17.
A C T I O N S
O F
V I T A M I N S
A
A N D
C
677
O n the other hand, bone appeared unaffected by papain, whereas
it rapidly disintegrated in response to vitamin A. From this it was
thought that the action of the vitamin on the skeleton was not due
merely to the release of a single papain-like protease. In the light of
these results, the following working hypothesis (Fell and Thomas, 1960)
was formulated: "that vitamin A may enhance the activity of a number
of cellular enzymes, one of which resembles vitamin A in its effect;
such an enhanced hydrolytic activity might be due to the activation of
enzymes, to their greater production or to their increased liberation
through an increased permeability of the cells or their organelles."
This hypothesis was the point of departure for a series of biochemical
studies on the action of vitamin A on cartilage in culture.
(d) Effect of vitamin A on composition and metabolism of cartilage in culture.
Experiments were made on the effects of vitamin A on the composition
and metabolism of the cartilaginous limb-bone rudiments from 7-day
chick embryos (Dingle, Lucy and Fell, 1961). After 6 days' cultivation
in the presence of the vitamin, the wet and dry weights and aminosugar content of the rudiments were half those of the controls in normal
medium. As compared with the controls, the amount of deoxyribonucleic acid was 8 0 % but the amount of ribonucleic acid was only 4 0 % .
Metabolic studies showed that the vitamin depressed the rate of oxygen
uptake, but increased that of lactic acid production.
(e) Effect of vitamin A on proteolytic activity of cartilage in culture. It had
long been noticed (Fell, unpublished) that vitamin A-treated explants
of various tissues liquified a plasma and embryo extract clot much more
than their controls in normal medium. This suggested an increased
proteolysis. Examination of clots with and without vitamin A, after
removal of the explants (limb-bone rudiments from 7-day chick
embryos), showed that during 48 h the vitamin A-treated rudiments
produced twice as much acid-soluble nitrogen in the medium as their
controls in normal medium (Dingle, Lucy and Fell, 1961). In the
absence of explants, "blank" clots incubated with and without the
vitamin showed no difference in acid-soluble nitrogen. Thus the results
indicated that the vitamin had increased the proteolytic activity of the
cartilage. This has recently been confirmed by Fell and Dingle (1963)
who found that when such explants were grown for 6-8 days on
plasma clots with and without vitamin A and then transferred to
chemically defined medium, in 18 h those treated with vitamin A
released up to seven times as much acid protease as their controls grown
without the vitamin. In spite of this, however, the vitamin A-treated
explants still contained up to twice as much proteolytic activity as the
controls; this means that the increased release was compensated by an
increased synthesis of the enzyme.
A C T I O N S
O F
V I T A M I N S
A
A N D
C
677
O n the other hand, bone appeared unaffected by papain, whereas
it rapidly disintegrated in response to vitamin A. From this it was
thought that the action of the vitamin on the skeleton was not due
merely to the release of a single papain-like protease. In the light of
these results, the following working hypothesis (Fell and Thomas, 1960)
was formulated: "that vitamin A may enhance the activity of a number
of cellular enzymes, one of which resembles vitamin A in its effect;
such an enhanced hydrolytic activity might be due to the activation of
enzymes, to their greater production or to their increased liberation
through an increased permeability of the cells or their organelles."
This hypothesis was the point of departure for a series of biochemical
studies on the action of vitamin A on cartilage in culture.
(d) Effect of vitamin A on composition and metabolism of cartilage in culture.
Experiments were made on the effects of vitamin A on the composition
and metabolism of the cartilaginous limb-bone rudiments from 7-day
chick embryos (Dingle, Lucy and Fell, 1961). After 6 days' cultivation
in the presence of the vitamin, the wet and dry weights and aminosugar content of the rudiments were half those of the controls in normal
medium. As compared with the controls, the amount of deoxyribonucleic acid was 8 0 % but the amount of ribonucleic acid was only 4 0 % .
Metabolic studies showed that the vitamin depressed the rate of oxygen
uptake, but increased that of lactic acid production.
(e) Effect of vitamin A on proteolytic activity of cartilage in culture. It had
long been noticed (Fell, unpublished) that vitamin A-treated explants
of various tissues liquified a plasma and embryo extract clot much more
than their controls in normal medium. This suggested an increased
proteolysis. Examination of clots with and without vitamin A, after
removal of the explants (limb-bone rudiments from 7-day chick
embryos), showed that during 48 h the vitamin A-treated rudiments
produced twice as much acid-soluble nitrogen in the medium as their
controls in normal medium (Dingle, Lucy and Fell, 1961). In the
absence of explants, "blank" clots incubated with and without the
vitamin showed no difference in acid-soluble nitrogen. Thus the results
indicated that the vitamin had increased the proteolytic activity of the
cartilage. This has recently been confirmed by Fell and Dingle (1963)
who found that when such explants were grown for 6-8 days on
plasma clots with and without vitamin A and then transferred to
chemically defined medium, in 18 h those treated with vitamin A
released up to seven times as much acid protease as their controls grown
without the vitamin. In spite of this, however, the vitamin A-treated
explants still contained up to twice as much proteolytic activity as the
controls; this means that the increased release was compensated by an
increased synthesis of the enzyme.