The ligand environment of zinc stored in vesicles

Zinc serves regulatory functions in cells and thus, several mechanisms exist for tight control of its homeostasis. One mechanism is storage in and retrieval from vesicles, so-called zincosomes, but the chemical speciation of zincosomal zinc has remained enigmatic. Here, we determine the intravesicular zinc-coordination in isolated zincosomes in comparison to intact RAW264.7 murine macrophage cells. In elemental maps of a cell monolayer, generated by microbeam X-ray fluorescence, zincosomes were identified as spots of high zinc accumulation. A fingerprint for the binding motif obtained by μXANES (X-ray absorption near edge structure) matches the XANES from isolated vesicles; zinc is not free, but present as a complexed form (average coordination; 1.0 sulfur, 2,5 histidines 30 and 1.0 oxygen), resembling regulatory or catalytic zinc sites in proteins. Such coordination enables reversible binding, acting as a ‘zinc sink’, facilitating the accumulation of high amounts of zinc against a concentration gradient.