the cell surface and in the soluble fraction. At higher Fe2+ levels, the particulate fraction acquired progressively larger supplies of iron. Iron was bound by both the surface and cytosol sides of the plasma membrane in the intact cell; however, the latter ability was lost following cell disruption. Surface binding of iron both in intact cells at 0-degrees-C and in particulate fraction at or 37-degrees-C was biphasic: there were rapid iron binding and slow iron binding sites. The latter was heat sensitive. Iron from the former could be eluted by 2 mM cold Fe2+. On the basis of Scatchard plots, the particulate fraction could bind a maximum of about 30 nmol of iron/mg of protein with a dissociation constant of about 735 mM at 37-degrees-C at the rapid binding site.