THE ROLE OF TIN IN THE DIRECT LABELING OF PROTEINS WITH RE-188

In the process of direct labelling of proteins with Re-188, the influe nce of Sn(II) in the concentration range of 5 x 10(-4)-1 mg/mL of prot ein was studied using Sn-117m radiolabel in the presence of two transc helation buffers-sodium gluconate and sodium citrate. It was shown tha t Sn(II) readily binds to the thiol groups on the protein, and the fra ction of Sn bound to the protein was 5 to 10 times higher in citrate t han in gluconate for all Sn(II) concentrations studied. At saturation point of similar to 1 mu g (10(-8) M) Sn/mg protein in gluconate, 16% of the protein thiol groups were bound to Sn, and at similar to 2.4 mu g (2 x 10(-8) M) in citrate, 32% of thiols were bound to Sn. A mechan ism was proposed for the involvement of Sn(II) in labelling of pre-red uced proteins with Re-188 via formation of protein-tin-Re-188(V) react ion intermediate. It was further shown that the amount of Sn(II) in re action mixture must exceed a certain level in order to achieve high la belling yields, and this level of Sn(II) was found to be different for citrate and gluconate buffers.