An 18-month old child of a migrant farm worker was hospitalised because of weight loss, vomiting and acute abdominal pain. It was noted that the child had mild muscular inco-ordination and weakness of the muscles and feet. A blood smear showed a moderate increase in reticulocyte count. The red blood cell count was 4x106 cells/mm3 and the haematocrit was 37%. A 24-hour urine sample contained 840μg (6.4μmol) of δ-aminolevulinic acid and 1.2mg (1.8μmol) of coproporphyrin III. Lead poisoning was suspected. A 24-hour urine sample was analysed and contained 0.24mg (1.1μmol) lead. X-ray examination of the patient's long bones showed electron-dense deposits at the epiphysis. What is the rationale for treatment of lead poisoning with penicillamine?