68
ENRICO URBANI
Duspiva (1942) advanced the view that dipeptidases participate in
breaking down the peptides continuously produced through the renewal
and synthesis of cellular proteins. It was further observed that in many
materials these enzymes and the pentose-nucleic acids increase or
decrease together (Urbani, 1954, 1955b, 1960): this fact calls to memory
Binkley's (1952) observations on the possibility that some nucleic acids
may hydrolyse certain dipeptides.
As already mentioned, the optimum pH value for dipeptidase activity
is close to that of the living protoplasm, but an acid medium such as
autolytic protoplasm inactivates and destroys dipeptidases. For these
and other reasons, we are led to think that dipeptidases and tripeptidases are active in the living cell, probably in connection with protein
synthesis; in any case, their role is not limited exclusively to the phenomena of posthumous autolysis. The results of our studies on embryonic
development and the findings mentioned earlier or recorded in the
literature support the view that the activity of these enzymes is closely
related to the anabolic and constructive processes (Binkley, 1950;
Brächet, 1947, 1957, 1960a; Patterson et al, 1949; Rotschild and
Junqueira, 1951).
Intracellular proteinases differ from peptidases in their characteristics, in that they hydrolyse proteins of high molecular weight in both
acid and alkaline media. There are probably different proteinases with
different chemical structures. Lenney (1956), for instance, isolated from
yeast two proteinases with different pH optima, one with and the other
without sulphydryl groups in their molecules.
As will be seen, it is sometimes possible to identify proteinases which
hydrolyse the same substrates in both acid and alkaline media and
which bear some resemblance to trypsin in vertebrates. Proteinases are
localized in cell cytoplasm (Holter and Lovtrup, 1949), and probably
in the lysosomes together with other acid hydrolases (de Duve, 1959):
they may also be found in the nuclei of some cells (Dounce, 1954); but
in Amoeba proteus they are not affected by experimental enucleation
(Urbani, 1952). It has been suggested that they participate exclusively
in posthumous phenomena; although based on the fact that they show
optimal activity in an acid medium—and this is therefore not applicable
to alkaline proteinases—this opinion has met with adverse criticism
from many authors. Danilewski (1886) showed in vitro synthesis by
these enzymes, which reacted on a protein hydrolysate, producing a
protein of relatively high molecular weight which he called plastein.
After him, many leading enzymologists investigated the possibility of
in vivo protein synthesis by proteinases (Bergmann and FraenkelConrat, 1938; Binkley, 1950; Borsook, 1952; Linderström-Lang, 1952;
Northrop, 1947; Terroine, 1955; Viertanen and co-workers, 1949;
ENRICO URBANI
Duspiva (1942) advanced the view that dipeptidases participate in
breaking down the peptides continuously produced through the renewal
and synthesis of cellular proteins. It was further observed that in many
materials these enzymes and the pentose-nucleic acids increase or
decrease together (Urbani, 1954, 1955b, 1960): this fact calls to memory
Binkley's (1952) observations on the possibility that some nucleic acids
may hydrolyse certain dipeptides.
As already mentioned, the optimum pH value for dipeptidase activity
is close to that of the living protoplasm, but an acid medium such as
autolytic protoplasm inactivates and destroys dipeptidases. For these
and other reasons, we are led to think that dipeptidases and tripeptidases are active in the living cell, probably in connection with protein
synthesis; in any case, their role is not limited exclusively to the phenomena of posthumous autolysis. The results of our studies on embryonic
development and the findings mentioned earlier or recorded in the
literature support the view that the activity of these enzymes is closely
related to the anabolic and constructive processes (Binkley, 1950;
Brächet, 1947, 1957, 1960a; Patterson et al, 1949; Rotschild and
Junqueira, 1951).
Intracellular proteinases differ from peptidases in their characteristics, in that they hydrolyse proteins of high molecular weight in both
acid and alkaline media. There are probably different proteinases with
different chemical structures. Lenney (1956), for instance, isolated from
yeast two proteinases with different pH optima, one with and the other
without sulphydryl groups in their molecules.
As will be seen, it is sometimes possible to identify proteinases which
hydrolyse the same substrates in both acid and alkaline media and
which bear some resemblance to trypsin in vertebrates. Proteinases are
localized in cell cytoplasm (Holter and Lovtrup, 1949), and probably
in the lysosomes together with other acid hydrolases (de Duve, 1959):
they may also be found in the nuclei of some cells (Dounce, 1954); but
in Amoeba proteus they are not affected by experimental enucleation
(Urbani, 1952). It has been suggested that they participate exclusively
in posthumous phenomena; although based on the fact that they show
optimal activity in an acid medium—and this is therefore not applicable
to alkaline proteinases—this opinion has met with adverse criticism
from many authors. Danilewski (1886) showed in vitro synthesis by
these enzymes, which reacted on a protein hydrolysate, producing a
protein of relatively high molecular weight which he called plastein.
After him, many leading enzymologists investigated the possibility of
in vivo protein synthesis by proteinases (Bergmann and FraenkelConrat, 1938; Binkley, 1950; Borsook, 1952; Linderström-Lang, 1952;
Northrop, 1947; Terroine, 1955; Viertanen and co-workers, 1949;