Turnover of intestinal brush-border proteins during postnatal development in rat

Protein turnover in brush-border membranes of rats during postnatal development has been studied by the double isotope technique. Unlike adult animals where only large proteins (mol wt > 140,000) show relatively rapid turnover, most brush-border proteins in 12-day-old rats show high 3H-to-14C ratios of leucine incorporation, consistent with rapid turnover. Lysosomal proteases, including cathepsin B, are partly responsible for this rapid turnover. This conclusion is based on the following findings: 1) In vivo treatment of 12-day-old animals with leupeptin, an inhibitor of cathepsin B, alters relative turnover rates, enzyme activity, and content of many brush-border proteins. Activities of maltase and trehalase rise while lactase falls. 2) Cathepsin B activity falls rapidly in intestine after the animals are 16 days of age, at a time when luminal pancreatic proteases are rising. Moreover, cathepsin B activity shows less latency in distal intestine at 12 and 16 days than at later ages or in proximal intestine. It is suggested that during postnatal development lysosomal enzymes, e.g., cathepsin B, play an important role in the turnover of intestinal brush-border proteins.