9. P R O T E I N M E T A B O L I S M IN C U L T U R E
309
raise the activity of alkaline phosphatase, indicating that arginine
acted as a specific inducer of arginase. Hillis and Bang (1962), using
organ-culture techniques, have reported survival of human embryonic
liver tissue for three weeks, during which time the cellular morphology
resembled that of normal embryonic liver, and the tissue possessed
arginase activity. The authors state that the increasing activity of the
cells with the age of the donor embryos would tend to support the
belief that the arginase activities demonstrated, were native to the
cells and not substrate-induced.
C. T U R N O V E R A N D C A T A B O L I S M
The synthesis and degradation of protein in rabbit macrophages
have been studied by measuring the incorporation and release of
labelled valine (Harris and Watts, 1958). Valine was incorporated into
tissue protein and released from the protein into the medium simultaneously, indicating that protein turnover was occurring on a large
scale in the macrophage. The specific activity of labelled macrophage
protein fell progressively in "cold" medium in 8 h by approximately
3 0 % . Since release of valine, unlike the incorporation, was relatively
insensitive to azide, and breakdown involved liberation of peptide-like
compounds and free amino acids, it was suggested that the degradative
process resembled proteolysis.
Recent biochemical research has elucidated many of the steps
involved in protein synthesis, but little is known about the catabolism of
proteins especially in relation to protein turnover. One possible
mechanism for the degradation o f protein is by the action of the proteolytic enzymes, e.g. cathepsins, which are known to be present in tissues.
Increased protein breakdown may result from the activation of an
enzyme system that is normally latent. An example of this is the
increased acid-soluble nitrogen that was observed in the used medium
as a result of adding an excess of vitamin A to cultures of cartilaginous
limb-bone rudiments from the embryonic chick; this finding indicated
a greater proteolytic activity in the presence of the vitamin (Dingle,
Lucy and Fell, 1961). The liberation of a proteolytic enzyme, possibly
a cathepsin, from the lysosomes of the chondrocytes was thought to be
responsible for the degradation of the protein-polysaccharide complex
of cartilage matrix that occurs in vitro under the influence of an excess
of vitamin A (Fell, Lucy and Dingle, 1961; Lucy, Dingle and Fell,
1961; Dingle, 1961). Recently it has been found that not only is a
proteolytic enzyme released into the culture medium from cartilage
treated with an excess of vitamin A, but the treated explants also
309
raise the activity of alkaline phosphatase, indicating that arginine
acted as a specific inducer of arginase. Hillis and Bang (1962), using
organ-culture techniques, have reported survival of human embryonic
liver tissue for three weeks, during which time the cellular morphology
resembled that of normal embryonic liver, and the tissue possessed
arginase activity. The authors state that the increasing activity of the
cells with the age of the donor embryos would tend to support the
belief that the arginase activities demonstrated, were native to the
cells and not substrate-induced.
C. T U R N O V E R A N D C A T A B O L I S M
The synthesis and degradation of protein in rabbit macrophages
have been studied by measuring the incorporation and release of
labelled valine (Harris and Watts, 1958). Valine was incorporated into
tissue protein and released from the protein into the medium simultaneously, indicating that protein turnover was occurring on a large
scale in the macrophage. The specific activity of labelled macrophage
protein fell progressively in "cold" medium in 8 h by approximately
3 0 % . Since release of valine, unlike the incorporation, was relatively
insensitive to azide, and breakdown involved liberation of peptide-like
compounds and free amino acids, it was suggested that the degradative
process resembled proteolysis.
Recent biochemical research has elucidated many of the steps
involved in protein synthesis, but little is known about the catabolism of
proteins especially in relation to protein turnover. One possible
mechanism for the degradation o f protein is by the action of the proteolytic enzymes, e.g. cathepsins, which are known to be present in tissues.
Increased protein breakdown may result from the activation of an
enzyme system that is normally latent. An example of this is the
increased acid-soluble nitrogen that was observed in the used medium
as a result of adding an excess of vitamin A to cultures of cartilaginous
limb-bone rudiments from the embryonic chick; this finding indicated
a greater proteolytic activity in the presence of the vitamin (Dingle,
Lucy and Fell, 1961). The liberation of a proteolytic enzyme, possibly
a cathepsin, from the lysosomes of the chondrocytes was thought to be
responsible for the degradation of the protein-polysaccharide complex
of cartilage matrix that occurs in vitro under the influence of an excess
of vitamin A (Fell, Lucy and Dingle, 1961; Lucy, Dingle and Fell,
1961; Dingle, 1961). Recently it has been found that not only is a
proteolytic enzyme released into the culture medium from cartilage
treated with an excess of vitamin A, but the treated explants also