Effect of diet on tissue protease activity.

Rats 1, 3, 12, and 24 months old were fed diets low in protein (8% casein), and proteolytic activity in tissue from brain, liver, and lung was determined. After a low-protein diet was fed for 4 weeks to 1-month-old rats, there was a significant increase in cathepsin D activity in liver, and calpain activity was increased in lung. Little change was seen in proteolytic activity in brain. In 12-month-old rats, there was an increase in cathepsin D activity in brain and liver. In 24-month-old rats, cathepsin D activity in the liver and calpain activity in lung were increased. There was no change in proteolytic activity in the brain. When animals were fed diets supplemented with fatty acids or antioxidants for 2 months, in 3-month-old rats calpain activity was increased in brain but decreased in lung. Cathepsin D activity was significantly increased in young and adult animals in brain and in liver. These observations suggest that diet changes result in significant alteration in tissue calpain and cathepsin D levels, and possibly activity, in vivo. Generally, changes are greater for cathepsin D than for calpain, and are smaller in brain than in other tissues.