Effect of the insulin iodination on the reactivity of the inter-chain disulphide bonds towards sodium sulphite

Abstract

Insulin iodination interferes with the ability of the interchain S.S bonds to react with sulphite at pH7. In insulin samples containing more than 5 iodine atoms/monomer unit, only one S.S bond/molecule reacts. The effect must be related to the substitution of the iodine into the tyrosyl groups, which probably causes a conformational rearrangement resulting in a steric hindrance of one of the interchain S.S bonds. The effect is removed by increasing the pH or by adding urea (8m) to the reaction mixture. The unreactivity of the S.S bond and the biological inactivation occur at the same ;critical' iodination level, suggesting that a same primary alteration of the molecule is responsible for both the effects.

Immunological and biological properties of iodoinsulin labeled with one or less atoms of iodine per molecule

“…Consequently, as insulin becomes iodinated, more of the iodinated forms will be present as monomers and dimers and therefore subject to further iodination. Results consistent with the postulate that the A-19 tyrosine is labeled first and then the A-14 is preferentially iodinated have been presented recently by Rosa, Massaglia, Pennisi, Cozzani, and Rossi (34).…”

Iodination of insulin in aqueous and organic solvents

“…(a) Constant-current electrolysis. Constant-current electrolysis (Rosa et al 1967) was used for iodination of insulin in aqueous solution at pH7-5, in alcohol-water mixtures and in 8M-urea. The following procedure was used for iodination of insulin at pH7-5: insulin (30mg.)…”

“…Iodine reacts at the two ortho positions of the phenolate ion; consequently the reactivity of the tyrosyl residues in insulin will reflect differences in their ionization behaviour or in the environment of their aromatic rings or both. Rosa, Massaglia, Pennisi, Cozzani & Rossi (1967) have found that iodination of insulin has a marked effect on the reactivity of the interchain disulphide bonds. Specifically, iodination results in the formation of an iodoinsulin species in which one of the interchain disulphide bonds does not react with sulphite, thiols and GSH-insulin transhydrogenase (Rosa, Rossi & Donato, 1968).…”

Cross-Linking of Insulin Induced by Iodination

“…The presence of degraded insulin (Berson, Ya10w~ Bauman, Rotschild andNewer1y, 1956) and aggregates (Arnou1d, Cantraine, Ooms, De1croix andFranckson, 1967) in iodinated insulin preparations was also noted but their structure not e1ucidated. To this end, insulin was iodinated in quantities sufficient to iso1ate and characterize the by-products of iodination.60-240 mg (10-401.1 M) insulin purified on Sephadex G-50 co1umns to remove proinsulin (Steiner, Hallund, Rubenstein, ChQ and Bayliss, 1968) was reacted with iodine-127 according to the methods of Izzo, Ba1e, Izzo and Roncone (1964) and Rosa, Massaglia, Pennisi, Cozzani and Rossi (1967) to yie1d lightly iodinated preparations. The dia1yz;ed and freeze-dried samp1es were then applied on to Sephadex G-50 co1-umns in 33% acetic acid which prevents the aggregation of insulin (Sluyterman, 1955; Humbe1, 1963).…”

“…60-240 mg (10-401.1 M) insulin purified on Sephadex G-50 co1umns to remove proinsulin (Steiner, Hallund, Rubenstein, ChQ and Bayliss, 1968) was reacted with iodine-127 according to the methods of Izzo, Ba1e, Izzo and Roncone (1964) and Rosa, Massaglia, Pennisi, Cozzani and Rossi (1967) to yie1d lightly iodinated preparations. The dia1yz;ed and freeze-dried samp1es were then applied on to Sephadex G-50 co1-umns in 33% acetic acid which prevents the aggregation of insulin (Sluyterman, 1955;Humbe1, 1963).…”