Activation of protease activity in rat peritoneal macrophages in protein deficiency: characterization of cathepsin D

Rat peritoneal macrophages contained high proteolytic activity that was significantly enhanced under the stress induced by protein deficiency. The aspartyl protease cathepsin D which has been known to be the most active protease in endocytic processes was extracted from the macrophages recovered from control (20% protein fed) and protein deficient (4% protein fed) rats and was affinity purified and characterized further. The cathepsin D from the control sample exhibited better recovery, purification and higher specific activity compared to that from the deficient groups. Apparently the pH optima and heat stability of the enzyme from both the groups were similar. The SDS PAGE profile clearly indicated the presence of greater amounts of active forms of cathepsin D in the deficient samples in vivo itself which reflected in a reduction in Km value of the enzyme. Subtle differences observed in the activity of these macrophage proteases in the protein deficient rats may be partly responsible for the enhanced degradation of macrophage membrane proteins reported earlier.