Enzymatic determination of zinc below one part per billion

Abstract

A stable and very sensitive reagent for the determination of zinc is obtained by removing zinc from pig kidney aminopeptidase, a commercially available metalloenzyme. Up to a given limit, the enzymatic activity of the reagent is strictly proportional to the concentration of zinc ions in the assay system. Aminopeptidase activity is determined by measuring the rate of release of p-nitroaniline from the chromogenic substrate L-leucine- p-nitroanilide. Thus, with a simple recording photometer, rapid and accurate determinations of free zinc ions in concentrations ranging from 5 pg-10 μg ml -1 can be achieved. The selectivity of the method is such that 0.1 ng Zn ml -1 can be assayed in the presence of a 10–100-fold molar amounts of a wide array of other cations with a relative error below 10%. The analytical procedure has been extended to the assay of Cu 2+, Co 2+ and Ni 2+ in the ng ml -1 range.