Chymostatin is an effective inhibitor of intracellular proteinases in vitro. In the present experiment male rats were injected intraperitonealy during a 3 days period twice daily with a solution containing 0,9 mg Chymostatin per 100 g live weight. Reference animals received a control injection containing the same solvents but no chymostatin. During this period a daily nitrogen balance was made and metabolic faecal nitrogen and true digestibility of nitrogen were estimated using 15N-labelled animals. Furthermore, apparent biological half lives of proteins in liver and intestinal tissues were determined following the decay curves for radioactivity in proteins 48 hours after injection of L-[5-3H]-arginine und L-[guanido-14C]-arginine. The fractional rate of protein synthesis in tissues was measured by a 6 hours continuous infusion technique with L-[U-14C]-tyrosine and L-[U-14C]-leucine. Among the parameters estimated only the apparent biological half lives of proteins in liver and intestinal tissues were influenced by chymostatin. However, the prolonged half lives seemed to be rather an effect of an increased reutilisation of amino acids resulting from the intracellular protein breakdown than a decreased rate of protein degradation. The in vivo effect of the proteinase inhibitor was by far inferior compared with the action in vitro. Factors like distribution, degradation and excretion of the inhibitor could be responsible for the moderate in vivo action of chymostatin.

Animals↗

PubMedDate not supplied

Surface hydrophobicity and intracellular degradation of proteins.

Cellular proteins turn over with rates varying more than 1000 fold and all organelles are involved in this process of renewal. Although the surface of protein molecules bears many peptide bonds, which could be potential cleavage sites for proteases, only a small fraction of all cellular proteins is subject to very rapid turnover, with half-lives of less than one hour in mammalian cells. Many of these proteins play key roles in basic regulatory mechanisms. One of the features that make proteins short-lived is surface hydrophobicity which is increased in nascent polypeptide chains before association with chaperones, in oligomeric proteins before the association of the monomers, and in many enzymes in absence of their substrates. Cellular proteases tend to act most rapidly on peptide bonds involving (or near to) apolar amino acids. A striking correlation has been found between the half-lives of cellular proteins and their surface hydrophobicity. Clusters of hydrophobic residues appear to be necessary for ubiquitination. Apolar amino acid residues that are hidden can be exposed after oxidation of proteins or after binding of ubiquitin molecules. Additionally, tetraubiquitin exposes many hydrophobic residues which are essential for targeting of the ubiquitinated substrate proteins to a 50 kDa-subunit of the 26S-protease.

Amino Acid Sequence↗