Studies of iron overload. Lysosomal proteolysis of rat liver ferritin

To learn more about pathological iron storage in the liver, two sorts of lysosomes were isolated from rat livers in Percoll - sucrose or sucrose gradients: siderosomes (= iron-loaded terminal lysosomes) and light lysosomes (secondary and terminal). Such cell fractions were obtained from acutely iron-loaded and control rat livers. After lysis with Triton X-100 the preparations were assayed for proteolytic activity against rat liver ferritin (RLF) and denatured bovine hemoglobin (DBH), for buffer-soluble ferritin protein content, total protein and non-heme iron. At pH 3.6 both fractions displayed considerable proteolytic activity (cathepsin D activity) against DBH and endogenous proteins but little activity against RLF. By contrast, proteolytic activity against RLF was maximal at the highest pH tested, 6.5, at which DBH was practically insusceptible. The behavior of proteolytic activity against ferritin at pH 6.5 makes it likely that a single enzyme was involved that acted by Michaelis-Menten kinetics. However, no more than 2.5% of endogenous ferritin protein in the organelles was buffer-soluble. 41 to 89 hours after an intramuscular dose of 50 mg Fe, given as iron dextran, the non-heme iron content of light lysosomes and siderosomes had increased markedly and the ratio of non-heme Fe to buffer-soluble ferritin protein also became much elevated in the organelles; but the ratio of buffer-soluble ferritin to total protein did not rise significantly. The rise in organellar non-heme Fe exceeded iron saturation of rat liver ferritin and thus reflected conversion of ferritin to hemosiderin, which is buffer-insoluble.(ABSTRACT TRUNCATED AT 250 WORDS)