Insulin was enzymatically moniodinated with 127-I or 125-I, and an improved method of purification by anion exchange chromatography was employed. (127-I)Monoiodoinsulin was identified by spectrophotometric analysis and its molar extinction coefficient determined to be 6.31 times 10-3 M-1 cm minus 1. The observed specific activity of carrier-free (125-I)monoidoinsulin was close to the theoretical value (378mCi/mg). The monoiodotyrosyl residue of monoidoinsulin was shown to be solvent-exposed. The ionic properties of the substituted hormone were altered at pH values close to the pK of monoiodotyrosine (8.85), but the pI was unchanged (5.65). (127-I)Monoiodoinsulin formed rhombohedral crystals and co-crystallized with native insulin. Monoidoinsulin was indistinguishable from insulin with respect to binding to two out of three guinea pig anti-insulin sera, to binding to IM9 cultured human lymphocytes, and to binding to isolated rat hepatocyte plasma membranes. The potency of monoidoinsulin was not statistically different from that of insulin in the rat fat cell bioassay and in the mouse convulsion assay. An insulin-degrading enzyme extracted from rat liver degraded monoiodoinsulin less readily than native insulin; monoiodoinsulin was a competitive inhibitor of insulin degradation, and the Km values were 30 nM AND 78 NM for monoidoinsulin and native insulin, respectively. It is concluded that monoidination does not markedly alter the three-dimensional structure of the molecule and that only a few sensitive biological systems are able to distinguish the monoidinated from the native hormone.

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