Measurement of colloidal iron binding at low pH in cartilage using the proton microprobe

Quantitative micro-PIXE analysis was performed on mouse embryo epiphyseal cartilage and on the rib cartilage of mature animals after incubation of sections with colloidal iron at pH 1.8. The iron content as well as that of sulphur and phosphorus and Fe/S, Fe/P ratios were determined. It was found that colloidal iron content was higher in the cartilage than in other tissues. The cartilage also displayed the highest content of sulphur. The Fe/S ratio was however not constant, being highest in the degeneration zone close to the mineralization front, where the binding of iron was strongest while the amount of sulphur decreased. This indicates that factors other than number of sulphate groups influence the binding of positively charged molecules to glycosaminoglycans. This is confirmed by differences in the results obtained for embryonic and mature rib cartilage.