Nutrition and protease activity.

Clearly, both the metabolism of many proteins and the size of protein pools are affected or regulated by the nutrient intake. At least some of these effects are probably due to changes in the activities of proteolytic enzymes. Indeed, starvation, protein-calorie malnutrition and protein malnutrition probably increase activities of a number of proteases in plasma, liver, skeletal muscle and heart. Pancreatic secretion of proteases and other enzymes is impaired. However, the changes in proteolytic activities often follow rather than precede alterations in the net rate of protein degradation. Thus, the contribution of the proteolytic enzymes to changes in protein degradation in these conditions is not clear. The effects of nutrition on proteases also are difficult to evaluate because the protease activities have been measured in vitro. These activities may not necessarily reflect the protease activity in vivo, where lysosomes, membrane barriers and transport systems are intact, and inhibitors and facilitators of proteinases may act differently from the in vitro state. Also, proteinase activities often are expressed per weight of tissue or per mg of protein. Tissue weight and protein content often fall during protein and/or calorie malnutrition. Hence, the previous reports of increased protease activities in malnutrition may be incorrect. The observed increase is only relative to the fall in weight and protein content of the tissue. When expressed in terms of total tissue or organ mass, there was no rise in protease activities in many of these studies.(ABSTRACT TRUNCATED AT 250 WORDS)