INTRACELLULAR PROTEIN BREAKDOWN .8. DOUBLE-LABELED PROTEINS AS SUBSTRATES

  • 1 January 1976
    • journal article
    • research article
    • Vol. 35  (3-4) , 301-307
Abstract
Double-labeled proteins from rat liver cytosol (14C in long-lived, 3H in short-lived proteins after in vivo labeling) are used as substrates for unlabeled proteinases in vitro. Differences in the degradation rates of short-lived and long-lived proteins in vitro by different proteinases and after addition of different effectors allow conclusions concerning their importance for the in vivo turnover of substrate proteins. The main activity (> 90%) of soluble lysosomal proteinases at pH 6.1 and pH 6.9 is caused by thiolproteinases, which degrade preferentially short-lived cytosol proteins. These proteinases are inhibited by leupeptin. Autolysis of double-labeled cell fractions shows a remarkably faster breakdown of short-lived substrate proteins only in the soluble part of lysosomes. Microsomal fractions degrade in vitro preferentially long-lived substrate proteins.