[On the complex formation of proteins with cu ions under acidic conditions]

In order to determine the influence of copper additives on the protein digestibility, the complex formation of proteins with Cu ions under acidic conditions (pH=2.2) was investigated. Isolated soja-protein (300 mg) resp. pepsin was mixed with 10 mumol (2.49 mg) CuSO4-5H2O and dialysed against low molecular complexing agents with increasing affinity towards copper ions. The experiments with soja-protein showed, that most of the Cu ions dialysed (97% of total Cu2+ content) without any addition of complexing agents, pepsin, however, had a lower percentage (83% of total Cu2+ content). The remaining copper ions in pepsin could only be removed by alanin and ammonium-tetramethylendithiocarbamate. Therefore it is concluded, that in the acidic range Cu ions only form complexes with pepsin, but not with soja-protein. This effect is due to the very low isoelectric point of pepsin. The determination of iron, nickel, and zinc in native proteins revealed, that at pH=2.2 iron and nickel are stronger linked to pepsin than to soja-protein; only zinc was loosely bound to both proteins.