Degradation of serum albumin by rat liver and kidney lysosomes

Lysosomes were isolated from the livers and from the kidneys of rats treated or not treated with the cysteine proteinase inhibitor leupeptin, and the levels of the intralysosomal serum albumin of the leupeptin-treated rats were compared with those of the saline-treated control rats. Leupeptin caused an intralysosomal accumulation of albumin in vivo because of its potent inhibition of lysosomal protein degradation. In fact, the lysosomes isolated from the livers and kidneys of leupeptin-treated rats almost completely lost their ability to degrade rat albumin in vitro. These findings show that the lysosomes are subcellular sites of the degradation of unlabeled serum albumin in these tissues. They also suggest that cysteine proteinases sensitive to leupeptin are involved in the lysosomal degradation of albumin. Albumin was degraded by total lysosomal enzymes in vitro. It was also degraded by the lysosomal extract being devoid of cathepsins H and J, prepared from rat kidney. The degradation of albumin by total lysosomal enzymes in vitro was greatly suppressed by a cysteine proteinase inhibitor, cystatin alpha, with no inhibition of cathepsins B and L. It was slightly suppressed by N-(L-3-trans-propylcarbamoyloxirane-2-carbonyl)-L-isoleucyl-L-prol ine (CA-074), a selective inhibitor of cathepsin B, and by pepstatin, an inhibitor of cathepsin D, whereas it was markedly suppressed by a combination of cystatin alpha and either CA-074 or pepstatin. These and associated findings show that cystatin alpha-sensitive cysteine proteinase(s), which is distinct from cathepsins B, H, L, and J, and cathepsins B and D are involved in the lysosomal degradation of albumin.