2014-06-13

#selenium helps preserve #vitaminC

1.20: news.health/counterexample to elder's vit'c use:
nutrition.org 1989:
. in the elderly, supplementing with vit'c
resulted in raising the ratio of
dehydro-ascorbate (oxidized C) to ascorbate;
this may be due to their lesser glutathione function
which is needed to reduce dehyrdo-C to vit'C .
news.health/se/#selenium helps preserve #vitaminC:
Journal of Biological Chemistry 1997:
Ascorbic acid, or vitamin C,
is important as a cofactor in enzyme reactions,
and in the defense against oxidant stress.
Mammals efficiently regenerate ascorbic acid
from its two-electron oxidized form,
dehydroascorbate (dehydro-C)
Recycling of ascorbate from its oxidized forms
is essential to maintain stores of the vitamin in human cells.
This recycling of dehydro-C to ascorbate
is known to be mediated by Glutathione (GSH)
--[which is regenerated by a selenium compound]--
either by direct chemical reduction,
or with the assistance of one or more
GSH-dependent enzymes.
However, a recent report showing that
rapid dehydro-C reduction in HL-60 cells
was unaffected by GSH depletion
brings up the possibility that other mechanisms
may contribute to dehydro-C recycling.
For example, NADPH-dependent dehydro-C reduction
has been reported to occur in rat liver
through the action of 3α-hydroxysteroid dehydrogenase.
. Mammalian thioredoxin reductase (TR) (EC 1.6.4.5)
is a selenoprotein,
that, in conjunction with thioredoxin (Trx),
forms an effective system for reduction of
protein disulfides.
. TR can also reduce many other substrates
without the assistance of Trx,
including lipoic acid, vitamin K3, ... .
Given the broad substrate specificity of TR,
. it was surprising that dehydro-C
was reportednot to be a substrate of TR.
. Our results show that the thioredoxin system
can reduce dehydro-C in the presence of NADPH,
and that a decrease in this reduction
may contribute to lowered ascorbate concentrations
in livers of selenium-deficient rats.
Selenium deficiency lowered
liver thioredoxin reductase activity by 88%,
glutathione peroxidase activity by 99%,
and ascorbate content by 33%,
but did not affect GSH content.
These results show that the thioredoxin system
can reduce dehydroascorbate,
and that this function is required for
maintenance of liver ascorbate content.