Zinc prevents the structural and functional properties of free radical treated-insulin

We have previously reported that zinc deficiency could increase in vivo lipid peroxidation and decrease rat insulin sensitivity. In the present paper, we address the hypothesis of the role of zinc on insulin molecule in relation to free radical damage. From native recombinant human insulin, we prepared a zinc-depleted insulin. Both preparations were subjected to controlled free radical attack by incubation in the presence of 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH). To obtain minimally oxidized insulin, the oxidation process was monitored by measuring the intrinsic fluorescence of the insulin preparations. For 2.5 mM of AAPH, the autofluorescence of zinc-depleted insulin markedly decreased as compared to that of native insulin. These data are in favor of conformational changes of the insulin molecule which were further studied by quenching of fluorescence by means of potassium iodide. Using the euglycaemic hyperinsulinic glucose clamp technique in rats, the in vivo activities of the different insulin preparations, showed that oxidized zinc-depleted insulin had a marked reduced activity as compared to oxidized native insulin. From our results, we suggest that structural modification of the insulin molecule took place after zinc depletion and free radical treatment. Moreover, zinc depletion appeared to increase the susceptibility of insulin to free radicals.