Preparation and properties of (monoiodotyrosine)-insulin

Labeling of insulin with non-radioactive 127I and application to incorporation of radioactive 125I for use in receptor-binding experiments by high-performance liquid chromatography

Conditions for the labeling of insulin with radioactive iodine isotopes were investigated by means of incorporation of non-radioactive 127I into the peptide. Either the chloramine-T (CT) or lactoperoxidase-hydrogen peroxide (LPO) technique was applied and reversed-phase high-performance liquid chromatography (RP-HPLC) was used for analysis of the reaction products. The LPO method provided the 127I-labeled peptide within 15-30 min, whereas the CT alternative yielded the labeled substrate even within 15 s. However, the latter reaction can only be controlled in a reproducible manner with difficulty and undesired side-reactions became increasingly prominent when the reaction time of 15 s was exceeded for only a few seconds. In another experiment, the LPO technique was applied for radiolabeling insulin with 125I. The product was first purified by size-exclusion chromatography (SEC) and then subjected to RP-HPLC. SEC yielded two peaks. The smaller one, which eluted at a slightly higher Kd value (accounting for about 14% of total radioactivity) predominantly consisted of material eluting at the column's void volume under the conditions of RP-HPLC, whereas the main SEC fraction (accounting for about 86% of total radioactivity) yielded a single peak, as shown by HPLC. The radioactive material attributable to the main SEC fraction revealed the expected receptor-binding properties, as evidenced by displacement experiments with non-radioactive insulin, as well as the action of tetradecanoyl phorbol acetate on the binding characteristics and thus indicating formation of a labeled hormone retaining biological activity.

Monoiodoinsulin specifically substituted in Tyr A14 or Tyr A19

Monoiodoinsulin was prepared using ion exchange chromatography. The isolated monoiodoinsulin showed on polyacrylamide gel electrophoresis two bands with different intensities related to the initial method of iodination. Each of the two bands were isolated from the gel, and determination of the iodine distribution among the tyrosyl groups showed that one band contained monoiodoinsulin substituted in Try A19 contaminated with monoiodoinsulin substituted in the B-chain. The other band contained essentially A14 monoiodoinsulin. Polyacrylamide gel electrophoresis is a convenient method to prepare homogeneous A14 monoiodoinsulin with biological activity indistinguishable from that of native insulin.

Homogeneous mono-(125)i-insulins. Preparation and characterization of mono-(125)i-(tyr a14)-and mono-(125)i-(tyr a19)-insulin

Mono-(125)I-(Tyr A19)-insulin (Mono*A19) was prepared by iodinating MC porcine insulin with (125)I in acid medium using iodate (as oxidizing agent), followed by anion-exchange chromatography. Mono-(125)I-(Tyr A14)-insulin (Mono*A14) was prepared by iodinating MC porcine insulin with (125)I, using H(2)O(2)/lactoperoxidase at neutral pH, followed by anion-exchange chromatography. The specific radioactivities were in the ranges of 120-200 and 220-300 mCi/mg for Mono*A19 and Mono*A14, respectively. Analyses of the intramolecular distributions of (125)I demonstrated that the preparations were 97-98% radiochemically pure. In both preparations, 98-99% of the radioactivity was capable of binding to insulin antibodies for up to 6 months of storage of the tracers. The IRI concentration decreased with the duration of storage. The greatest observed fall in IRI concentration was 70%. The ime course could be explained by the assumption that the disintegration of a (125)I-nucleus destroys the immunoreactivity of the insulin molecule in which the decay occurs.

B1-3,5-diiodotyrosine insulin: a valid tracer for insulin

Insulin, specifically substituted at the PheB1 position with 3,5-diiodotyrosine, has been tested in several biological and immunological systems. Immunoreactivity was assessed using antisera specific for different parts of the insulin molecule. Biological activity in vitro was estimated on isolated rat fat cells. In vivo bioactivity (hypoglycaemia) and metabolism (metabolic and urinary clearance rates, half-life, apparent distribution space) were measured by infusion of the material into greyhounds. The results indicated that this B1-labelled insulin preparation was biologically fully active and, unlike randomly labelled preparations of iodoinsulin, was metabolised with kinetics indistinguishable from those of the unlabelled hormone. We suggest that this material is a valid tracer for insulin, fulfilling the criteria of high specific activity and biological identity to the native hormone.