α-1-Antitrypsin metabolism in the protein-deficient weanling rat

Abstract

1. Protein-deficient weanling rats fed on a 30 g casein/kg diet for 3 weeks lost albumin but maintained the level of serum α-1-antitrypsin, the most abundant protease inhibitor in blood.2.α-1-Antitrypsins from malnourished rats and control rats (given 250 g casein/kg diet) differed; the protease inhibitor from protein-deficient animals: (1) was more acidic, (2) appeared slightly larger (57400 v. 56000 daltons) on sodium dodecyl sulphate (SDS)-polyacrylamide gels, (3) had a more acidic Pitype and increased anodal mobility at pH 8.9, (4) bound more concanavalin-A and contained more carbohydrate, specifically two to three extra sialic acid residues. The amino sugar and neutral sugar contents of both preparations of α-1-antitrypsin were the same.3. Analysis of the products of cyanogen-bromide cleavage revealed that α-1-antitrypsin preparations from protein-deficient rats contain an extra glycopeptide that was not present in α-1-antitrypsin from control animals.4. In vivo studies showed that the increased sialic acid content of α-1-antitrypsin of protein-deficient rats did not alter the half-life of the molecule in the blood of control rats. However, the fractional catabolic rate of α-1-antitrypsin from either well-nourished or protein-deficient rats was significantly (P < 0.01) lower in protein- deficient rats than in control rats (0.0247/h v. 0.0406/h).5. The decreased fractional catabolic rate could not be explained by changes in hepatic mannosyl-, galactosyl- or N-acetylhexosaminyl receptors since liver perfusion studies showed that bovine serum albumin, when covalently modified separately with each of these ligands, was extracted from the perfusion medium as rapidly or more rapidly by livers from malnourished animals.6. Perfused livers from protein-deficient rats secrete three times more α-1-antitrypsin than do livers from well-nourished animals.7. The decreased fractional catabolic rate and increased rate of biosynthesis and secretion of the glycoprotein by livers from protein-deficient animals may account for the maintenance of α-1-antitrypsin levels during protein malnutrition.