Distribution of copper and zinc in mammalian eyes. Occurrence of metals in melanin fractions from eye tissues.

Several methods have been used in the investigation of trace elements in eye tissues. BurdonCooper (1928) and Burdon-Cooper & Lewis (1929) give the results of emission spectrographic analysis of the mineral constituents of normal and cataractous human-eye lenses. The technique failed to demonstrate the presence of zinc, iron and manganese, which Tauber & Krause (1943) were later able to detect (and estimate) by other methods. The latter authors pointed out that the persistent spectral lines of the three elements could easily be obscured by lines due to elements such as calcium and phosphorus, which are present in the ash as major constituents. The first stage in the present investigation was the application ofmodernmethods ofemissionspectrum analysis to the problem of detecting and determining trace metals in eye tissues (A.L.S. and M.H.S., see Shakir, 1948). So far as detectionwas concerned, excellent results were obtained, but the range covered by zinc, copper and manganese contents of various eye tissues proved to be too narrow for emission spectra to give quantitative information of sufficient accuracy. Accordingly, a good deal of exploratory work was carried out using organic reagents forming metallic complexes which could be estimated by spectrophotometric methods. Selected procedures have now been improved and the results revised and extended (J.M.B.). In the course of the work based on emission spectroscopy, semi-quantitative estimations of trace elements were made on dialysed and undialysed tissues from fish eyes (Shakir, 1948). The presence of comparatively large quantities of dialysable inorganic material results in a cloud of vaporized salts around the electrodes, and this has a marked effect on the recorded intensities of the spectral lines shown by the 'trace' elements. This effect introduces uncertainty into the validity of comparing the results with dialysed and undialysed preparations. A fivefold difference in trace metal concentration is certainly demonstrable, but it is difficult to be sure about small differences. The emission spectrum technique is thus not sufficiently sensitive for further study of the rather small differences recorded by Tauber & Krause (1943) in respect of the concentrations of zinc, copper and manganese in cattle-eye tissues. In spite of such limitations, it seemed clear that tissues from fish eyes contained non-dialysable zinc and copper in excess ofthe retained amounts ofother trace metals. The preliminary work with dithizone zinc and copper complexes indicated that it might be possible to establish significanit differences between the various eye tissues in mammals. Repetition and extension of the work shows this to be the case. Leiner & Leiner (1944) had shown for zinc in fish eyes that such differences occur. Although the absolute concentrations found in each tissue varied greatly from species to species, a list showing the different types of tissues in order of decreasing zinc content was praciically the same for all the fishes studied. The results of Tauber & Krause (1943) on cattle eyes do not show a similar distribution of zinc, but the analytical method described in their paper seems inadequate. Their determinations of copper concentrations in eye tissues appear to be the only ones recorded in the literature. It was therefore decided that the whole problem of zinc and copper in eye tissues deserved further study. The first part of this paper embodies the experimental details and results of determinations of zinc and copper concentrations in all the eye tissues of several mammals. The results of these determinations show that the iris and choroid tissues contain the highest concentrations of zinc and copper. It therefore became necessary to investigate the more precise location of these metals in irises and choroids. The second part of the paper deals with the experimental methods and results of a fractionation of irises, and a study of a pigment fraction obtained from irises and choroids.