Bound trace element content of bovine retinal disk membranes as determined by particle-induced x-ray emission

Particle-induced x-ray emission (PIXE) was used to determine the trace element content of bovine retinal disk membranes. PIXE is a multielemental analytical technique capable of the simultaneous detection and quantization of all elements from sodium and above in atomic number. The multielemental capability of PIXE allows the analysis time per element to be very low if a number of elements are detected in each sample. In addition, the multielemental capability of PIXE can be used to determine elemental content with respect to an internal reference. Here the content of detected trace element per rhodopsin was determined without recourse to an external rhodopsin assay. This was accomplished by using the sulfur content of rhodopsin as an internal reference. Detected trace element contents per rhodopsin were 1.58 +/- 0.049 Ca, 0.081 +/- 0.024 Fe, 0.393 +/- 0.200 Cu, and 0.150 +/- 0.031 Zn. Upper limits were placed on the amount of manganese, molybdenum, and nickel per rhodopsin as 0.019, 0.019, and 0.006, respectively. Two proteins known to be present in disk membranes, retinol dehydrogenase and a large protein, approximately 238,000 mol wt, are considered as potential metallo-proteins. No correlation was observed between the content of any detected element and bleaching levels.