17.
A C T I O N S O F V I T A M I N S
A A N D C
679
other than organ culture. Using cartilaginous limb-bone rudiments
taken directly from 8-9-day chick embryos, Lucy, Dingle and Fell
(1961) were able to show that normal chondrocytes contain an enzyme
that is capable of producing an effect on cartilage matrix that closely
resembles that caused by excess of vitamin A. It appears to act on the
protein moiety of the protein-mucopolysaccharide complex. This
enzyme is in a particulate form, has an acid p H optimum, and appears
to be a cathepsin identical with that released by the explanted cartilage
in culture.
Gianetto and de Duve (1955) had shown that in rat liver, cathepsins
and other hydrolases are contained in particles which they termed
lysosomes. It occurred to Dingle (1961) that perhaps vitamin A acted
on similar granules in the chondrocytes, causing an abnormally active
release of the hydrolases, including cathepsins, that are incorporated
in these organelles. Experiments on the action of vitamin A on lysosomal
particles isolated from various tissues including cartilage, showed that
this idea was correct. Moreover, the molecular requirements for the
release of the cathepsin from the particles, had a similar specificity to
those for producing the histological changes in skin and cartilage in
culture.
Further work revealed that vitamin A acts on isolated mitochondria
(de Luca and Dingle, unpublished) causing an inhibition of their
respiratory activity. This may explain the fact recorded above, that in
the presence of vitamin A cartilaginous rudiments consume less oxygen
but produce more lactic acid than controls in normal medium. These
changes in the physiology of isolated mitochondria are accompanied
by severe swelling (Lucy, Luscombe and Dingle, 1963). Cell membranes
may also be affected, and erythrocytes are rapidly lysed by vitamin A
(Dingle and Lucy, 1962).
Little is known about the mechanism through which vitamin A
acts on chick epidermis and other epithelia. As described above,
however, Fitton Jackson and Fell (1963) have shown that the fine
filamentous material with which the cytoplasm of the basal cells in
the controls is packed, is not formed in the vitamin A-treated epidermis.
Autoradiographic experiments (Pelc and Fell, 1960) afford evidence
that the synthesis of sulphated mucopolysaccharides was almost as
active in the basal cells of the controls as in those of the vitamin Atreated explants. This synthesis was inhibited by keratinization in the
upper layers of the control epithelium but not in the superficial cells
of the epidermis exposed to the vitamin which inhibited keratinization.
Thus it is conceivable that, in embryonic chick skin, mucus secretion
may be the automatic result of inhibited keratinization; this, however,
is mere speculation.
A C T I O N S O F V I T A M I N S
A A N D C
679
other than organ culture. Using cartilaginous limb-bone rudiments
taken directly from 8-9-day chick embryos, Lucy, Dingle and Fell
(1961) were able to show that normal chondrocytes contain an enzyme
that is capable of producing an effect on cartilage matrix that closely
resembles that caused by excess of vitamin A. It appears to act on the
protein moiety of the protein-mucopolysaccharide complex. This
enzyme is in a particulate form, has an acid p H optimum, and appears
to be a cathepsin identical with that released by the explanted cartilage
in culture.
Gianetto and de Duve (1955) had shown that in rat liver, cathepsins
and other hydrolases are contained in particles which they termed
lysosomes. It occurred to Dingle (1961) that perhaps vitamin A acted
on similar granules in the chondrocytes, causing an abnormally active
release of the hydrolases, including cathepsins, that are incorporated
in these organelles. Experiments on the action of vitamin A on lysosomal
particles isolated from various tissues including cartilage, showed that
this idea was correct. Moreover, the molecular requirements for the
release of the cathepsin from the particles, had a similar specificity to
those for producing the histological changes in skin and cartilage in
culture.
Further work revealed that vitamin A acts on isolated mitochondria
(de Luca and Dingle, unpublished) causing an inhibition of their
respiratory activity. This may explain the fact recorded above, that in
the presence of vitamin A cartilaginous rudiments consume less oxygen
but produce more lactic acid than controls in normal medium. These
changes in the physiology of isolated mitochondria are accompanied
by severe swelling (Lucy, Luscombe and Dingle, 1963). Cell membranes
may also be affected, and erythrocytes are rapidly lysed by vitamin A
(Dingle and Lucy, 1962).
Little is known about the mechanism through which vitamin A
acts on chick epidermis and other epithelia. As described above,
however, Fitton Jackson and Fell (1963) have shown that the fine
filamentous material with which the cytoplasm of the basal cells in
the controls is packed, is not formed in the vitamin A-treated epidermis.
Autoradiographic experiments (Pelc and Fell, 1960) afford evidence
that the synthesis of sulphated mucopolysaccharides was almost as
active in the basal cells of the controls as in those of the vitamin Atreated explants. This synthesis was inhibited by keratinization in the
upper layers of the control epithelium but not in the superficial cells
of the epidermis exposed to the vitamin which inhibited keratinization.
Thus it is conceivable that, in embryonic chick skin, mucus secretion
may be the automatic result of inhibited keratinization; this, however,
is mere speculation.