Zn.sup.2+ chelation by serum albumin improves hexameric Zn.sup.2+-insulin dissociation into monomers after exocytosis

• Published in: PloS one 2017-11, Vol.12 (11), p.e0187547
• Main Authors: Pertusa, José A. G, León-Quinto, Trinidad, Berná, Genoveva, Tejedo, Juan R, Hmadcha, Abdelkrim, Bedoya, Francisco J, Martín, Franz, Soria, Bernat
• Format: Article
• Language: English
• Subjects: Albumin; Analysis; Exocytosis; Health aspects; Insulin; Physiological aspects
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0187547

[beta]-cells release hexameric Zn.sup.2+ -insulin into the extracellular space, but monomeric Zn.sup.2+ -free insulin appears to be the only biologically active form. The mechanisms implicated in dissociation of the hexamer remain unclear, but they seem to be Zn.sup.2+ concentration-dependent. In this study, we investigate the influence of albumin binding to Zn.sup.2+ on Zn.sup.2+ -insulin dissociation into Zn.sup.2+ -free insulin and its physiological, methodological and therapeutic relevance. Glucose and K.sup.+ -induced insulin release were analyzed in isolated mouse islets by static incubation and perifusion experiments in the presence and absence of albumin and Zn.sup.2+ chelators. Insulin tolerance tests were performed in rats using different insulin solutions with and without Zn.sup.2+ and/or albumin. Albumin-free buffer does not alter quantification by RIA of Zn.sup.2+ -free insulin but strongly affects RIA measurements of Zn.sup.2+ -insulin. In contrast, accurate determination of Zn.sup.2+ -insulin was obtained only when bovine serum albumin or Zn.sup.2+ chelators were present in the assay buffer solution. Albumin and Zn.sup.2+ chelators do not modify insulin release but do affect insulin determination. Preincubation with albumin or Zn.sup.2+ chelators promotes the conversion of "slow" Zn.sup.2+ -insulin into "fast" insulin. Consequently, insulin diffusion from large islets is ameliorated in the presence of Zn.sup.2+ chelators. These observations support the notion that the Zn.sup.2+ -binding properties of albumin improve the dissociation of Zn.sup.2+ -insulin into subunits after exocytosis, which may be useful in insulin determination, insulin pharmacokinetic assays and islet transplantation.