The preparation and characterization of mono-iodinated photoreactive analogs of insulin

Structure-based stabilization of insulin as a therapeutic protein assembly via enhanced aromatic-aromatic interactions

Key contributions to protein structure and stability are provided by weakly polar interactions, which arise from asymmetric electronic distributions within amino acids and peptide bonds. Of particular interest are aromatic side chains whose directional π-systems commonly stabilize protein interiors and interfaces. Here, we consider aromatic-aromatic interactions within a model protein assembly: the dimer interface of insulin. Semi-classical simulations of aromatic-aromatic interactions at this interface suggested that substitution of residue TyrB26 by Trp would preserve native structure while enhancing dimerization (and hence hexamer stability). The crystal structure of a [TrpB26]insulin analog (determined as a T3Rf3 zinc hexamer at a resolution of 2.25 Å) was observed to be essentially identical to that of WT insulin. Remarkably and yet in general accordance with theoretical expectations, spectroscopic studies demonstrated a 150-fold increase in the in vitro lifetime of the variant hexamer, a critical pharmacokinetic parameter influencing design of long-acting formulations. Functional studies in diabetic rats indeed revealed prolonged action following subcutaneous injection. The potency of the TrpB26-modified analog was equal to or greater than an unmodified control. Thus, exploiting a general quantum-chemical feature of protein structure and stability, our results exemplify a mechanism-based approach to the optimization of a therapeutic protein assembly.

Hormone binding site of the insulin receptor: analysis using photoaffinity-mediated avidin complexing

A trifunctional reagent was designed which allows derivatization of ligands, particularly peptides and proteins, for subsequent photoaffinity labelling of receptors and specific isolation of the covalent complex or its fragments. B29-(2-nitro-4-azidophenyl)-biocytinyl-insulin (NB-insulin) was synthesized, radioiodinated, and the B26-mono-iodo derivative isolated by HPLC. It was used to photoaffinity label human placental membranes and the purified insulin receptor. Extensive digestion of the covalent insulin-receptor complex with trypsin (EC 3.4.21.4) led to the generation of a fragment of Mr 14,000. Specific complexing with avidin, derivatized avidin or streptavidin could be demonstrated for the photoaffinity labelled alpha-subunit and the 14,000 core fragment. The latter was isolated (approx. 100 pmol from 3-4 placentae) by streptavidin affinity chromatography and HPLC. According to microsequencing based on the known primary structure of the insulin receptor, the N-terminus of the core peptide appears to be Leu20-His21-Glu22-Leu23. We thus conclude: a part of the insulin-binding region of the receptor is located close to the N-terminus of its alpha-subunit in a remarkably stable domain of the sequence 20--(approx.) 120.

Effect of experimental diabetes on insulin binding by renal basolateral membranes

Removal of insulin from the peritubular vessels involves binding of insulin to specific receptors in the basolateral membranes (BLM); this is followed by phosphorylation of the receptor which may mediate the actions of the hormone. In most tissues receptor number is regulated by plasma insulin levels and is increased in insulinopenic diabetics. To determine whether cortical BLM insulin receptors are similarly regulated, we studied insulin binding to receptors in BLM from normal control rats and rats with streptozotocin diabetes of varying severity. Specific binding of insulin did not differ between control and modestly insulinopenic diabetics but was increased significantly in the severely insulinopenic diabetics. Insulin treatment returned binding to normal. Scatchard analysis suggested an increase in the binding capacity of the severe diabetic BLM rather than an increase in affinity for insulin. This latter was confirmed by competitive experiments in which similar displacement curves were obtained with control and diabetic membranes. Insulin removed by glomerular filtration binds to specific receptors in the luminal membranes but unlike BLM receptors, phosphorylation of these luminal receptors has not been observed. To determine whether luminal and BLM receptors differ structurally, binding sites in both membranes were affinity labelled with 125I-insulin and the cross linking agent, disuccinimidyl suberate, and subjected to SDS-polyacrylamide gel electrophoresis in the presence of a reducing agent. Autoradiograms revealed that the major specifically labelled subunit in both membranes is a 135,000 Mr species which is more abundant in the BLM. We conclude that insulin receptors in cortical BLM respond to severe insulinopenic diabetes as do receptors in most other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)