Although large quantities of zinc have been measured in pancreatic islets and correlated with islet B-cell function, potential artifacts in histochemical techniques and theexistence of zinc-free insulin in certain species leave its role in insulin storage and secretion unclear. In this investigation, we examined the kinetics of uptake of 65zinc into isolated rat islets and the distribution of both 65zinc and endogenous islet zinc in islet subfractions. In addition, the chemically formed 65zinc-sodium insulin kinetics ofcomplexes were studied.
Net uptake of 65zinc was slow, being almost linear for the first 4 h and not reaching isotopic equilibrium over the 24 h of culture. Culture of islets in high glucose (25.6 mM) resulted in a diminished net 65zinc uptake and significantly decreased content of both endogenous zinc and insulin when compared with culture in low glucose (5.6 mM). Analysis of 65inc and endogenous zinc distribution in islet subfractions revealed a low percentage of 65zinc and endogenous zinc in the granule-enriched fractions: about 90% of total 65zinc and 80% of endogenous zinc were associated with nongranular fractions. A comparison of the specific activity of the granular fractions with the lighter sucrose fractions revealed a nonequilibration of the 65zinc taken up with the granular zinc pool. Chemical analysis of 65zinc-insulin complexes revealed an almost instantaneous binding of 65zinc to zinc-free insulin. Dissociation of preformed 65zincinsulin complexes was slow, but some dissociation occurred with a loweringof the pH.
We demonstrated that net zinc uptake by cultured islets is a slow phenomenon reducedby a high glucose environment. Although chemical analyses suggest a strong affinity of 65zinc for insulin, only small quantities of 65zinc, as well as endogenous zinc, associated with the granule-enriched fractions, and equilibration of granular 65zinc with islet 65zinc pools did not occur over 24 h. Thus, zinc may be needed in other metabolic processes besides insulin crystallization in the Bcell.
