The role of ferritin in the in vivo localization of Gallium-67

On the mechanism of gallium accumulation in tumours. Isolation of N-hydroxypeptides

With the use of A SEP-PAK Alumina N cartridge, peptide fractions containing the hydroxamate moiety have been isolated from tumour-tissue homogenate. It is therefore postulated that N-hydroxypeptides might play a role in the accumulation of gallium-67 in tumours. The biodistribution of several 67Ga-labelled hydroxamate peptide fractions is determined. Although their biodistribution differs from that of 67Ga-citrate no enhancement in tumour accumulation is observed indicating that they do not act like tumour-specific siderophores.

The role of ferritin in the intracellular distribution of gallium 67

The binding of gallium 67 or iron 59 to ferritin in vitro was investigated using equilibrium dialysis. Gallium 67 did not bind to apo-ferritin until the protein was transformed into ferritin in the presence of iron citrate. Apotransferrin inhibited the binding of 67Ga to ferritin, especially in the presence of sodium bicarbonate and citrate, thus indicating that 67Ga has not gained access to ferritin from its complex with transferrin. Similar inhibition was observed for ferritin-59Fe. The release of 59Fe from its transferrin complex was enhanced by ATP, citrate, or ascorbic acid, while these reagents did not stimulate the dissociation of 67Ga from its transferrin complex. On the other hand, 67Ga injected intravenously in vivo was not found in the ferritin fractions of rat liver, kidney, and tumor. The difference between experimental results in vivo and in vitro supports the hypothesis that 67Ga in the cytoplasm is not labile enough to be bound to ferritin. We have indicated a significant role of ferritin in distinguishing between 67Ga and 59Fe in the cell, and provided some clues to interpret the chemical forms of 67Ga in the cytoplasm.

Relation between 67Ga uptake and iron metabolism in rat tissues

The relationship between 67Ga uptake and iron metabolism was investigated in rat tissues. 67Ga and 59Fe(II) both accumulated in the mitochondrial-lysosomal fraction after being injected. Moreover, they both showed especially high affinity for heparan sulfate (HS) among various acid mucopolysaccharides (AMPS). When iron (ferrous citrate) was injected IV before, simultaneously with, and after 67Ga citrate IV injection, 67Ga uptake was significantly inhibited in normal rat liver in all cases. Elevated 67Ga uptake in the liver of CCl4-treated rats was also lowered to the control level by iron pretreatment. High zinc intake remarkably elevated the 67Ga uptake in rat liver. The contents of iron in the liver and liver AMPS of 0.75% zinc-fed rats were lowered in comparison with those in controls. Thus, the elevation of 67Ga uptake in the liver of zinc-fed rats might be due to the decrease of iron bound to HS.