Degradation of serum albumin by rat liver and kidney lysosomes

Lysosomes were isolated from the livers and from the kidneys of rats treate d or not treated with the cysteine proteinase inhibitor leupeptin, and the levels of the intralysosomal serum albumin of the leupeptin-treated rats we re compared with those of the saline-treated control rats. Leupeptin caused an intralysosomal accumulation of albumin in vivo because of its potent in hibition of lysosomal protein degradation. In fact, the lysosomes isolated from the livers and kidneys of leupeptin-treated rats almost completely los t their ability to degrade rat albumin in vitro. These findings show that t he lysosomes are subcellular sites of the degradation of unlabeled serum al bumin in these tissues. They also suggest that cysteine proteinases sensiti ve to leupeptin are involved in the lysosomal degradation of albumin. Album in was degraded by total lysosomal enzymes in vitro. It was also degraded b y the lysosomal extract being devoid of cathepsins H and J, prepared from r at kidney. The degradation of albumin by total lysosomal enzymes in vitro w as greatly suppressed by a cysteine proteinase inhibitor, cystatin alpha. w ith no inhibition of cathepsins B and L. It was slightly suppressed by N-(L -3-transpropylcarbamoyloxirane-2-carbonyl)-L-isoleucyl-L-proline (CA-074), a selective inhibitor of cathepsin B, and by pepstatin, an inhibitor of cat hepsin D, whereas it was markedly suppressed by a combination of cystatin a lpha and either CA-074 or pepstatin. These and associated findings show tha t cystatin alpha-sensitive cysteine proteinase(s), which is distinct from c athepsins B, H, L, and J, and cathepsins B and D are involved in the lysoso mal degradation of albumin.