The role of tin in the direct labelling of proteins with Rhenium-188.

In the process of direct labelling of proteins with 188Re, the influence of Sn(II) in the concentration range of 5 x 10(-4)-l mg/mL of protein was studied using 117mSn radiolabel in the presence of two transchelation buffers-sodium gluconate and sodium citrate. It was shown that Sn(II) readily binds to the thiol groups on the protein, and the fraction of Sn bound to the protein was 5 to 10 times higher in citrate than in gluconate for all Sn(II) concentrations studied. At saturation point of approximately 1 microgram (10(-8) M) Sn/mg protein in gluconate, 16% of the protein thiol groups were bound to Sn, and at approximately 2.4 micrograms (2 x 10(-8) M) in citrate, 32% of thiols were bound to Sn. A mechanism was proposed for the involvement of Sn(II) in labelling of pre-reduced proteins with 188Re via formation of protein-tin-188Re(V) reaction intermediate. It was further shown that the amount of Sn(II) in reaction mixture must exceed a certain level in order to achieve high labelling yields, and this level of Sn(II) was found to be different for citrate and gluconate buffers.