The Interaction between Xanthurenic Acid-Insulin Complex and Zinc Ions

An equimolar mixture of a xanthurenic acid-insulin complex and ZnSO4 was separated into an insulin peak and a peak containing xanthurenic acid (XA) and Zn by Sephadex G-75 column chromatography. XA and di-[L-histidino]-zinc (II) readily combined to produce di-[L-histidino]-di-xanthurenato zinc (II) (His2-Zn2+-XA2). By increase in the concentration of Zn2+ ions, XA was removed from di-[L-histidino]-di-xanthurenato zinc (II) (His2-Zn2+-XA2) as XA-Zn2+. The XA-insulin complex showed decreased relative intensity of fluorescence compared with the Zn-insulin when excited at a wavelength of 284 nm. The difference spectrum between native Zn-insulin and XA-insulin complexes showed a slight red shift. A difference in the CD spectrum between native Zn-insulin and XA-insulin complexes was observed.