Metal–protein binding losses in proteomic studies by PAGE–LA-ICP-MS

Some experiments to study the influence of electrophoresis conditions and subsequent LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometry) determination of two metal-binding proteins with different metal–protein affinities (superoxide dismutase, containing Cu and Zn, and alcohol dehydrogenase, containing Zn) are performed. In metal-binding proteins with weak metal–protein affinities, metal losses can happen during electrophoretic separation. It has been demonstrated that the detection of these metals bound to the proteins depends, not only on the nature of the electrophoretic process (naturing or non-denaturing) and post-separation gel treatment, but also on the trailing ion chosen and current applied in the electrophoretic method used. Non-denaturing methods are preferred to denaturing ones in the case of alcohol dehydrogenase being BN-PAGE (Blue Native-Polyacrylamide Gel Electrophoresis) with the use of Tricine as trailing ion the most recommended method. The concentration obtained for Zn in ADH applying BN-PAGE–LA-ICP-MS was 2.6 ± 0.30 mg g−1 very close to the one obtained for ADH solution by ICP-MS (3 ± 0.23 mg g1). For superoxide dismutase either denaturing or non-denaturing electrophoresis conditions can be used, but a denaturing method based on the use of Tricine as trailing ion is recommended to preserve metals–protein binding when the use of non-denaturing conditions must be avoided. The found concentration for Cu and Zn in SOD after applying SDS–Tris–Tricine-PAGE–LA-ICP-MS was 2.5 ± 0.33 and 2.4 ± 0.37 mg g−1 respectively, more or less close (especially for Cu) to the one obtained in SOD solution by ICP-MS (3 ± 0.21 and 3.7 ± 0.32 mg g−1 for Cu and Zn). We observe that as higher current is applied the possibility of metal–protein binding losses is higher. In all cases staining of the gel prior to LA-ICP-MS is not recommended.