Comparison of hepatic cathepsin D activity in response to tumor growth and to caloric restriction in mice.

In malignant disease, the caloric intake is often reduced. We have studied the role of this nutritional deprivation in increased lysosomal enzyme activities in liver and muscle tissue in tumorbearing mice. Three study groups were used: Group 1, sarcoma-bearing mice (MCG 101); Group 2, starved mice; Group 3, mice pair fed according to the ad libitum intake of the tumor-bearing animals. Cathepsin D activity (Vmax)was increased in the homogenates of liver tissue in all study groups. /S-Glucuronidase activity was increased in liver homogenates from tumor-bearing mice and starved mice. In muscle tissue homogenates, only the tumorbearing mice showed increased activities of these enzymes. Gel filtration of the crude cathepsin D fraction prepared from liver tissue showed only one main peak of enzyme activity in the study groups and in controls. This argues against the possibility that increased cathepsin D activity in liver tissue was due to migrating cells of lymphoid origin. The characteristics of the purified cathepsin D activity from liver homogenates differed among the groups. In starved ani mals, the partially purified enzyme showed significantly higher Vmaxand Km than in tumor-bearing mice and controls. The apparent increased activity of the enzyme in liver homogenates from tumor-bearing animals disappeared after purification. It is concluded that the increased activities of lysosomal enzymes in liver and skeletal muscle tissue from tumor hosts are not due to poor nutrition only.